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1=====================
2BPF Type Format (BTF)
3=====================
4
51. Introduction
6===============
7
8BTF (BPF Type Format) is the metadata format which encodes the debug info
9related to BPF program/map. The name BTF was used initially to describe data
10types. The BTF was later extended to include function info for defined
11subroutines, and line info for source/line information.
12
13The debug info is used for map pretty print, function signature, etc. The
14function signature enables better bpf program/function kernel symbol. The line
15info helps generate source annotated translated byte code, jited code and
16verifier log.
17
18The BTF specification contains two parts,
19 * BTF kernel API
20 * BTF ELF file format
21
22The kernel API is the contract between user space and kernel. The kernel
23verifies the BTF info before using it. The ELF file format is a user space
24contract between ELF file and libbpf loader.
25
26The type and string sections are part of the BTF kernel API, describing the
27debug info (mostly types related) referenced by the bpf program. These two
28sections are discussed in details in :ref:`BTF_Type_String`.
29
30.. _BTF_Type_String:
31
322. BTF Type and String Encoding
33===============================
34
35The file ``include/uapi/linux/btf.h`` provides high-level definition of how
36types/strings are encoded.
37
38The beginning of data blob must be::
39
40 struct btf_header {
41 __u16 magic;
42 __u8 version;
43 __u8 flags;
44 __u32 hdr_len;
45
46 /* All offsets are in bytes relative to the end of this header */
47 __u32 type_off; /* offset of type section */
48 __u32 type_len; /* length of type section */
49 __u32 str_off; /* offset of string section */
50 __u32 str_len; /* length of string section */
51 };
52
53The magic is ``0xeB9F``, which has different encoding for big and little
54endian systems, and can be used to test whether BTF is generated for big- or
55little-endian target. The ``btf_header`` is designed to be extensible with
56``hdr_len`` equal to ``sizeof(struct btf_header)`` when a data blob is
57generated.
58
592.1 String Encoding
60-------------------
61
62The first string in the string section must be a null string. The rest of
63string table is a concatenation of other null-terminated strings.
64
652.2 Type Encoding
66-----------------
67
68The type id ``0`` is reserved for ``void`` type. The type section is parsed
69sequentially and type id is assigned to each recognized type starting from id
70``1``. Currently, the following types are supported::
71
72 #define BTF_KIND_INT 1 /* Integer */
73 #define BTF_KIND_PTR 2 /* Pointer */
74 #define BTF_KIND_ARRAY 3 /* Array */
75 #define BTF_KIND_STRUCT 4 /* Struct */
76 #define BTF_KIND_UNION 5 /* Union */
77 #define BTF_KIND_ENUM 6 /* Enumeration up to 32-bit values */
78 #define BTF_KIND_FWD 7 /* Forward */
79 #define BTF_KIND_TYPEDEF 8 /* Typedef */
80 #define BTF_KIND_VOLATILE 9 /* Volatile */
81 #define BTF_KIND_CONST 10 /* Const */
82 #define BTF_KIND_RESTRICT 11 /* Restrict */
83 #define BTF_KIND_FUNC 12 /* Function */
84 #define BTF_KIND_FUNC_PROTO 13 /* Function Proto */
85 #define BTF_KIND_VAR 14 /* Variable */
86 #define BTF_KIND_DATASEC 15 /* Section */
87 #define BTF_KIND_FLOAT 16 /* Floating point */
88 #define BTF_KIND_DECL_TAG 17 /* Decl Tag */
89 #define BTF_KIND_TYPE_TAG 18 /* Type Tag */
90 #define BTF_KIND_ENUM64 19 /* Enumeration up to 64-bit values */
91
92Note that the type section encodes debug info, not just pure types.
93``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
94
95Each type contains the following common data::
96
97 struct btf_type {
98 __u32 name_off;
99 /* "info" bits arrangement
100 * bits 0-15: vlen (e.g. # of struct's members)
101 * bits 16-23: unused
102 * bits 24-28: kind (e.g. int, ptr, array...etc)
103 * bits 29-30: unused
104 * bit 31: kind_flag, currently used by
105 * struct, union, fwd, enum and enum64.
106 */
107 __u32 info;
108 /* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64.
109 * "size" tells the size of the type it is describing.
110 *
111 * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
112 * FUNC, FUNC_PROTO, DECL_TAG and TYPE_TAG.
113 * "type" is a type_id referring to another type.
114 */
115 union {
116 __u32 size;
117 __u32 type;
118 };
119 };
120
121For certain kinds, the common data are followed by kind-specific data. The
122``name_off`` in ``struct btf_type`` specifies the offset in the string table.
123The following sections detail encoding of each kind.
124
1252.2.1 BTF_KIND_INT
126~~~~~~~~~~~~~~~~~~
127
128``struct btf_type`` encoding requirement:
129 * ``name_off``: any valid offset
130 * ``info.kind_flag``: 0
131 * ``info.kind``: BTF_KIND_INT
132 * ``info.vlen``: 0
133 * ``size``: the size of the int type in bytes.
134
135``btf_type`` is followed by a ``u32`` with the following bits arrangement::
136
137 #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
138 #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
139 #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
140
141The ``BTF_INT_ENCODING`` has the following attributes::
142
143 #define BTF_INT_SIGNED (1 << 0)
144 #define BTF_INT_CHAR (1 << 1)
145 #define BTF_INT_BOOL (1 << 2)
146
147The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
148bool, for the int type. The char and bool encoding are mostly useful for
149pretty print. At most one encoding can be specified for the int type.
150
151The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
152type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
153The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
154for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
155
156The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
157for this int. For example, a bitfield struct member has:
158
159 * btf member bit offset 100 from the start of the structure,
160 * btf member pointing to an int type,
161 * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
162
163Then in the struct memory layout, this member will occupy ``4`` bits starting
164from bits ``100 + 2 = 102``.
165
166Alternatively, the bitfield struct member can be the following to access the
167same bits as the above:
168
169 * btf member bit offset 102,
170 * btf member pointing to an int type,
171 * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4``
172
173The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
174bitfield encoding. Currently, both llvm and pahole generate
175``BTF_INT_OFFSET() = 0`` for all int types.
176
1772.2.2 BTF_KIND_PTR
178~~~~~~~~~~~~~~~~~~
179
180``struct btf_type`` encoding requirement:
181 * ``name_off``: 0
182 * ``info.kind_flag``: 0
183 * ``info.kind``: BTF_KIND_PTR
184 * ``info.vlen``: 0
185 * ``type``: the pointee type of the pointer
186
187No additional type data follow ``btf_type``.
188
1892.2.3 BTF_KIND_ARRAY
190~~~~~~~~~~~~~~~~~~~~
191
192``struct btf_type`` encoding requirement:
193 * ``name_off``: 0
194 * ``info.kind_flag``: 0
195 * ``info.kind``: BTF_KIND_ARRAY
196 * ``info.vlen``: 0
197 * ``size/type``: 0, not used
198
199``btf_type`` is followed by one ``struct btf_array``::
200
201 struct btf_array {
202 __u32 type;
203 __u32 index_type;
204 __u32 nelems;
205 };
206
207The ``struct btf_array`` encoding:
208 * ``type``: the element type
209 * ``index_type``: the index type
210 * ``nelems``: the number of elements for this array (``0`` is also allowed).
211
212The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``,
213``u64``, ``unsigned __int128``). The original design of including
214``index_type`` follows DWARF, which has an ``index_type`` for its array type.
215Currently in BTF, beyond type verification, the ``index_type`` is not used.
216
217The ``struct btf_array`` allows chaining through element type to represent
218multidimensional arrays. For example, for ``int a[5][6]``, the following type
219information illustrates the chaining:
220
221 * [1]: int
222 * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
223 * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
224
225Currently, both pahole and llvm collapse multidimensional array into
226one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
227equal to ``30``. This is because the original use case is map pretty print
228where the whole array is dumped out so one-dimensional array is enough. As
229more BTF usage is explored, pahole and llvm can be changed to generate proper
230chained representation for multidimensional arrays.
231
2322.2.4 BTF_KIND_STRUCT
233~~~~~~~~~~~~~~~~~~~~~
2342.2.5 BTF_KIND_UNION
235~~~~~~~~~~~~~~~~~~~~
236
237``struct btf_type`` encoding requirement:
238 * ``name_off``: 0 or offset to a valid C identifier
239 * ``info.kind_flag``: 0 or 1
240 * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION
241 * ``info.vlen``: the number of struct/union members
242 * ``info.size``: the size of the struct/union in bytes
243
244``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.::
245
246 struct btf_member {
247 __u32 name_off;
248 __u32 type;
249 __u32 offset;
250 };
251
252``struct btf_member`` encoding:
253 * ``name_off``: offset to a valid C identifier
254 * ``type``: the member type
255 * ``offset``: <see below>
256
257If the type info ``kind_flag`` is not set, the offset contains only bit offset
258of the member. Note that the base type of the bitfield can only be int or enum
259type. If the bitfield size is 32, the base type can be either int or enum
260type. If the bitfield size is not 32, the base type must be int, and int type
261``BTF_INT_BITS()`` encodes the bitfield size.
