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1=========================================
2How to get printk format specifiers right
3=========================================
4
5.. _printk-specifiers:
6
7:Author: Randy Dunlap <rdunlap@infradead.org>
8:Author: Andrew Murray <amurray@mpc-data.co.uk>
9
10
11Integer types
12=============
13
14::
15
16 If variable is of Type, use printk format specifier:
17 ------------------------------------------------------------
18 char %d or %x
19 unsigned char %u or %x
20 short int %d or %x
21 unsigned short int %u or %x
22 int %d or %x
23 unsigned int %u or %x
24 long %ld or %lx
25 unsigned long %lu or %lx
26 long long %lld or %llx
27 unsigned long long %llu or %llx
28 size_t %zu or %zx
29 ssize_t %zd or %zx
30 s8 %d or %x
31 u8 %u or %x
32 s16 %d or %x
33 u16 %u or %x
34 s32 %d or %x
35 u32 %u or %x
36 s64 %lld or %llx
37 u64 %llu or %llx
38
39
40If <type> is architecture-dependent for its size (e.g., cycles_t, tcflag_t) or
41is dependent on a config option for its size (e.g., blk_status_t), use a format
42specifier of its largest possible type and explicitly cast to it.
43
44Example::
45
46 printk("test: latency: %llu cycles\n", (unsigned long long)time);
47
48Reminder: sizeof() returns type size_t.
49
50The kernel's printf does not support %n. Floating point formats (%e, %f,
51%g, %a) are also not recognized, for obvious reasons. Use of any
52unsupported specifier or length qualifier results in a WARN and early
53return from vsnprintf().
54
55Pointer types
56=============
57
58A raw pointer value may be printed with %p which will hash the address
59before printing. The kernel also supports extended specifiers for printing
60pointers of different types.
61
62Some of the extended specifiers print the data on the given address instead
63of printing the address itself. In this case, the following error messages
64might be printed instead of the unreachable information::
65
66 (null) data on plain NULL address
67 (efault) data on invalid address
68 (einval) invalid data on a valid address
69
70Plain Pointers
71--------------
72
73::
74
75 %p abcdef12 or 00000000abcdef12
76
77Pointers printed without a specifier extension (i.e unadorned %p) are
78hashed to prevent leaking information about the kernel memory layout. This
79has the added benefit of providing a unique identifier. On 64-bit machines
80the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it
81gathers enough entropy.
82
83When possible, use specialised modifiers such as %pS or %pB (described below)
84to avoid the need of providing an unhashed address that has to be interpreted
85post-hoc. If not possible, and the aim of printing the address is to provide
86more information for debugging, use %p and boot the kernel with the
87``no_hash_pointers`` parameter during debugging, which will print all %p
88addresses unmodified. If you *really* always want the unmodified address, see
89%px below.
90
91If (and only if) you are printing addresses as a content of a virtual file in
92e.g. procfs or sysfs (using e.g. seq_printf(), not printk()) read by a
93userspace process, use the %pK modifier described below instead of %p or %px.
94
95Error Pointers
96--------------
97
98::
99
100 %pe -ENOSPC
101
102For printing error pointers (i.e. a pointer for which IS_ERR() is true)
103as a symbolic error name. Error values for which no symbolic name is
104known are printed in decimal, while a non-ERR_PTR passed as the
105argument to %pe gets treated as ordinary %p.
106
107Symbols/Function Pointers
108-------------------------
109
110::
111
112 %pS versatile_init+0x0/0x110
113 %ps versatile_init
114 %pSR versatile_init+0x9/0x110
115 (with __builtin_extract_return_addr() translation)
116 %pB prev_fn_of_versatile_init+0x88/0x88
117
118
119The ``S`` and ``s`` specifiers are used for printing a pointer in symbolic
120format. They result in the symbol name with (S) or without (s)
121offsets. If KALLSYMS are disabled then the symbol address is printed instead.
122
123The ``B`` specifier results in the symbol name with offsets and should be
124used when printing stack backtraces. The specifier takes into
125consideration the effect of compiler optimisations which may occur
126when tail-calls are used and marked with the noreturn GCC attribute.
127
128If the pointer is within a module, the module name and optionally build ID is
129printed after the symbol name with an extra ``b`` appended to the end of the
130specifier.