262
263If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
264bitfield size and bit offset. The bitfield size and bit offset are calculated
265as below.::
266
267 #define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
268 #define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)
269
270In this case, if the base type is an int type, it must be a regular int type:
271
272 * ``BTF_INT_OFFSET()`` must be 0.
273 * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
274
275Commit 9d5f9f701b18 introduced ``kind_flag`` and explains why both modes
276exist.
277
2782.2.6 BTF_KIND_ENUM
279~~~~~~~~~~~~~~~~~~~
280
281``struct btf_type`` encoding requirement:
282 * ``name_off``: 0 or offset to a valid C identifier
283 * ``info.kind_flag``: 0 for unsigned, 1 for signed
284 * ``info.kind``: BTF_KIND_ENUM
285 * ``info.vlen``: number of enum values
286 * ``size``: 1/2/4/8
287
288``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
289
290 struct btf_enum {
291 __u32 name_off;
292 __s32 val;
293 };
294
295The ``btf_enum`` encoding:
296 * ``name_off``: offset to a valid C identifier
297 * ``val``: any value
298
299If the original enum value is signed and the size is less than 4,
300that value will be sign extended into 4 bytes. If the size is 8,
301the value will be truncated into 4 bytes.
302
3032.2.7 BTF_KIND_FWD
304~~~~~~~~~~~~~~~~~~
305
306``struct btf_type`` encoding requirement:
307 * ``name_off``: offset to a valid C identifier
308 * ``info.kind_flag``: 0 for struct, 1 for union
309 * ``info.kind``: BTF_KIND_FWD
310 * ``info.vlen``: 0
311 * ``type``: 0
312
313No additional type data follow ``btf_type``.
314
3152.2.8 BTF_KIND_TYPEDEF
316~~~~~~~~~~~~~~~~~~~~~~
317
318``struct btf_type`` encoding requirement:
319 * ``name_off``: offset to a valid C identifier
320 * ``info.kind_flag``: 0
321 * ``info.kind``: BTF_KIND_TYPEDEF
322 * ``info.vlen``: 0
323 * ``type``: the type which can be referred by name at ``name_off``
324
325No additional type data follow ``btf_type``.
326
3272.2.9 BTF_KIND_VOLATILE
328~~~~~~~~~~~~~~~~~~~~~~~
329
330``struct btf_type`` encoding requirement:
331 * ``name_off``: 0
332 * ``info.kind_flag``: 0
333 * ``info.kind``: BTF_KIND_VOLATILE
334 * ``info.vlen``: 0
335 * ``type``: the type with ``volatile`` qualifier
336
337No additional type data follow ``btf_type``.
338
3392.2.10 BTF_KIND_CONST
340~~~~~~~~~~~~~~~~~~~~~
341
342``struct btf_type`` encoding requirement:
343 * ``name_off``: 0
344 * ``info.kind_flag``: 0
345 * ``info.kind``: BTF_KIND_CONST
346 * ``info.vlen``: 0
347 * ``type``: the type with ``const`` qualifier
348
349No additional type data follow ``btf_type``.
350
3512.2.11 BTF_KIND_RESTRICT
352~~~~~~~~~~~~~~~~~~~~~~~~
353
354``struct btf_type`` encoding requirement:
355 * ``name_off``: 0
356 * ``info.kind_flag``: 0
357 * ``info.kind``: BTF_KIND_RESTRICT
358 * ``info.vlen``: 0
359 * ``type``: the type with ``restrict`` qualifier
360
361No additional type data follow ``btf_type``.
362
3632.2.12 BTF_KIND_FUNC
364~~~~~~~~~~~~~~~~~~~~
365
366``struct btf_type`` encoding requirement:
367 * ``name_off``: offset to a valid C identifier
368 * ``info.kind_flag``: 0
369 * ``info.kind``: BTF_KIND_FUNC
370 * ``info.vlen``: linkage information (BTF_FUNC_STATIC, BTF_FUNC_GLOBAL
371 or BTF_FUNC_EXTERN - see :ref:`BTF_Function_Linkage_Constants`)
372 * ``type``: a BTF_KIND_FUNC_PROTO type
373
374No additional type data follow ``btf_type``.
375
376A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
377signature is defined by ``type``. The subprogram is thus an instance of that
378type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
379:ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
380(ABI).
381
382Currently, only linkage values of BTF_FUNC_STATIC and BTF_FUNC_GLOBAL are
383supported in the kernel.
384
3852.2.13 BTF_KIND_FUNC_PROTO
386~~~~~~~~~~~~~~~~~~~~~~~~~~
387
388``struct btf_type`` encoding requirement:
389 * ``name_off``: 0
390 * ``info.kind_flag``: 0
391 * ``info.kind``: BTF_KIND_FUNC_PROTO
392 * ``info.vlen``: # of parameters
393 * ``type``: the return type
394
395``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.::
396
397 struct btf_param {
398 __u32 name_off;
399 __u32 type;
400 };
401
402If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then
403``btf_param.name_off`` must point to a valid C identifier except for the
404possible last argument representing the variable argument. The btf_param.type
405refers to parameter type.
406
407If the function has variable arguments, the last parameter is encoded with
408``name_off = 0`` and ``type = 0``.
409
4102.2.14 BTF_KIND_VAR
411~~~~~~~~~~~~~~~~~~~
412
413``struct btf_type`` encoding requirement:
414 * ``name_off``: offset to a valid C identifier
415 * ``info.kind_flag``: 0
416 * ``info.kind``: BTF_KIND_VAR
417 * ``info.vlen``: 0
418 * ``type``: the type of the variable
419
420``btf_type`` is followed by a single ``struct btf_variable`` with the
421following data::
422
423 struct btf_var {
424 __u32 linkage;
425 };
426
427``btf_var.linkage`` may take the values: BTF_VAR_STATIC, BTF_VAR_GLOBAL_ALLOCATED or BTF_VAR_GLOBAL_EXTERN -
428see :ref:`BTF_Var_Linkage_Constants`.
429
430Not all type of global variables are supported by LLVM at this point.
431The following is currently available:
432
433 * static variables with or without section attributes
434 * global variables with section attributes
435
436The latter is for future extraction of map key/value type id's from a
437map definition.
438
4392.2.15 BTF_KIND_DATASEC
440~~~~~~~~~~~~~~~~~~~~~~~
441
442``struct btf_type`` encoding requirement:
443 * ``name_off``: offset to a valid name associated with a variable or
444 one of .data/.bss/.rodata
445 * ``info.kind_flag``: 0
446 * ``info.kind``: BTF_KIND_DATASEC
447 * ``info.vlen``: # of variables
448 * ``size``: total section size in bytes (0 at compilation time, patched
449 to actual size by BPF loaders such as libbpf)
450
451``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::
452
453 struct btf_var_secinfo {
454 __u32 type;
455 __u32 offset;
456 __u32 size;
457 };
458
459``struct btf_var_secinfo`` encoding:
460 * ``type``: the type of the BTF_KIND_VAR variable
461 * ``offset``: the in-section offset of the variable
462 * ``size``: the size of the variable in bytes
463
4642.2.16 BTF_KIND_FLOAT
465~~~~~~~~~~~~~~~~~~~~~
466
467``struct btf_type`` encoding requirement:
468 * ``name_off``: any valid offset
469 * ``info.kind_flag``: 0
470 * ``info.kind``: BTF_KIND_FLOAT
471 * ``info.vlen``: 0
472 * ``size``: the size of the float type in bytes: 2, 4, 8, 12 or 16.
473
474No additional type data follow ``btf_type``.
475
4762.2.17 BTF_KIND_DECL_TAG
477~~~~~~~~~~~~~~~~~~~~~~~~
478
479``struct btf_type`` encoding requirement:
480 * ``name_off``: offset to a non-empty string
481 * ``info.kind_flag``: 0
482 * ``info.kind``: BTF_KIND_DECL_TAG
483 * ``info.vlen``: 0
484 * ``type``: ``struct``, ``union``, ``func``, ``var`` or ``typedef``
485
486``btf_type`` is followed by ``struct btf_decl_tag``.::
487
488 struct btf_decl_tag {
489 __u32 component_idx;
490 };
491
492The ``name_off`` encodes btf_decl_tag attribute string.
493The ``type`` should be ``struct``, ``union``, ``func``, ``var`` or ``typedef``.
494For ``var`` or ``typedef`` type, ``btf_decl_tag.component_idx`` must be ``-1``.
495For the other three types, if the btf_decl_tag attribute is
496applied to the ``struct``, ``union`` or ``func`` itself,
497``btf_decl_tag.component_idx`` must be ``-1``. Otherwise,
498the attribute is applied to a ``struct``/``union`` member or
499a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a
500valid index (starting from 0) pointing to a member or an argument.
501
5022.2.18 BTF_KIND_TYPE_TAG
503~~~~~~~~~~~~~~~~~~~~~~~~
504
505``struct btf_type`` encoding requirement:
506 * ``name_off``: offset to a non-empty string
507 * ``info.kind_flag``: 0
508 * ``info.kind``: BTF_KIND_TYPE_TAG
509 * ``info.vlen``: 0
510 * ``type``: the type with ``btf_type_tag`` attribute
511
512Currently, ``BTF_KIND_TYPE_TAG`` is only emitted for pointer types.
513It has the following btf type chain:
514::
515
516 ptr -> [type_tag]*
517 -> [const | volatile | restrict | typedef]*
518 -> base_type
519
520Basically, a pointer type points to zero or more
521type_tag, then zero or more const/volatile/restrict/typedef
522and finally the base type. The base type is one of
523int, ptr, array, struct, union, enum, func_proto and float types.
524
5252.2.19 BTF_KIND_ENUM64
526~~~~~~~~~~~~~~~~~~~~~~
527
528``struct btf_type`` encoding requirement:
529 * ``name_off``: 0 or offset to a valid C identifier
530 * ``info.kind_flag``: 0 for unsigned, 1 for signed
531 * ``info.kind``: BTF_KIND_ENUM64
532 * ``info.vlen``: number of enum values
533 * ``size``: 1/2/4/8
534
535``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum64``.::
536
537 struct btf_enum64 {
538 __u32 name_off;
539 __u32 val_lo32;
540 __u32 val_hi32;
541 };
542
543The ``btf_enum64`` encoding:
544 * ``name_off``: offset to a valid C identifier
545 * ``val_lo32``: lower 32-bit value for a 64-bit value
546 * ``val_hi32``: high 32-bit value for a 64-bit value
547
548If the original enum value is signed and the size is less than 8,
549that value will be sign extended into 8 bytes.
550
5512.3 Constant Values
552-------------------
553
554.. _BTF_Function_Linkage_Constants:
555
5562.3.1 Function Linkage Constant Values
557~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
558.. table:: Function Linkage Values and Meanings
559
560 =================== ===== ===========
561 kind value description
562 =================== ===== ===========
563 ``BTF_FUNC_STATIC`` 0x0 definition of subprogram not visible outside containing compilation unit
564 ``BTF_FUNC_GLOBAL`` 0x1 definition of subprogram visible outside containing compilation unit
565 ``BTF_FUNC_EXTERN`` 0x2 declaration of a subprogram whose definition is outside the containing compilation unit
566 =================== ===== ===========
567
568
569.. _BTF_Var_Linkage_Constants:
570
5712.3.2 Variable Linkage Constant Values
572~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
573.. table:: Variable Linkage Values and Meanings
574
575 ============================ ===== ===========
576 kind value description
577 ============================ ===== ===========
578 ``BTF_VAR_STATIC`` 0x0 definition of global variable not visible outside containing compilation unit
579 ``BTF_VAR_GLOBAL_ALLOCATED`` 0x1 definition of global variable visible outside containing compilation unit
580 ``BTF_VAR_GLOBAL_EXTERN`` 0x2 declaration of global variable whose definition is outside the containing compilation unit
581 ============================ ===== ===========
582
5833. BTF Kernel API
584=================
585
586The following bpf syscall command involves BTF:
587 * BPF_BTF_LOAD: load a blob of BTF data into kernel
588 * BPF_MAP_CREATE: map creation with btf key and value type info.
589 * BPF_PROG_LOAD: prog load with btf function and line info.
590 * BPF_BTF_GET_FD_BY_ID: get a btf fd
591 * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info
592 and other btf related info are returned.
593
594The workflow typically looks like:
595::
596
597 Application:
598 BPF_BTF_LOAD
599 |
600 v
601 BPF_MAP_CREATE and BPF_PROG_LOAD
602 |
603 V
604 ......
605
606 Introspection tool:
607 ......
608 BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's)
609 |
610 V
611 BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd)
612 |
613 V
614 BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id)
615 | |
616 V |
617 BPF_BTF_GET_FD_BY_ID (get btf_fd) |
618 | |
619 V |
620 BPF_OBJ_GET_INFO_BY_FD (get btf) |
621 | |
622 V V
623 pretty print types, dump func signatures and line info, etc.
624
625
6263.1 BPF_BTF_LOAD
627----------------
628
629Load a blob of BTF data into kernel. A blob of data, described in
630:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
631is returned to a userspace.
632
6333.2 BPF_MAP_CREATE
634------------------
635
636A map can be created with ``btf_fd`` and specified key/value type id.::
637
638 __u32 btf_fd; /* fd pointing to a BTF type data */
639 __u32 btf_key_type_id; /* BTF type_id of the key */
640 __u32 btf_value_type_id; /* BTF type_id of the value */
641
642In libbpf, the map can be defined with extra annotation like below:
643::
644
645 struct {
646 __uint(type, BPF_MAP_TYPE_ARRAY);
647 __type(key, int);
648 __type(value, struct ipv_counts);
649 __uint(max_entries, 4);
650 } btf_map SEC(".maps");
651
652During ELF parsing, libbpf is able to extract key/value type_id's and assign
653them to BPF_MAP_CREATE attributes automatically.
654
655.. _BPF_Prog_Load:
656
6573.3 BPF_PROG_LOAD
658-----------------
659
660During prog_load, func_info and line_info can be passed to kernel with proper
661values for the following attributes:
662::
663
664 __u32 insn_cnt;
665 __aligned_u64 insns;
666 ......
667 __u32 prog_btf_fd; /* fd pointing to BTF type data */
668 __u32 func_info_rec_size; /* userspace bpf_func_info size */
669 __aligned_u64 func_info; /* func info */
670 __u32 func_info_cnt; /* number of bpf_func_info records */
671 __u32 line_info_rec_size; /* userspace bpf_line_info size */
672 __aligned_u64 line_info; /* line info */
673 __u32 line_info_cnt; /* number of bpf_line_info records */
674
675The func_info and line_info are an array of below, respectively.::
676
677 struct bpf_func_info {
678 __u32 insn_off; /* [0, insn_cnt - 1] */
679 __u32 type_id; /* pointing to a BTF_KIND_FUNC type */
680 };
681 struct bpf_line_info {
682 __u32 insn_off; /* [0, insn_cnt - 1] */
683 __u32 file_name_off; /* offset to string table for the filename */
684 __u32 line_off; /* offset to string table for the source line */
685 __u32 line_col; /* line number and column number */
686 };
687
688func_info_rec_size is the size of each func_info record, and
689line_info_rec_size is the size of each line_info record. Passing the record
690size to kernel make it possible to extend the record itself in the future.
691
692Below are requirements for func_info:
693 * func_info[0].insn_off must be 0.
694 * the func_info insn_off is in strictly increasing order and matches
695 bpf func boundaries.
696
697Below are requirements for line_info:
698 * the first insn in each func must have a line_info record pointing to it.
699 * the line_info insn_off is in strictly increasing order.
700
701For line_info, the line number and column number are defined as below:
702::
703
704 #define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10)
705 #define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff)
706
7073.4 BPF_{PROG,MAP}_GET_NEXT_ID
708------------------------------
709
710In kernel, every loaded program, map or btf has a unique id. The id won't
711change during the lifetime of a program, map, or btf.
712
713The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
714each command, to user space, for bpf program or maps, respectively, so an
715inspection tool can inspect all programs and maps.
716
7173.5 BPF_{PROG,MAP}_GET_FD_BY_ID
718-------------------------------
719
720An introspection tool cannot use id to get details about program or maps.
721A file descriptor needs to be obtained first for reference-counting purpose.
722
7233.6 BPF_OBJ_GET_INFO_BY_FD
724--------------------------
725
726Once a program/map fd is acquired, an introspection tool can get the detailed
727information from kernel about this fd, some of which are BTF-related. For
728example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids.
729``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated
730bpf byte codes, and jited_line_info.
731
7323.7 BPF_BTF_GET_FD_BY_ID
733------------------------
734
735With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
736syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
737command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
738kernel with BPF_BTF_LOAD, can be retrieved.
739
740With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
741tool has full btf knowledge and is able to pretty print map key/values, dump
742func signatures and line info, along with byte/jit codes.
743
7444. ELF File Format Interface
745============================
746
7474.1 .BTF section
748----------------
749
750The .BTF section contains type and string data. The format of this section is
751same as the one describe in :ref:`BTF_Type_String`.
752
753.. _BTF_Ext_Section:
754
7554.2 .BTF.ext section
756--------------------
757
758The .BTF.ext section encodes func_info, line_info and CO-RE relocations
759which needs loader manipulation before loading into the kernel.
760
761The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
762and ``tools/lib/bpf/btf.c``.