131
132::
133
134 %pS versatile_init+0x0/0x110 [module_name]
135 %pSb versatile_init+0x0/0x110 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
136 %pSRb versatile_init+0x9/0x110 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
137 (with __builtin_extract_return_addr() translation)
138 %pBb prev_fn_of_versatile_init+0x88/0x88 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
139
140Probed Pointers from BPF / tracing
141----------------------------------
142
143::
144
145 %pks kernel string
146 %pus user string
147
148The ``k`` and ``u`` specifiers are used for printing prior probed memory from
149either kernel memory (k) or user memory (u). The subsequent ``s`` specifier
150results in printing a string. For direct use in regular vsnprintf() the (k)
151and (u) annotation is ignored, however, when used out of BPF's bpf_trace_printk(),
152for example, it reads the memory it is pointing to without faulting.
153
154Kernel Pointers
155---------------
156
157::
158
159 %pK 01234567 or 0123456789abcdef
160
161For printing kernel pointers which should be hidden from unprivileged
162users. The behaviour of %pK depends on the kptr_restrict sysctl - see
163Documentation/admin-guide/sysctl/kernel.rst for more details.
164
165This modifier is *only* intended when producing content of a file read by
166userspace from e.g. procfs or sysfs, not for dmesg. Please refer to the
167section about %p above for discussion about how to manage hashing pointers
168in printk().
169
170Unmodified Addresses
171--------------------
172
173::
174
175 %px 01234567 or 0123456789abcdef
176
177For printing pointers when you *really* want to print the address. Please
178consider whether or not you are leaking sensitive information about the
179kernel memory layout before printing pointers with %px. %px is functionally
180equivalent to %lx (or %lu). %px is preferred because it is more uniquely
181grep'able. If in the future we need to modify the way the kernel handles
182printing pointers we will be better equipped to find the call sites.
183
184Before using %px, consider if using %p is sufficient together with enabling the
185``no_hash_pointers`` kernel parameter during debugging sessions (see the %p
186description above). One valid scenario for %px might be printing information
187immediately before a panic, which prevents any sensitive information to be
188exploited anyway, and with %px there would be no need to reproduce the panic
189with no_hash_pointers.
190
191Pointer Differences
192-------------------
193
194::
195
196 %td 2560
197 %tx a00
198
199For printing the pointer differences, use the %t modifier for ptrdiff_t.
200
201Example::
202
203 printk("test: difference between pointers: %td\n", ptr2 - ptr1);
204
205Struct Resources
206----------------
207
208::
209
210 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or
211 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
212 %pR [mem 0x60000000-0x6fffffff pref] or
213 [mem 0x0000000060000000-0x000000006fffffff pref]
214
215For printing struct resources. The ``R`` and ``r`` specifiers result in a
216printed resource with (R) or without (r) a decoded flags member.
217
218Passed by reference.
219
220Physical address types phys_addr_t
221----------------------------------
222
223::
224
225 %pa[p] 0x01234567 or 0x0123456789abcdef
226
227For printing a phys_addr_t type (and its derivatives, such as
228resource_size_t) which can vary based on build options, regardless of the
229width of the CPU data path.
230
231Passed by reference.
232
233DMA address types dma_addr_t
234----------------------------
235
236::
237
238 %pad 0x01234567 or 0x0123456789abcdef
239
240For printing a dma_addr_t type which can vary based on build options,
241regardless of the width of the CPU data path.
242
243Passed by reference.
244
245Raw buffer as an escaped string
246-------------------------------
247
248::
249
250 %*pE[achnops]
251
252For printing raw buffer as an escaped string. For the following buffer::
253
254 1b 62 20 5c 43 07 22 90 0d 5d
255
256A few examples show how the conversion would be done (excluding surrounding
257quotes)::
258
259 %*pE "\eb \C\a"\220\r]"
260 %*pEhp "\x1bb \C\x07"\x90\x0d]"
261 %*pEa "\e\142\040\\\103\a\042\220\r\135"
262
263The conversion rules are applied according to an optional combination
264of flags (see :c:func:`string_escape_mem` kernel documentation for the
265details):
266
267 - a - ESCAPE_ANY
268 - c - ESCAPE_SPECIAL
269 - h - ESCAPE_HEX
270 - n - ESCAPE_NULL
271 - o - ESCAPE_OCTAL
272 - p - ESCAPE_NP
273 - s - ESCAPE_SPACE
274
275By default ESCAPE_ANY_NP is used.
276
277ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
278printing SSIDs.
279
280If field width is omitted then 1 byte only will be escaped.