763
764The current header of .BTF.ext section::
765
766 struct btf_ext_header {
767 __u16 magic;
768 __u8 version;
769 __u8 flags;
770 __u32 hdr_len;
771
772 /* All offsets are in bytes relative to the end of this header */
773 __u32 func_info_off;
774 __u32 func_info_len;
775 __u32 line_info_off;
776 __u32 line_info_len;
777
778 /* optional part of .BTF.ext header */
779 __u32 core_relo_off;
780 __u32 core_relo_len;
781 };
782
783It is very similar to .BTF section. Instead of type/string section, it
784contains func_info, line_info and core_relo sub-sections.
785See :ref:`BPF_Prog_Load` for details about func_info and line_info
786record format.
787
788The func_info is organized as below.::
789
790 func_info_rec_size /* __u32 value */
791 btf_ext_info_sec for section #1 /* func_info for section #1 */
792 btf_ext_info_sec for section #2 /* func_info for section #2 */
793 ...
794
795``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when
796.BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of
797func_info for each specific ELF section.::
798
799 struct btf_ext_info_sec {
800 __u32 sec_name_off; /* offset to section name */
801 __u32 num_info;
802 /* Followed by num_info * record_size number of bytes */
803 __u8 data[0];
804 };
805
806Here, num_info must be greater than 0.
807
808The line_info is organized as below.::
809
810 line_info_rec_size /* __u32 value */
811 btf_ext_info_sec for section #1 /* line_info for section #1 */
812 btf_ext_info_sec for section #2 /* line_info for section #2 */
813 ...
814
815``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when
816.BTF.ext is generated.
817
818The interpretation of ``bpf_func_info->insn_off`` and
819``bpf_line_info->insn_off`` is different between kernel API and ELF API. For
820kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
821bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
822beginning of section (``btf_ext_info_sec->sec_name_off``).
823
824The core_relo is organized as below.::
825
826 core_relo_rec_size /* __u32 value */
827 btf_ext_info_sec for section #1 /* core_relo for section #1 */
828 btf_ext_info_sec for section #2 /* core_relo for section #2 */
829
830``core_relo_rec_size`` specifies the size of ``bpf_core_relo``
831structure when .BTF.ext is generated. All ``bpf_core_relo`` structures
832within a single ``btf_ext_info_sec`` describe relocations applied to
833section named by ``btf_ext_info_sec->sec_name_off``.
834
835See :ref:`Documentation/bpf/llvm_reloc.rst <btf-co-re-relocations>`
836for more information on CO-RE relocations.
837
8384.3 .BTF_ids section
839--------------------
840
841The .BTF_ids section encodes BTF ID values that are used within the kernel.
842
843This section is created during the kernel compilation with the help of
844macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can
845use them to create lists and sets (sorted lists) of BTF ID values.
846
847The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values,
848with following syntax::
849
850 BTF_ID_LIST(list)
851 BTF_ID(type1, name1)
852 BTF_ID(type2, name2)
853
854resulting in following layout in .BTF_ids section::
855
856 __BTF_ID__type1__name1__1:
857 .zero 4
858 __BTF_ID__type2__name2__2:
859 .zero 4
860
861The ``u32 list[];`` variable is defined to access the list.
862
863The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we
864want to define unused entry in BTF_ID_LIST, like::
865
866 BTF_ID_LIST(bpf_skb_output_btf_ids)
867 BTF_ID(struct, sk_buff)
868 BTF_ID_UNUSED
869 BTF_ID(struct, task_struct)
870
871The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values
872and their count, with following syntax::
873
874 BTF_SET_START(set)
875 BTF_ID(type1, name1)
876 BTF_ID(type2, name2)
877 BTF_SET_END(set)
878
879resulting in following layout in .BTF_ids section::
880
881 __BTF_ID__set__set:
882 .zero 4
883 __BTF_ID__type1__name1__3:
884 .zero 4
885 __BTF_ID__type2__name2__4:
886 .zero 4
887
888The ``struct btf_id_set set;`` variable is defined to access the list.
889
890The ``typeX`` name can be one of following::
891
892 struct, union, typedef, func
893
894and is used as a filter when resolving the BTF ID value.
895
896All the BTF ID lists and sets are compiled in the .BTF_ids section and
897resolved during the linking phase of kernel build by ``resolve_btfids`` tool.
898
8994.4 .BTF.base section
900---------------------
901Split BTF - where the .BTF section only contains types not in the associated
902base .BTF section - is an extremely efficient way to encode type information
903for kernel modules, since they generally consist of a few module-specific
904types along with a large set of shared kernel types. The former are encoded
905in split BTF, while the latter are encoded in base BTF, resulting in more
906compact representations. A type in split BTF that refers to a type in
907base BTF refers to it using its base BTF ID, and split BTF IDs start
908at last_base_BTF_ID + 1.
909
910The downside of this approach however is that this makes the split BTF
911somewhat brittle - when the base BTF changes, base BTF ID references are
912no longer valid and the split BTF itself becomes useless. The role of the
913.BTF.base section is to make split BTF more resilient for cases where
914the base BTF may change, as is the case for kernel modules not built every
915time the kernel is for example. .BTF.base contains named base types; INTs,
916FLOATs, STRUCTs, UNIONs, ENUM[64]s and FWDs. INTs and FLOATs are fully
917described in .BTF.base sections, while composite types like structs
918and unions are not fully defined - the .BTF.base type simply serves as
919a description of the type the split BTF referred to, so structs/unions
920have 0 members in the .BTF.base section. ENUM[64]s are similarly recorded
921with 0 members. Any other types are added to the split BTF. This
922distillation process then leaves us with a .BTF.base section with
923such minimal descriptions of base types and .BTF split section which refers
924to those base types. Later, we can relocate the split BTF using both the
925information stored in the .BTF.base section and the new .BTF base; the type
926information in the .BTF.base section allows us to update the split BTF
927references to point at the corresponding new base BTF IDs.
928
929BTF relocation happens on kernel module load when a kernel module has a
930.BTF.base section, and libbpf also provides a btf__relocate() API to
931accomplish this.
932
933As an example consider the following base BTF::
934
935 [1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
936 [2] STRUCT 'foo' size=8 vlen=2
937 'f1' type_id=1 bits_offset=0
938 'f2' type_id=1 bits_offset=32
939
940...and associated split BTF::
941
942 [3] PTR '(anon)' type_id=2
943
944i.e. split BTF describes a pointer to struct foo { int f1; int f2 };
945
946.BTF.base will consist of::
947
948 [1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
949 [2] STRUCT 'foo' size=8 vlen=0
950
951If we relocate the split BTF later using the following new base BTF::
952
953 [1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
954 [2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
955 [3] STRUCT 'foo' size=8 vlen=2
956 'f1' type_id=2 bits_offset=0
957 'f2' type_id=2 bits_offset=32
958
959...we can use our .BTF.base description to know that the split BTF reference
960is to struct foo, and relocation results in new split BTF::
961
962 [4] PTR '(anon)' type_id=3
963
964Note that we had to update BTF ID and start BTF ID for the split BTF.
965
966So we see how .BTF.base plays the role of facilitating later relocation,
967leading to more resilient split BTF.
968
969.BTF.base sections will be generated automatically for out-of-tree kernel module
970builds - i.e. where KBUILD_EXTMOD is set (as it would be for "make M=path/2/mod"
971cases). .BTF.base generation requires pahole support for the "distilled_base"
972BTF feature; this is available in pahole v1.28 and later.
973
9745. Using BTF
975============
976
9775.1 bpftool map pretty print
978----------------------------
979
980With BTF, the map key/value can be printed based on fields rather than simply
981raw bytes. This is especially valuable for large structure or if your data
982structure has bitfields. For example, for the following map,::
983
984 enum A { A1, A2, A3, A4, A5 };
985 typedef enum A ___A;
986 struct tmp_t {
987 char a1:4;
988 int a2:4;
989 int :4;
990 __u32 a3:4;
991 int b;
992 ___A b1:4;
993 enum A b2:4;
994 };
995 struct {
996 __uint(type, BPF_MAP_TYPE_ARRAY);
997 __type(key, int);
998 __type(value, struct tmp_t);
999 __uint(max_entries, 1);
1000 } tmpmap SEC(".maps");
1001
1002bpftool is able to pretty print like below:
1003::
1004
1005 [{
1006 "key": 0,
1007 "value": {
1008 "a1": 0x2,
1009 "a2": 0x4,
1010 "a3": 0x6,
1011 "b": 7,
1012 "b1": 0x8,
1013 "b2": 0xa
1014 }
1015 }
1016 ]
1017
10185.2 bpftool prog dump
1019---------------------
1020
1021The following is an example showing how func_info and line_info can help prog
1022dump with better kernel symbol names, function prototypes and line
1023information.::
1024
1025 $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
1026 [...]