281
282Raw buffer as a hex string
283--------------------------
284
285::
286
287 %*ph 00 01 02 ... 3f
288 %*phC 00:01:02: ... :3f
289 %*phD 00-01-02- ... -3f
290 %*phN 000102 ... 3f
291
292For printing small buffers (up to 64 bytes long) as a hex string with a
293certain separator. For larger buffers consider using
294:c:func:`print_hex_dump`.
295
296MAC/FDDI addresses
297------------------
298
299::
300
301 %pM 00:01:02:03:04:05
302 %pMR 05:04:03:02:01:00
303 %pMF 00-01-02-03-04-05
304 %pm 000102030405
305 %pmR 050403020100
306
307For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
308specifiers result in a printed address with (M) or without (m) byte
309separators. The default byte separator is the colon (:).
310
311Where FDDI addresses are concerned the ``F`` specifier can be used after
312the ``M`` specifier to use dash (-) separators instead of the default
313separator.
314
315For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
316specifier to use reversed byte order suitable for visual interpretation
317of Bluetooth addresses which are in the little endian order.
318
319Passed by reference.
320
321IPv4 addresses
322--------------
323
324::
325
326 %pI4 1.2.3.4
327 %pi4 001.002.003.004
328 %p[Ii]4[hnbl]
329
330For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
331specifiers result in a printed address with (i4) or without (I4) leading
332zeros.
333
334The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
335host, network, big or little endian order addresses respectively. Where
336no specifier is provided the default network/big endian order is used.
337
338Passed by reference.
339
340IPv6 addresses
341--------------
342
343::
344
345 %pI6 0001:0002:0003:0004:0005:0006:0007:0008
346 %pi6 00010002000300040005000600070008
347 %pI6c 1:2:3:4:5:6:7:8
348
349For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
350specifiers result in a printed address with (I6) or without (i6)
351colon-separators. Leading zeros are always used.
352
353The additional ``c`` specifier can be used with the ``I`` specifier to
354print a compressed IPv6 address as described by
355https://tools.ietf.org/html/rfc5952
356
357Passed by reference.
358
359IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
360---------------------------------------------------------
361
362::
363
364 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
365 %piS 001.002.003.004 or 00010002000300040005000600070008
366 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
367 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
368 %p[Ii]S[pfschnbl]
369
370For printing an IP address without the need to distinguish whether it's of
371type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
372specified through ``IS`` or ``iS``, can be passed to this format specifier.
373
374The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
375(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
376flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
377
378In case of an IPv6 address the compressed IPv6 address as described by
379https://tools.ietf.org/html/rfc5952 is being used if the additional
380specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
381case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
382https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
383
384In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
385specifiers can be used as well and are ignored in case of an IPv6
386address.
387
388Passed by reference.
389
390Further examples::
391
392 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
393 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
394 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
395
396UUID/GUID addresses
397-------------------
398
399::
400
401 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
402 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
403 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
404 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
405
406For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``,
407``b`` and ``B`` specifiers are used to specify a little endian order in
408lower (l) or upper case (L) hex notation - and big endian order in lower (b)
409or upper case (B) hex notation.
410
411Where no additional specifiers are used the default big endian
412order with lower case hex notation will be printed.
413
414Passed by reference.
415
416dentry names
417------------
418
419::
420
421 %pd{,2,3,4}
422 %pD{,2,3,4}
423
424For printing dentry name; if we race with :c:func:`d_move`, the name might
425be a mix of old and new ones, but it won't oops. %pd dentry is a safer
426equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
427last components. %pD does the same thing for struct file.
428
429Passed by reference.
430
431block_device names
432------------------
433
434::
435
436 %pg sda, sda1 or loop0p1
437
438For printing name of block_device pointers.
439
440struct va_format
441----------------
442
443::
444
445 %pV
446
447For printing struct va_format structures. These contain a format string
448and va_list as follows::
449
450 struct va_format {
451 const char *fmt;
452 va_list *va;
453 };
454
455Implements a "recursive vsnprintf".
456
457Do not use this feature without some mechanism to verify the
458correctness of the format string and va_list arguments.
459
460Passed by reference.
461
462Device tree nodes
463-----------------
464
465::
466
467 %pOF[fnpPcCF]
468
469
470For printing device tree node structures. Default behaviour is
471equivalent to %pOFf.
472
473 - f - device node full_name
474 - n - device node name
475 - p - device node phandle
476 - P - device node path spec (name + @unit)
477 - F - device node flags
478 - c - major compatible string
479 - C - full compatible string
480
481The separator when using multiple arguments is ':'
482
483Examples::
484
485 %pOF /foo/bar@0 - Node full name
486 %pOFf /foo/bar@0 - Same as above
487 %pOFfp /foo/bar@0:10 - Node full name + phandle
488 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
489 major compatible string +
490 node flags
491 D - dynamic
492 d - detached
493 P - Populated
494 B - Populated bus
495
496Passed by reference.
497
498Fwnode handles
499--------------
500
501::
502
503 %pfw[fP]
504
505For printing information on fwnode handles. The default is to print the full
506node name, including the path. The modifiers are functionally equivalent to
507%pOF above.
508
509 - f - full name of the node, including the path
510 - P - the name of the node including an address (if there is one)
511
512Examples (ACPI)::
513
514 %pfwf \_SB.PCI0.CIO2.port@1.endpoint@0 - Full node name
515 %pfwP endpoint@0 - Node name
516
517Examples (OF)::
518
519 %pfwf /ocp@68000000/i2c@48072000/camera@10/port/endpoint - Full name
520 %pfwP endpoint - Node name
521
522Time and date
523-------------
524
525::
526
527 %pt[RT] YYYY-mm-ddTHH:MM:SS
528 %pt[RT]s YYYY-mm-dd HH:MM:SS
529 %pt[RT]d YYYY-mm-dd
530 %pt[RT]t HH:MM:SS
531 %pt[RT][dt][r][s]
532
533For printing date and time as represented by::
534
535 R struct rtc_time structure
536 T time64_t type
537
538in human readable format.
539
540By default year will be incremented by 1900 and month by 1.
541Use %pt[RT]r (raw) to suppress this behaviour.
542
543The %pt[RT]s (space) will override ISO 8601 separator by using ' ' (space)
544instead of 'T' (Capital T) between date and time. It won't have any effect
545when date or time is omitted.
546
547Passed by reference.
548
549struct clk
550----------
551
552::
553
554 %pC pll1
555 %pCn pll1
556
557For printing struct clk structures. %pC and %pCn print the name of the clock
558(Common Clock Framework) or a unique 32-bit ID (legacy clock framework).
559
560Passed by reference.
561
562bitmap and its derivatives such as cpumask and nodemask
563-------------------------------------------------------
564
565::
566
567 %*pb 0779
568 %*pbl 0,3-6,8-10
569
570For printing bitmap and its derivatives such as cpumask and nodemask,
571%*pb outputs the bitmap with field width as the number of bits and %*pbl
572output the bitmap as range list with field width as the number of bits.
573
574The field width is passed by value, the bitmap is passed by reference.
575Helper macros cpumask_pr_args() and nodemask_pr_args() are available to ease
576printing cpumask and nodemask.
577
578Flags bitfields such as page flags, gfp_flags
579---------------------------------------------
580
581::
582
583 %pGp 0x17ffffc0002036(referenced|uptodate|lru|active|private|node=0|zone=2|lastcpupid=0x1fffff)
584 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN
585 %pGv read|exec|mayread|maywrite|mayexec|denywrite
586
587For printing flags bitfields as a collection of symbolic constants that
588would construct the value. The type of flags is given by the third
589character. Currently supported are [p]age flags, [v]ma_flags (both
590expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
591names and print order depends on the particular type.
592
593Note that this format should not be used directly in the
594:c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags()
595functions from <trace/events/mmflags.h>.
596
597Passed by reference.
598
599Network device features
600-----------------------
601
602::
603
604 %pNF 0x000000000000c000
605
606For printing netdev_features_t.
607
608Passed by reference.
609
610V4L2 and DRM FourCC code (pixel format)
611---------------------------------------
612
613::
614
615 %p4cc
616
617Print a FourCC code used by V4L2 or DRM, including format endianness and
618its numerical value as hexadecimal.
619
620Passed by reference.
621
622Examples::
623
624 %p4cc BG12 little-endian (0x32314742)
625 %p4cc Y10 little-endian (0x20303159)
626 %p4cc NV12 big-endian (0xb231564e)
627
628Rust
629----
630
631::
632
633 %pA
634
635Only intended to be used from Rust code to format ``core::fmt::Arguments``.
636Do *not* use it from C.
637
638Thanks
639======
640
641If you add other %p extensions, please extend <lib/test_printf.c> with
642one or more test cases, if at all feasible.
643
644Thank you for your cooperation and attention.