1027 int test_long_fname_2(struct dummy_tracepoint_args * arg):
1028 bpf_prog_44a040bf25481309_test_long_fname_2:
1029 ; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
1030 0: push %rbp
1031 1: mov %rsp,%rbp
1032 4: sub $0x30,%rsp
1033 b: sub $0x28,%rbp
1034 f: mov %rbx,0x0(%rbp)
1035 13: mov %r13,0x8(%rbp)
1036 17: mov %r14,0x10(%rbp)
1037 1b: mov %r15,0x18(%rbp)
1038 1f: xor %eax,%eax
1039 21: mov %rax,0x20(%rbp)
1040 25: xor %esi,%esi
1041 ; int key = 0;
1042 27: mov %esi,-0x4(%rbp)
1043 ; if (!arg->sock)
1044 2a: mov 0x8(%rdi),%rdi
1045 ; if (!arg->sock)
1046 2e: cmp $0x0,%rdi
1047 32: je 0x0000000000000070
1048 34: mov %rbp,%rsi
1049 ; counts = bpf_map_lookup_elem(&btf_map, &key);
1050 [...]
1051
10525.3 Verifier Log
1053----------------
1054
1055The following is an example of how line_info can help debugging verification
1056failure.::
1057
1058 /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
1059 * is modified as below.
1060 */
1061 data = (void *)(long)xdp->data;
1062 data_end = (void *)(long)xdp->data_end;
1063 /*
1064 if (data + 4 > data_end)
1065 return XDP_DROP;
1066 */
1067 *(u32 *)data = dst->dst;
1068
1069 $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
1070 ; data = (void *)(long)xdp->data;
1071 224: (79) r2 = *(u64 *)(r10 -112)
1072 225: (61) r2 = *(u32 *)(r2 +0)
1073 ; *(u32 *)data = dst->dst;
1074 226: (63) *(u32 *)(r2 +0) = r1
1075 invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
1076 R2 offset is outside of the packet
1077
10786. BTF Generation
1079=================
1080
1081You need latest pahole
1082
1083 https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
1084
1085or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't
1086support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,::
1087
1088 -bash-4.4$ cat t.c
1089 struct t {
1090 int a:2;
1091 int b:3;
1092 int c:2;
1093 } g;
1094 -bash-4.4$ gcc -c -O2 -g t.c
1095 -bash-4.4$ pahole -JV t.o
1096 File t.o:
1097 [1] STRUCT t kind_flag=1 size=4 vlen=3
1098 a type_id=2 bitfield_size=2 bits_offset=0
1099 b type_id=2 bitfield_size=3 bits_offset=2
1100 c type_id=2 bitfield_size=2 bits_offset=5
1101 [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
1102
1103The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target
1104only. The assembly code (-S) is able to show the BTF encoding in assembly
1105format.::
1106
1107 -bash-4.4$ cat t2.c
1108 typedef int __int32;
1109 struct t2 {
1110 int a2;
1111 int (*f2)(char q1, __int32 q2, ...);
1112 int (*f3)();
1113 } g2;
1114 int main() { return 0; }
1115 int test() { return 0; }
1116 -bash-4.4$ clang -c -g -O2 --target=bpf t2.c
1117 -bash-4.4$ readelf -S t2.o
1118 ......
1119 [ 8] .BTF PROGBITS 0000000000000000 00000247
1120 000000000000016e 0000000000000000 0 0 1
1121 [ 9] .BTF.ext PROGBITS 0000000000000000 000003b5
1122 0000000000000060 0000000000000000 0 0 1
1123 [10] .rel.BTF.ext REL 0000000000000000 000007e0
1124 0000000000000040 0000000000000010 16 9 8
1125 ......
1126 -bash-4.4$ clang -S -g -O2 --target=bpf t2.c
1127 -bash-4.4$ cat t2.s
1128 ......
1129 .section .BTF,"",@progbits
1130 .short 60319 # 0xeb9f
1131 .byte 1
1132 .byte 0
1133 .long 24
1134 .long 0
1135 .long 220
1136 .long 220
1137 .long 122
1138 .long 0 # BTF_KIND_FUNC_PROTO(id = 1)
1139 .long 218103808 # 0xd000000
1140 .long 2
1141 .long 83 # BTF_KIND_INT(id = 2)
1142 .long 16777216 # 0x1000000
1143 .long 4
1144 .long 16777248 # 0x1000020
1145 ......
1146 .byte 0 # string offset=0
1147 .ascii ".text" # string offset=1
1148 .byte 0
1149 .ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7
1150 .byte 0
1151 .ascii "int main() { return 0; }" # string offset=33
1152 .byte 0
1153 .ascii "int test() { return 0; }" # string offset=58
1154 .byte 0
1155 .ascii "int" # string offset=83
1156 ......
1157 .section .BTF.ext,"",@progbits
1158 .short 60319 # 0xeb9f
1159 .byte 1
1160 .byte 0
1161 .long 24
1162 .long 0
1163 .long 28
1164 .long 28
1165 .long 44
1166 .long 8 # FuncInfo
1167 .long 1 # FuncInfo section string offset=1
1168 .long 2
1169 .long .Lfunc_begin0
1170 .long 3
1171 .long .Lfunc_begin1
1172 .long 5
1173 .long 16 # LineInfo
1174 .long 1 # LineInfo section string offset=1
1175 .long 2
1176 .long .Ltmp0
1177 .long 7
1178 .long 33
1179 .long 7182 # Line 7 Col 14
1180 .long .Ltmp3
1181 .long 7
1182 .long 58
1183 .long 8206 # Line 8 Col 14
1184
11857. Testing
1186==========
1187
1188The kernel BPF selftest `tools/testing/selftests/bpf/prog_tests/btf.c`_
1189provides an extensive set of BTF-related tests.
1190
1191.. Links
1192.. _tools/testing/selftests/bpf/prog_tests/btf.c:
1193 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/btf.c
1=====================
2BPF Type Format (BTF)
3=====================
4
51. Introduction
6***************
7
8BTF (BPF Type Format) is the metadata format which encodes the debug info
9related to BPF program/map. The name BTF was used initially to describe data
10types. The BTF was later extended to include function info for defined
11subroutines, and line info for source/line information.
12
13The debug info is used for map pretty print, function signature, etc. The
14function signature enables better bpf program/function kernel symbol. The line
15info helps generate source annotated translated byte code, jited code and
16verifier log.
17
18The BTF specification contains two parts,
19 * BTF kernel API
20 * BTF ELF file format
21
22The kernel API is the contract between user space and kernel. The kernel
23verifies the BTF info before using it. The ELF file format is a user space
24contract between ELF file and libbpf loader.
25
26The type and string sections are part of the BTF kernel API, describing the
27debug info (mostly types related) referenced by the bpf program. These two
28sections are discussed in details in :ref:`BTF_Type_String`.
29
30.. _BTF_Type_String:
31
322. BTF Type and String Encoding
33*******************************
34
35The file ``include/uapi/linux/btf.h`` provides high-level definition of how
36types/strings are encoded.
37
38The beginning of data blob must be::
39
40 struct btf_header {
41 __u16 magic;
42 __u8 version;
43 __u8 flags;
44 __u32 hdr_len;
45
46 /* All offsets are in bytes relative to the end of this header */
47 __u32 type_off; /* offset of type section */
48 __u32 type_len; /* length of type section */
49 __u32 str_off; /* offset of string section */
50 __u32 str_len; /* length of string section */
51 };
52
53The magic is ``0xeB9F``, which has different encoding for big and little
54endian systems, and can be used to test whether BTF is generated for big- or
55little-endian target. The ``btf_header`` is designed to be extensible with
56``hdr_len`` equal to ``sizeof(struct btf_header)`` when a data blob is
57generated.
58
592.1 String Encoding
60===================
61
62The first string in the string section must be a null string. The rest of
63string table is a concatenation of other null-terminated strings.
64
652.2 Type Encoding
66=================
67
68The type id ``0`` is reserved for ``void`` type. The type section is parsed
69sequentially and type id is assigned to each recognized type starting from id
70``1``. Currently, the following types are supported::
71
72 #define BTF_KIND_INT 1 /* Integer */
73 #define BTF_KIND_PTR 2 /* Pointer */
74 #define BTF_KIND_ARRAY 3 /* Array */
75 #define BTF_KIND_STRUCT 4 /* Struct */
76 #define BTF_KIND_UNION 5 /* Union */
77 #define BTF_KIND_ENUM 6 /* Enumeration */
78 #define BTF_KIND_FWD 7 /* Forward */
79 #define BTF_KIND_TYPEDEF 8 /* Typedef */
80 #define BTF_KIND_VOLATILE 9 /* Volatile */
81 #define BTF_KIND_CONST 10 /* Const */
82 #define BTF_KIND_RESTRICT 11 /* Restrict */
83 #define BTF_KIND_FUNC 12 /* Function */
84 #define BTF_KIND_FUNC_PROTO 13 /* Function Proto */
85 #define BTF_KIND_VAR 14 /* Variable */
86 #define BTF_KIND_DATASEC 15 /* Section */
87
88Note that the type section encodes debug info, not just pure types.