1=========================================
2How to get printk format specifiers right
3=========================================
4
5.. _printk-specifiers:
6
7:Author: Randy Dunlap <rdunlap@infradead.org>
8:Author: Andrew Murray <amurray@mpc-data.co.uk>
9
10
11Integer types
12=============
13
14::
15
16 If variable is of Type, use printk format specifier:
17 ------------------------------------------------------------
18 signed char %d or %hhx
19 unsigned char %u or %x
20 char %u or %x
21 short int %d or %hx
22 unsigned short int %u or %x
23 int %d or %x
24 unsigned int %u or %x
25 long %ld or %lx
26 unsigned long %lu or %lx
27 long long %lld or %llx
28 unsigned long long %llu or %llx
29 size_t %zu or %zx
30 ssize_t %zd or %zx
31 s8 %d or %hhx
32 u8 %u or %x
33 s16 %d or %hx
34 u16 %u or %x
35 s32 %d or %x
36 u32 %u or %x
37 s64 %lld or %llx
38 u64 %llu or %llx
39
40
41If <type> is architecture-dependent for its size (e.g., cycles_t, tcflag_t) or
42is dependent on a config option for its size (e.g., blk_status_t), use a format
43specifier of its largest possible type and explicitly cast to it.
44
45Example::
46
47 printk("test: latency: %llu cycles\n", (unsigned long long)time);
48
49Reminder: sizeof() returns type size_t.
50
51The kernel's printf does not support %n. Floating point formats (%e, %f,
52%g, %a) are also not recognized, for obvious reasons. Use of any
53unsupported specifier or length qualifier results in a WARN and early
54return from vsnprintf().
55
56Pointer types
57=============
58
59A raw pointer value may be printed with %p which will hash the address
60before printing. The kernel also supports extended specifiers for printing
61pointers of different types.
62
63Some of the extended specifiers print the data on the given address instead
64of printing the address itself. In this case, the following error messages
65might be printed instead of the unreachable information::
66
67 (null) data on plain NULL address
68 (efault) data on invalid address
69 (einval) invalid data on a valid address
70
71Plain Pointers
72--------------
73
74::
75
76 %p abcdef12 or 00000000abcdef12
77
78Pointers printed without a specifier extension (i.e unadorned %p) are
79hashed to prevent leaking information about the kernel memory layout. This
80has the added benefit of providing a unique identifier. On 64-bit machines
81the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it
82gathers enough entropy.
83
84When possible, use specialised modifiers such as %pS or %pB (described below)
85to avoid the need of providing an unhashed address that has to be interpreted
86post-hoc. If not possible, and the aim of printing the address is to provide
87more information for debugging, use %p and boot the kernel with the
88``no_hash_pointers`` parameter during debugging, which will print all %p
89addresses unmodified. If you *really* always want the unmodified address, see
90%px below.
91
92If (and only if) you are printing addresses as a content of a virtual file in
93e.g. procfs or sysfs (using e.g. seq_printf(), not printk()) read by a
94userspace process, use the %pK modifier described below instead of %p or %px.
95
96Error Pointers
97--------------
98
99::
100
101 %pe -ENOSPC
102
103For printing error pointers (i.e. a pointer for which IS_ERR() is true)
104as a symbolic error name. Error values for which no symbolic name is
105known are printed in decimal, while a non-ERR_PTR passed as the
106argument to %pe gets treated as ordinary %p.
107
108Symbols/Function Pointers
109-------------------------
110
111::
112
113 %pS versatile_init+0x0/0x110
114 %ps versatile_init
115 %pSR versatile_init+0x9/0x110
116 (with __builtin_extract_return_addr() translation)
117 %pB prev_fn_of_versatile_init+0x88/0x88
118
119
120The ``S`` and ``s`` specifiers are used for printing a pointer in symbolic
121format. They result in the symbol name with (S) or without (s)
122offsets. If KALLSYMS are disabled then the symbol address is printed instead.
123
124The ``B`` specifier results in the symbol name with offsets and should be
125used when printing stack backtraces. The specifier takes into
126consideration the effect of compiler optimisations which may occur
127when tail-calls are used and marked with the noreturn GCC attribute.
128
129If the pointer is within a module, the module name and optionally build ID is
130printed after the symbol name with an extra ``b`` appended to the end of the
131specifier.
132
133::
134
135 %pS versatile_init+0x0/0x110 [module_name]
136 %pSb versatile_init+0x0/0x110 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
137 %pSRb versatile_init+0x9/0x110 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
138 (with __builtin_extract_return_addr() translation)
139 %pBb prev_fn_of_versatile_init+0x88/0x88 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
140
141Probed Pointers from BPF / tracing
142----------------------------------
143
144::
145
146 %pks kernel string
147 %pus user string
148
149The ``k`` and ``u`` specifiers are used for printing prior probed memory from
150either kernel memory (k) or user memory (u). The subsequent ``s`` specifier
151results in printing a string. For direct use in regular vsnprintf() the (k)
152and (u) annotation is ignored, however, when used out of BPF's bpf_trace_printk(),
153for example, it reads the memory it is pointing to without faulting.
154
155Kernel Pointers
156---------------
157
158::
159
160 %pK 01234567 or 0123456789abcdef
161
162For printing kernel pointers which should be hidden from unprivileged
163users. The behaviour of %pK depends on the kptr_restrict sysctl - see
164Documentation/admin-guide/sysctl/kernel.rst for more details.
165
166This modifier is *only* intended when producing content of a file read by
167userspace from e.g. procfs or sysfs, not for dmesg. Please refer to the
168section about %p above for discussion about how to manage hashing pointers
169in printk().
170
171Unmodified Addresses
172--------------------
173
174::
175
176 %px 01234567 or 0123456789abcdef
177
178For printing pointers when you *really* want to print the address. Please
179consider whether or not you are leaking sensitive information about the
180kernel memory layout before printing pointers with %px. %px is functionally
181equivalent to %lx (or %lu). %px is preferred because it is more uniquely
182grep'able. If in the future we need to modify the way the kernel handles
183printing pointers we will be better equipped to find the call sites.
184
185Before using %px, consider if using %p is sufficient together with enabling the
186``no_hash_pointers`` kernel parameter during debugging sessions (see the %p
187description above). One valid scenario for %px might be printing information
188immediately before a panic, which prevents any sensitive information to be
189exploited anyway, and with %px there would be no need to reproduce the panic
190with no_hash_pointers.
191
192Pointer Differences
193-------------------
194
195::
196
197 %td 2560
198 %tx a00
199
200For printing the pointer differences, use the %t modifier for ptrdiff_t.
201
202Example::
203
204 printk("test: difference between pointers: %td\n", ptr2 - ptr1);
205
206Struct Resources
207----------------
208
209::
210
211 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or
212 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
213 %pR [mem 0x60000000-0x6fffffff pref] or
214 [mem 0x0000000060000000-0x000000006fffffff pref]
215
216For printing struct resources. The ``R`` and ``r`` specifiers result in a
217printed resource with (R) or without (r) a decoded flags member.
218
219Passed by reference.
220
221Physical address types phys_addr_t
222----------------------------------
223
224::
225
226 %pa[p] 0x01234567 or 0x0123456789abcdef
227
228For printing a phys_addr_t type (and its derivatives, such as
229resource_size_t) which can vary based on build options, regardless of the
230width of the CPU data path.
231
232Passed by reference.
233
234DMA address types dma_addr_t
235----------------------------
236
237::
238
239 %pad 0x01234567 or 0x0123456789abcdef
240
241For printing a dma_addr_t type which can vary based on build options,
242regardless of the width of the CPU data path.
243
244Passed by reference.
245
246Raw buffer as an escaped string
247-------------------------------
248
249::
250
251 %*pE[achnops]
252
253For printing raw buffer as an escaped string. For the following buffer::
254
255 1b 62 20 5c 43 07 22 90 0d 5d
256
257A few examples show how the conversion would be done (excluding surrounding
258quotes)::
259
260 %*pE "\eb \C\a"\220\r]"
261 %*pEhp "\x1bb \C\x07"\x90\x0d]"
262 %*pEa "\e\142\040\\\103\a\042\220\r\135"
263
264The conversion rules are applied according to an optional combination
265of flags (see :c:func:`string_escape_mem` kernel documentation for the
266details):
267
268 - a - ESCAPE_ANY
269 - c - ESCAPE_SPECIAL
270 - h - ESCAPE_HEX
271 - n - ESCAPE_NULL
272 - o - ESCAPE_OCTAL
273 - p - ESCAPE_NP
274 - s - ESCAPE_SPACE
275
276By default ESCAPE_ANY_NP is used.
277
278ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
279printing SSIDs.
280
281If field width is omitted then 1 byte only will be escaped.
282
283Raw buffer as a hex string
284--------------------------
285
286::
287
288 %*ph 00 01 02 ... 3f
289 %*phC 00:01:02: ... :3f
290 %*phD 00-01-02- ... -3f
291 %*phN 000102 ... 3f
292
293For printing small buffers (up to 64 bytes long) as a hex string with a
294certain separator. For larger buffers consider using
295:c:func:`print_hex_dump`.