89``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
90
91Each type contains the following common data::
92
93 struct btf_type {
94 __u32 name_off;
95 /* "info" bits arrangement
96 * bits 0-15: vlen (e.g. # of struct's members)
97 * bits 16-23: unused
98 * bits 24-27: kind (e.g. int, ptr, array...etc)
99 * bits 28-30: unused
100 * bit 31: kind_flag, currently used by
101 * struct, union and fwd
102 */
103 __u32 info;
104 /* "size" is used by INT, ENUM, STRUCT and UNION.
105 * "size" tells the size of the type it is describing.
106 *
107 * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
108 * FUNC and FUNC_PROTO.
109 * "type" is a type_id referring to another type.
110 */
111 union {
112 __u32 size;
113 __u32 type;
114 };
115 };
116
117For certain kinds, the common data are followed by kind-specific data. The
118``name_off`` in ``struct btf_type`` specifies the offset in the string table.
119The following sections detail encoding of each kind.
120
1212.2.1 BTF_KIND_INT
122~~~~~~~~~~~~~~~~~~
123
124``struct btf_type`` encoding requirement:
125 * ``name_off``: any valid offset
126 * ``info.kind_flag``: 0
127 * ``info.kind``: BTF_KIND_INT
128 * ``info.vlen``: 0
129 * ``size``: the size of the int type in bytes.
130
131``btf_type`` is followed by a ``u32`` with the following bits arrangement::
132
133 #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
134 #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
135 #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
136
137The ``BTF_INT_ENCODING`` has the following attributes::
138
139 #define BTF_INT_SIGNED (1 << 0)
140 #define BTF_INT_CHAR (1 << 1)
141 #define BTF_INT_BOOL (1 << 2)
142
143The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
144bool, for the int type. The char and bool encoding are mostly useful for
145pretty print. At most one encoding can be specified for the int type.
146
147The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
148type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
149The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
150for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
151
152The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
153for this int. For example, a bitfield struct member has:
154
155 * btf member bit offset 100 from the start of the structure,
156 * btf member pointing to an int type,
157 * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
158
159Then in the struct memory layout, this member will occupy ``4`` bits starting
160from bits ``100 + 2 = 102``.
161
162Alternatively, the bitfield struct member can be the following to access the
163same bits as the above:
164
165 * btf member bit offset 102,
166 * btf member pointing to an int type,
167 * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4``
168
169The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
170bitfield encoding. Currently, both llvm and pahole generate
171``BTF_INT_OFFSET() = 0`` for all int types.
172
1732.2.2 BTF_KIND_PTR
174~~~~~~~~~~~~~~~~~~
175
176``struct btf_type`` encoding requirement:
177 * ``name_off``: 0
178 * ``info.kind_flag``: 0
179 * ``info.kind``: BTF_KIND_PTR
180 * ``info.vlen``: 0
181 * ``type``: the pointee type of the pointer
182
183No additional type data follow ``btf_type``.
184
1852.2.3 BTF_KIND_ARRAY
186~~~~~~~~~~~~~~~~~~~~
187
188``struct btf_type`` encoding requirement:
189 * ``name_off``: 0
190 * ``info.kind_flag``: 0
191 * ``info.kind``: BTF_KIND_ARRAY
192 * ``info.vlen``: 0
193 * ``size/type``: 0, not used
194
195``btf_type`` is followed by one ``struct btf_array``::
196
197 struct btf_array {
198 __u32 type;
199 __u32 index_type;
200 __u32 nelems;
201 };
202
203The ``struct btf_array`` encoding:
204 * ``type``: the element type
205 * ``index_type``: the index type
206 * ``nelems``: the number of elements for this array (``0`` is also allowed).
207
208The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``,
209``u64``, ``unsigned __int128``). The original design of including
210``index_type`` follows DWARF, which has an ``index_type`` for its array type.
211Currently in BTF, beyond type verification, the ``index_type`` is not used.
212
213The ``struct btf_array`` allows chaining through element type to represent
214multidimensional arrays. For example, for ``int a[5][6]``, the following type
215information illustrates the chaining:
216
217 * [1]: int
218 * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
219 * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
220
221Currently, both pahole and llvm collapse multidimensional array into
222one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
223equal to ``30``. This is because the original use case is map pretty print
224where the whole array is dumped out so one-dimensional array is enough. As
225more BTF usage is explored, pahole and llvm can be changed to generate proper
226chained representation for multidimensional arrays.
227
2282.2.4 BTF_KIND_STRUCT
229~~~~~~~~~~~~~~~~~~~~~
2302.2.5 BTF_KIND_UNION
231~~~~~~~~~~~~~~~~~~~~
232
233``struct btf_type`` encoding requirement:
234 * ``name_off``: 0 or offset to a valid C identifier
235 * ``info.kind_flag``: 0 or 1
236 * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION
237 * ``info.vlen``: the number of struct/union members
238 * ``info.size``: the size of the struct/union in bytes
239
240``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.::
241
242 struct btf_member {
243 __u32 name_off;
244 __u32 type;
245 __u32 offset;
246 };
247
248``struct btf_member`` encoding:
249 * ``name_off``: offset to a valid C identifier
250 * ``type``: the member type
251 * ``offset``: <see below>
252
253If the type info ``kind_flag`` is not set, the offset contains only bit offset
254of the member. Note that the base type of the bitfield can only be int or enum
255type. If the bitfield size is 32, the base type can be either int or enum
256type. If the bitfield size is not 32, the base type must be int, and int type
257``BTF_INT_BITS()`` encodes the bitfield size.
258
259If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
260bitfield size and bit offset. The bitfield size and bit offset are calculated
261as below.::
262
263 #define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
264 #define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)
265
266In this case, if the base type is an int type, it must be a regular int type:
267
268 * ``BTF_INT_OFFSET()`` must be 0.
269 * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
270
271The following kernel patch introduced ``kind_flag`` and explained why both
272modes exist:
273
274 https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3
275
2762.2.6 BTF_KIND_ENUM
277~~~~~~~~~~~~~~~~~~~
278
279``struct btf_type`` encoding requirement:
280 * ``name_off``: 0 or offset to a valid C identifier
281 * ``info.kind_flag``: 0
282 * ``info.kind``: BTF_KIND_ENUM
283 * ``info.vlen``: number of enum values
284 * ``size``: 4
285
286``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
287
288 struct btf_enum {
289 __u32 name_off;
290 __s32 val;
291 };
292
293The ``btf_enum`` encoding:
294 * ``name_off``: offset to a valid C identifier
295 * ``val``: any value
296
2972.2.7 BTF_KIND_FWD
298~~~~~~~~~~~~~~~~~~
299
300``struct btf_type`` encoding requirement:
301 * ``name_off``: offset to a valid C identifier
302 * ``info.kind_flag``: 0 for struct, 1 for union
303 * ``info.kind``: BTF_KIND_FWD
304 * ``info.vlen``: 0
305 * ``type``: 0
306
307No additional type data follow ``btf_type``.
308
3092.2.8 BTF_KIND_TYPEDEF
310~~~~~~~~~~~~~~~~~~~~~~
311
312``struct btf_type`` encoding requirement:
313 * ``name_off``: offset to a valid C identifier
314 * ``info.kind_flag``: 0
315 * ``info.kind``: BTF_KIND_TYPEDEF
316 * ``info.vlen``: 0
317 * ``type``: the type which can be referred by name at ``name_off``
318
319No additional type data follow ``btf_type``.
320
3212.2.9 BTF_KIND_VOLATILE
322~~~~~~~~~~~~~~~~~~~~~~~
323
324``struct btf_type`` encoding requirement:
325 * ``name_off``: 0
326 * ``info.kind_flag``: 0
327 * ``info.kind``: BTF_KIND_VOLATILE
328 * ``info.vlen``: 0
329 * ``type``: the type with ``volatile`` qualifier
330
331No additional type data follow ``btf_type``.
332
3332.2.10 BTF_KIND_CONST
334~~~~~~~~~~~~~~~~~~~~~
335
336``struct btf_type`` encoding requirement:
337 * ``name_off``: 0
338 * ``info.kind_flag``: 0
339 * ``info.kind``: BTF_KIND_CONST
340 * ``info.vlen``: 0
341 * ``type``: the type with ``const`` qualifier
342
343No additional type data follow ``btf_type``.
344
3452.2.11 BTF_KIND_RESTRICT
346~~~~~~~~~~~~~~~~~~~~~~~~
347
348``struct btf_type`` encoding requirement:
349 * ``name_off``: 0
350 * ``info.kind_flag``: 0
351 * ``info.kind``: BTF_KIND_RESTRICT
352 * ``info.vlen``: 0
353 * ``type``: the type with ``restrict`` qualifier
354
355No additional type data follow ``btf_type``.
356
3572.2.12 BTF_KIND_FUNC
358~~~~~~~~~~~~~~~~~~~~
359
360``struct btf_type`` encoding requirement:
361 * ``name_off``: offset to a valid C identifier
362 * ``info.kind_flag``: 0
363 * ``info.kind``: BTF_KIND_FUNC
364 * ``info.vlen``: 0
365 * ``type``: a BTF_KIND_FUNC_PROTO type
366
367No additional type data follow ``btf_type``.