296
297MAC/FDDI addresses
298------------------
299
300::
301
302 %pM 00:01:02:03:04:05
303 %pMR 05:04:03:02:01:00
304 %pMF 00-01-02-03-04-05
305 %pm 000102030405
306 %pmR 050403020100
307
308For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
309specifiers result in a printed address with (M) or without (m) byte
310separators. The default byte separator is the colon (:).
311
312Where FDDI addresses are concerned the ``F`` specifier can be used after
313the ``M`` specifier to use dash (-) separators instead of the default
314separator.
315
316For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
317specifier to use reversed byte order suitable for visual interpretation
318of Bluetooth addresses which are in the little endian order.
319
320Passed by reference.
321
322IPv4 addresses
323--------------
324
325::
326
327 %pI4 1.2.3.4
328 %pi4 001.002.003.004
329 %p[Ii]4[hnbl]
330
331For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
332specifiers result in a printed address with (i4) or without (I4) leading
333zeros.
334
335The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
336host, network, big or little endian order addresses respectively. Where
337no specifier is provided the default network/big endian order is used.
338
339Passed by reference.
340
341IPv6 addresses
342--------------
343
344::
345
346 %pI6 0001:0002:0003:0004:0005:0006:0007:0008
347 %pi6 00010002000300040005000600070008
348 %pI6c 1:2:3:4:5:6:7:8
349
350For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
351specifiers result in a printed address with (I6) or without (i6)
352colon-separators. Leading zeros are always used.
353
354The additional ``c`` specifier can be used with the ``I`` specifier to
355print a compressed IPv6 address as described by
356https://tools.ietf.org/html/rfc5952
357
358Passed by reference.
359
360IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
361---------------------------------------------------------
362
363::
364
365 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
366 %piS 001.002.003.004 or 00010002000300040005000600070008
367 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
368 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
369 %p[Ii]S[pfschnbl]
370
371For printing an IP address without the need to distinguish whether it's of
372type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
373specified through ``IS`` or ``iS``, can be passed to this format specifier.
374
375The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
376(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
377flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
378
379In case of an IPv6 address the compressed IPv6 address as described by
380https://tools.ietf.org/html/rfc5952 is being used if the additional
381specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
382case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
383https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
384
385In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
386specifiers can be used as well and are ignored in case of an IPv6
387address.
388
389Passed by reference.
390
391Further examples::
392
393 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
394 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
395 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
396
397UUID/GUID addresses
398-------------------
399
400::
401
402 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
403 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
404 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
405 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
406
407For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``,
408``b`` and ``B`` specifiers are used to specify a little endian order in
409lower (l) or upper case (L) hex notation - and big endian order in lower (b)
410or upper case (B) hex notation.
411
412Where no additional specifiers are used the default big endian
413order with lower case hex notation will be printed.
414
415Passed by reference.
416
417dentry names
418------------
419
420::
421
422 %pd{,2,3,4}
423 %pD{,2,3,4}
424
425For printing dentry name; if we race with :c:func:`d_move`, the name might
426be a mix of old and new ones, but it won't oops. %pd dentry is a safer
427equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
428last components. %pD does the same thing for struct file.
429
430Passed by reference.
431
432block_device names
433------------------
434
435::
436
437 %pg sda, sda1 or loop0p1
438
439For printing name of block_device pointers.
440
441struct va_format
442----------------
443
444::
445
446 %pV
447
448For printing struct va_format structures. These contain a format string
449and va_list as follows::
450
451 struct va_format {
452 const char *fmt;
453 va_list *va;
454 };
455
456Implements a "recursive vsnprintf".
457
458Do not use this feature without some mechanism to verify the
459correctness of the format string and va_list arguments.
460
461Passed by reference.
462
463Device tree nodes
464-----------------
465
466::
467
468 %pOF[fnpPcCF]
469
470
471For printing device tree node structures. Default behaviour is
472equivalent to %pOFf.
473
474 - f - device node full_name
475 - n - device node name
476 - p - device node phandle
477 - P - device node path spec (name + @unit)
478 - F - device node flags
479 - c - major compatible string
480 - C - full compatible string
481
482The separator when using multiple arguments is ':'
483
484Examples::
485
486 %pOF /foo/bar@0 - Node full name
487 %pOFf /foo/bar@0 - Same as above
488 %pOFfp /foo/bar@0:10 - Node full name + phandle
489 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
490 major compatible string +
491 node flags
492 D - dynamic
493 d - detached
494 P - Populated
495 B - Populated bus
496
497Passed by reference.
498
499Fwnode handles
500--------------
501
502::
503
504 %pfw[fP]
505
506For printing information on fwnode handles. The default is to print the full
507node name, including the path. The modifiers are functionally equivalent to
508%pOF above.
509
510 - f - full name of the node, including the path
511 - P - the name of the node including an address (if there is one)
512
513Examples (ACPI)::
514
515 %pfwf \_SB.PCI0.CIO2.port@1.endpoint@0 - Full node name
516 %pfwP endpoint@0 - Node name
517
518Examples (OF)::
519
520 %pfwf /ocp@68000000/i2c@48072000/camera@10/port/endpoint - Full name
521 %pfwP endpoint - Node name
522
523Time and date
524-------------
525
526::
527
528 %pt[RT] YYYY-mm-ddTHH:MM:SS
529 %pt[RT]s YYYY-mm-dd HH:MM:SS
530 %pt[RT]d YYYY-mm-dd
531 %pt[RT]t HH:MM:SS
532 %pt[RT][dt][r][s]
533
534For printing date and time as represented by::
535
536 R struct rtc_time structure
537 T time64_t type
538
539in human readable format.
540
541By default year will be incremented by 1900 and month by 1.
542Use %pt[RT]r (raw) to suppress this behaviour.
543
544The %pt[RT]s (space) will override ISO 8601 separator by using ' ' (space)
545instead of 'T' (Capital T) between date and time. It won't have any effect
546when date or time is omitted.
547
548Passed by reference.
549
550struct clk
551----------
552
553::
554
555 %pC pll1
556 %pCn pll1
557
558For printing struct clk structures. %pC and %pCn print the name of the clock
559(Common Clock Framework) or a unique 32-bit ID (legacy clock framework).
560
561Passed by reference.
562
563bitmap and its derivatives such as cpumask and nodemask
564-------------------------------------------------------
565
566::
567
568 %*pb 0779
569 %*pbl 0,3-6,8-10
570
571For printing bitmap and its derivatives such as cpumask and nodemask,
572%*pb outputs the bitmap with field width as the number of bits and %*pbl
573output the bitmap as range list with field width as the number of bits.
574
575The field width is passed by value, the bitmap is passed by reference.
576Helper macros cpumask_pr_args() and nodemask_pr_args() are available to ease
577printing cpumask and nodemask.
578
579Flags bitfields such as page flags, page_type, gfp_flags
580--------------------------------------------------------
581
582::
583
584 %pGp 0x17ffffc0002036(referenced|uptodate|lru|active|private|node=0|zone=2|lastcpupid=0x1fffff)
585 %pGt 0xffffff7f(buddy)
586 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN
587 %pGv read|exec|mayread|maywrite|mayexec|denywrite
588
589For printing flags bitfields as a collection of symbolic constants that
590would construct the value. The type of flags is given by the third
591character. Currently supported are:
592
593 - p - [p]age flags, expects value of type (``unsigned long *``)
594 - t - page [t]ype, expects value of type (``unsigned int *``)
595 - v - [v]ma_flags, expects value of type (``unsigned long *``)
596 - g - [g]fp_flags, expects value of type (``gfp_t *``)
597
598The flag names and print order depends on the particular type.
599
600Note that this format should not be used directly in the
601:c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags()
602functions from <trace/events/mmflags.h>.
603
604Passed by reference.
605
606Network device features
607-----------------------
608
609::
610
611 %pNF 0x000000000000c000
612
613For printing netdev_features_t.
614
615Passed by reference.
616
617V4L2 and DRM FourCC code (pixel format)
618---------------------------------------
619
620::
621
622 %p4cc
623
624Print a FourCC code used by V4L2 or DRM, including format endianness and
625its numerical value as hexadecimal.
626
627Passed by reference.
628
629Examples::
630
631 %p4cc BG12 little-endian (0x32314742)
632 %p4cc Y10 little-endian (0x20303159)
633 %p4cc NV12 big-endian (0xb231564e)
634
635Rust
636----
637
638::
639
640 %pA
641
642Only intended to be used from Rust code to format ``core::fmt::Arguments``.
643Do *not* use it from C.
644
645Thanks
646======
647
648If you add other %p extensions, please extend <lib/test_printf.c> with
649one or more test cases, if at all feasible.
650
651Thank you for your cooperation and attention.