368
369A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
370signature is defined by ``type``. The subprogram is thus an instance of that
371type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
372:ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
373(ABI).
374
3752.2.13 BTF_KIND_FUNC_PROTO
376~~~~~~~~~~~~~~~~~~~~~~~~~~
377
378``struct btf_type`` encoding requirement:
379 * ``name_off``: 0
380 * ``info.kind_flag``: 0
381 * ``info.kind``: BTF_KIND_FUNC_PROTO
382 * ``info.vlen``: # of parameters
383 * ``type``: the return type
384
385``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.::
386
387 struct btf_param {
388 __u32 name_off;
389 __u32 type;
390 };
391
392If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then
393``btf_param.name_off`` must point to a valid C identifier except for the
394possible last argument representing the variable argument. The btf_param.type
395refers to parameter type.
396
397If the function has variable arguments, the last parameter is encoded with
398``name_off = 0`` and ``type = 0``.
399
4002.2.14 BTF_KIND_VAR
401~~~~~~~~~~~~~~~~~~~
402
403``struct btf_type`` encoding requirement:
404 * ``name_off``: offset to a valid C identifier
405 * ``info.kind_flag``: 0
406 * ``info.kind``: BTF_KIND_VAR
407 * ``info.vlen``: 0
408 * ``type``: the type of the variable
409
410``btf_type`` is followed by a single ``struct btf_variable`` with the
411following data::
412
413 struct btf_var {
414 __u32 linkage;
415 };
416
417``struct btf_var`` encoding:
418 * ``linkage``: currently only static variable 0, or globally allocated
419 variable in ELF sections 1
420
421Not all type of global variables are supported by LLVM at this point.
422The following is currently available:
423
424 * static variables with or without section attributes
425 * global variables with section attributes
426
427The latter is for future extraction of map key/value type id's from a
428map definition.
429
4302.2.15 BTF_KIND_DATASEC
431~~~~~~~~~~~~~~~~~~~~~~~
432
433``struct btf_type`` encoding requirement:
434 * ``name_off``: offset to a valid name associated with a variable or
435 one of .data/.bss/.rodata
436 * ``info.kind_flag``: 0
437 * ``info.kind``: BTF_KIND_DATASEC
438 * ``info.vlen``: # of variables
439 * ``size``: total section size in bytes (0 at compilation time, patched
440 to actual size by BPF loaders such as libbpf)
441
442``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::
443
444 struct btf_var_secinfo {
445 __u32 type;
446 __u32 offset;
447 __u32 size;
448 };
449
450``struct btf_var_secinfo`` encoding:
451 * ``type``: the type of the BTF_KIND_VAR variable
452 * ``offset``: the in-section offset of the variable
453 * ``size``: the size of the variable in bytes
454
4553. BTF Kernel API
456*****************
457
458The following bpf syscall command involves BTF:
459 * BPF_BTF_LOAD: load a blob of BTF data into kernel
460 * BPF_MAP_CREATE: map creation with btf key and value type info.
461 * BPF_PROG_LOAD: prog load with btf function and line info.
462 * BPF_BTF_GET_FD_BY_ID: get a btf fd
463 * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info
464 and other btf related info are returned.
465
466The workflow typically looks like:
467::
468
469 Application:
470 BPF_BTF_LOAD
471 |
472 v
473 BPF_MAP_CREATE and BPF_PROG_LOAD
474 |
475 V
476 ......
477
478 Introspection tool:
479 ......
480 BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's)
481 |
482 V
483 BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd)
484 |
485 V
486 BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id)
487 | |
488 V |
489 BPF_BTF_GET_FD_BY_ID (get btf_fd) |
490 | |
491 V |
492 BPF_OBJ_GET_INFO_BY_FD (get btf) |
493 | |
494 V V
495 pretty print types, dump func signatures and line info, etc.
496
497
4983.1 BPF_BTF_LOAD
499================
500
501Load a blob of BTF data into kernel. A blob of data, described in
502:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
503is returned to a userspace.
504
5053.2 BPF_MAP_CREATE
506==================
507
508A map can be created with ``btf_fd`` and specified key/value type id.::
509
510 __u32 btf_fd; /* fd pointing to a BTF type data */
511 __u32 btf_key_type_id; /* BTF type_id of the key */
512 __u32 btf_value_type_id; /* BTF type_id of the value */
513
514In libbpf, the map can be defined with extra annotation like below:
515::
516
517 struct bpf_map_def SEC("maps") btf_map = {
518 .type = BPF_MAP_TYPE_ARRAY,
519 .key_size = sizeof(int),
520 .value_size = sizeof(struct ipv_counts),
521 .max_entries = 4,
522 };
523 BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts);
524
525Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name, key and
526value types for the map. During ELF parsing, libbpf is able to extract
527key/value type_id's and assign them to BPF_MAP_CREATE attributes
528automatically.
529
530.. _BPF_Prog_Load:
531
5323.3 BPF_PROG_LOAD
533=================
534
535During prog_load, func_info and line_info can be passed to kernel with proper
536values for the following attributes:
537::
538
539 __u32 insn_cnt;
540 __aligned_u64 insns;
541 ......
542 __u32 prog_btf_fd; /* fd pointing to BTF type data */
543 __u32 func_info_rec_size; /* userspace bpf_func_info size */
544 __aligned_u64 func_info; /* func info */
545 __u32 func_info_cnt; /* number of bpf_func_info records */
546 __u32 line_info_rec_size; /* userspace bpf_line_info size */
547 __aligned_u64 line_info; /* line info */
548 __u32 line_info_cnt; /* number of bpf_line_info records */
549
550The func_info and line_info are an array of below, respectively.::
551
552 struct bpf_func_info {
553 __u32 insn_off; /* [0, insn_cnt - 1] */
554 __u32 type_id; /* pointing to a BTF_KIND_FUNC type */
555 };
556 struct bpf_line_info {
557 __u32 insn_off; /* [0, insn_cnt - 1] */
558 __u32 file_name_off; /* offset to string table for the filename */
559 __u32 line_off; /* offset to string table for the source line */
560 __u32 line_col; /* line number and column number */
561 };
562
563func_info_rec_size is the size of each func_info record, and
564line_info_rec_size is the size of each line_info record. Passing the record
565size to kernel make it possible to extend the record itself in the future.
566
567Below are requirements for func_info:
568 * func_info[0].insn_off must be 0.
569 * the func_info insn_off is in strictly increasing order and matches
570 bpf func boundaries.
571
572Below are requirements for line_info:
573 * the first insn in each func must have a line_info record pointing to it.
574 * the line_info insn_off is in strictly increasing order.
575
576For line_info, the line number and column number are defined as below:
577::
578
579 #define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10)
580 #define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff)
581
5823.4 BPF_{PROG,MAP}_GET_NEXT_ID
583==============================
584
585In kernel, every loaded program, map or btf has a unique id. The id won't
586change during the lifetime of a program, map, or btf.
587
588The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
589each command, to user space, for bpf program or maps, respectively, so an
590inspection tool can inspect all programs and maps.
591
5923.5 BPF_{PROG,MAP}_GET_FD_BY_ID
593===============================
594
595An introspection tool cannot use id to get details about program or maps.
596A file descriptor needs to be obtained first for reference-counting purpose.
597
5983.6 BPF_OBJ_GET_INFO_BY_FD
599==========================
600
601Once a program/map fd is acquired, an introspection tool can get the detailed
602information from kernel about this fd, some of which are BTF-related. For
603example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids.
604``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated
605bpf byte codes, and jited_line_info.
606
6073.7 BPF_BTF_GET_FD_BY_ID
608========================
609
610With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
611syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
612command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
613kernel with BPF_BTF_LOAD, can be retrieved.
614
615With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
616tool has full btf knowledge and is able to pretty print map key/values, dump
617func signatures and line info, along with byte/jit codes.
618
6194. ELF File Format Interface
620****************************
621
6224.1 .BTF section
623================
624
625The .BTF section contains type and string data. The format of this section is
626same as the one describe in :ref:`BTF_Type_String`.
627
628.. _BTF_Ext_Section:
629
6304.2 .BTF.ext section
631====================
632
633The .BTF.ext section encodes func_info and line_info which needs loader
634manipulation before loading into the kernel.
635
636The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
637and ``tools/lib/bpf/btf.c``.
638
639The current header of .BTF.ext section::
640
641 struct btf_ext_header {
642 __u16 magic;
643 __u8 version;
644 __u8 flags;
645 __u32 hdr_len;
646
647 /* All offsets are in bytes relative to the end of this header */
648 __u32 func_info_off;
649 __u32 func_info_len;
650 __u32 line_info_off;
651 __u32 line_info_len;
652 };
653
654It is very similar to .BTF section. Instead of type/string section, it
655contains func_info and line_info section. See :ref:`BPF_Prog_Load` for details
656about func_info and line_info record format.
657
658The func_info is organized as below.::
659
660 func_info_rec_size
661 btf_ext_info_sec for section #1 /* func_info for section #1 */
662 btf_ext_info_sec for section #2 /* func_info for section #2 */
663 ...
664
665``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when
666.BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of
667func_info for each specific ELF section.::
668
669 struct btf_ext_info_sec {
670 __u32 sec_name_off; /* offset to section name */
671 __u32 num_info;
672 /* Followed by num_info * record_size number of bytes */
673 __u8 data[0];
674 };
675
676Here, num_info must be greater than 0.
677
678The line_info is organized as below.::
679
680 line_info_rec_size
681 btf_ext_info_sec for section #1 /* line_info for section #1 */
682 btf_ext_info_sec for section #2 /* line_info for section #2 */
683 ...
684
685``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when
686.BTF.ext is generated.
687
688The interpretation of ``bpf_func_info->insn_off`` and
689``bpf_line_info->insn_off`` is different between kernel API and ELF API. For
690kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
691bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
692beginning of section (``btf_ext_info_sec->sec_name_off``).
693
6944.2 .BTF_ids section
695====================
696
697The .BTF_ids section encodes BTF ID values that are used within the kernel.
698
699This section is created during the kernel compilation with the help of
700macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can
701use them to create lists and sets (sorted lists) of BTF ID values.
702
703The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values,
704with following syntax::
705
706 BTF_ID_LIST(list)
707 BTF_ID(type1, name1)
708 BTF_ID(type2, name2)
709
710resulting in following layout in .BTF_ids section::
711
712 __BTF_ID__type1__name1__1:
713 .zero 4
714 __BTF_ID__type2__name2__2:
715 .zero 4
716
717The ``u32 list[];`` variable is defined to access the list.
718
719The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we
720want to define unused entry in BTF_ID_LIST, like::
721
722 BTF_ID_LIST(bpf_skb_output_btf_ids)
723 BTF_ID(struct, sk_buff)
724 BTF_ID_UNUSED
725 BTF_ID(struct, task_struct)
726
727All the BTF ID lists and sets are compiled in the .BTF_ids section and
728resolved during the linking phase of kernel build by ``resolve_btfids`` tool.
729
7305. Using BTF
731************
732
7335.1 bpftool map pretty print
734============================
735
736With BTF, the map key/value can be printed based on fields rather than simply
737raw bytes. This is especially valuable for large structure or if your data
738structure has bitfields. For example, for the following map,::
739
740 enum A { A1, A2, A3, A4, A5 };
741 typedef enum A ___A;
742 struct tmp_t {
743 char a1:4;
744 int a2:4;
745 int :4;
746 __u32 a3:4;
747 int b;
748 ___A b1:4;
749 enum A b2:4;
750 };
751 struct bpf_map_def SEC("maps") tmpmap = {
752 .type = BPF_MAP_TYPE_ARRAY,
753 .key_size = sizeof(__u32),
754 .value_size = sizeof(struct tmp_t),
755 .max_entries = 1,
756 };
757 BPF_ANNOTATE_KV_PAIR(tmpmap, int, struct tmp_t);
758
759bpftool is able to pretty print like below:
760::
761
762 [{
763 "key": 0,
764 "value": {
765 "a1": 0x2,
766 "a2": 0x4,
767 "a3": 0x6,
768 "b": 7,
769 "b1": 0x8,
770 "b2": 0xa
771 }
772 }
773 ]
774
7755.2 bpftool prog dump
776=====================
777
778The following is an example showing how func_info and line_info can help prog
779dump with better kernel symbol names, function prototypes and line
780information.::
781
782 $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
783 [...]
784 int test_long_fname_2(struct dummy_tracepoint_args * arg):
785 bpf_prog_44a040bf25481309_test_long_fname_2:
786 ; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
787 0: push %rbp
788 1: mov %rsp,%rbp
789 4: sub $0x30,%rsp
790 b: sub $0x28,%rbp
791 f: mov %rbx,0x0(%rbp)
792 13: mov %r13,0x8(%rbp)
793 17: mov %r14,0x10(%rbp)
794 1b: mov %r15,0x18(%rbp)
795 1f: xor %eax,%eax
796 21: mov %rax,0x20(%rbp)
797 25: xor %esi,%esi
798 ; int key = 0;
799 27: mov %esi,-0x4(%rbp)
800 ; if (!arg->sock)
801 2a: mov 0x8(%rdi),%rdi
802 ; if (!arg->sock)
803 2e: cmp $0x0,%rdi
804 32: je 0x0000000000000070
805 34: mov %rbp,%rsi
806 ; counts = bpf_map_lookup_elem(&btf_map, &key);
807 [...]
808
8095.3 Verifier Log
810================
811
812The following is an example of how line_info can help debugging verification
813failure.::
814
815 /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
816 * is modified as below.
817 */
818 data = (void *)(long)xdp->data;
819 data_end = (void *)(long)xdp->data_end;
820 /*
821 if (data + 4 > data_end)
822 return XDP_DROP;
823 */
824 *(u32 *)data = dst->dst;
825
826 $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
827 ; data = (void *)(long)xdp->data;
828 224: (79) r2 = *(u64 *)(r10 -112)
829 225: (61) r2 = *(u32 *)(r2 +0)
830 ; *(u32 *)data = dst->dst;
831 226: (63) *(u32 *)(r2 +0) = r1
832 invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
833 R2 offset is outside of the packet
834
8356. BTF Generation
836*****************
837
838You need latest pahole
839
840 https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
841
842or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't
843support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,::
844
845 -bash-4.4$ cat t.c
846 struct t {
847 int a:2;
848 int b:3;
849 int c:2;
850 } g;
851 -bash-4.4$ gcc -c -O2 -g t.c
852 -bash-4.4$ pahole -JV t.o
853 File t.o:
854 [1] STRUCT t kind_flag=1 size=4 vlen=3
855 a type_id=2 bitfield_size=2 bits_offset=0
856 b type_id=2 bitfield_size=3 bits_offset=2
857 c type_id=2 bitfield_size=2 bits_offset=5
858 [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
859
860The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target
861only. The assembly code (-S) is able to show the BTF encoding in assembly
862format.::
863
864 -bash-4.4$ cat t2.c
865 typedef int __int32;
866 struct t2 {
867 int a2;
868 int (*f2)(char q1, __int32 q2, ...);
869 int (*f3)();
870 } g2;
871 int main() { return 0; }
872 int test() { return 0; }
873 -bash-4.4$ clang -c -g -O2 -target bpf t2.c
874 -bash-4.4$ readelf -S t2.o
875 ......
876 [ 8] .BTF PROGBITS 0000000000000000 00000247
877 000000000000016e 0000000000000000 0 0 1
878 [ 9] .BTF.ext PROGBITS 0000000000000000 000003b5
879 0000000000000060 0000000000000000 0 0 1
880 [10] .rel.BTF.ext REL 0000000000000000 000007e0
881 0000000000000040 0000000000000010 16 9 8
882 ......
883 -bash-4.4$ clang -S -g -O2 -target bpf t2.c
884 -bash-4.4$ cat t2.s
885 ......
886 .section .BTF,"",@progbits
887 .short 60319 # 0xeb9f
888 .byte 1
889 .byte 0
890 .long 24
891 .long 0
892 .long 220
893 .long 220
894 .long 122
895 .long 0 # BTF_KIND_FUNC_PROTO(id = 1)
896 .long 218103808 # 0xd000000
897 .long 2
898 .long 83 # BTF_KIND_INT(id = 2)
899 .long 16777216 # 0x1000000
900 .long 4
901 .long 16777248 # 0x1000020
902 ......
903 .byte 0 # string offset=0
904 .ascii ".text" # string offset=1
905 .byte 0
906 .ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7
907 .byte 0
908 .ascii "int main() { return 0; }" # string offset=33
909 .byte 0
910 .ascii "int test() { return 0; }" # string offset=58
911 .byte 0
912 .ascii "int" # string offset=83
913 ......
914 .section .BTF.ext,"",@progbits
915 .short 60319 # 0xeb9f
916 .byte 1
917 .byte 0
918 .long 24
919 .long 0
920 .long 28
921 .long 28
922 .long 44
923 .long 8 # FuncInfo
924 .long 1 # FuncInfo section string offset=1
925 .long 2
926 .long .Lfunc_begin0
927 .long 3
928 .long .Lfunc_begin1
929 .long 5
930 .long 16 # LineInfo
931 .long 1 # LineInfo section string offset=1
932 .long 2
933 .long .Ltmp0
934 .long 7
935 .long 33
936 .long 7182 # Line 7 Col 14
937 .long .Ltmp3
938 .long 7
939 .long 58
940 .long 8206 # Line 8 Col 14
941
9427. Testing
943**********
944
945Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests.