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1.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
2
3====================
4BPF LLVM Relocations
5====================
6
7This document describes LLVM BPF backend relocation types.
8
9Relocation Record
10=================
11
12LLVM BPF backend records each relocation with the following 16-byte
13ELF structure::
14
15 typedef struct
16 {
17 Elf64_Addr r_offset; // Offset from the beginning of section.
18 Elf64_Xword r_info; // Relocation type and symbol index.
19 } Elf64_Rel;
20
21For example, for the following code::
22
23 int g1 __attribute__((section("sec")));
24 int g2 __attribute__((section("sec")));
25 static volatile int l1 __attribute__((section("sec")));
26 static volatile int l2 __attribute__((section("sec")));
27 int test() {
28 return g1 + g2 + l1 + l2;
29 }
30
31Compiled with ``clang -target bpf -O2 -c test.c``, the following is
32the code with ``llvm-objdump -dr test.o``::
33
34 0: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
35 0000000000000000: R_BPF_64_64 g1
36 2: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
37 3: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll
38 0000000000000018: R_BPF_64_64 g2
39 5: 61 20 00 00 00 00 00 00 r0 = *(u32 *)(r2 + 0)
40 6: 0f 10 00 00 00 00 00 00 r0 += r1
41 7: 18 01 00 00 08 00 00 00 00 00 00 00 00 00 00 00 r1 = 8 ll
42 0000000000000038: R_BPF_64_64 sec
43 9: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
44 10: 0f 10 00 00 00 00 00 00 r0 += r1
45 11: 18 01 00 00 0c 00 00 00 00 00 00 00 00 00 00 00 r1 = 12 ll
46 0000000000000058: R_BPF_64_64 sec
47 13: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
48 14: 0f 10 00 00 00 00 00 00 r0 += r1
49 15: 95 00 00 00 00 00 00 00 exit
50
51There are four relations in the above for four ``LD_imm64`` instructions.
52The following ``llvm-readelf -r test.o`` shows the binary values of the four
53relocations::
54
55 Relocation section '.rel.text' at offset 0x190 contains 4 entries:
56 Offset Info Type Symbol's Value Symbol's Name
57 0000000000000000 0000000600000001 R_BPF_64_64 0000000000000000 g1
58 0000000000000018 0000000700000001 R_BPF_64_64 0000000000000004 g2
59 0000000000000038 0000000400000001 R_BPF_64_64 0000000000000000 sec
60 0000000000000058 0000000400000001 R_BPF_64_64 0000000000000000 sec
61
62Each relocation is represented by ``Offset`` (8 bytes) and ``Info`` (8 bytes).
63For example, the first relocation corresponds to the first instruction
64(Offset 0x0) and the corresponding ``Info`` indicates the relocation type
65of ``R_BPF_64_64`` (type 1) and the entry in the symbol table (entry 6).
66The following is the symbol table with ``llvm-readelf -s test.o``::
67
68 Symbol table '.symtab' contains 8 entries:
69 Num: Value Size Type Bind Vis Ndx Name
70 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
71 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS test.c
72 2: 0000000000000008 4 OBJECT LOCAL DEFAULT 4 l1
73 3: 000000000000000c 4 OBJECT LOCAL DEFAULT 4 l2
74 4: 0000000000000000 0 SECTION LOCAL DEFAULT 4 sec
75 5: 0000000000000000 128 FUNC GLOBAL DEFAULT 2 test
76 6: 0000000000000000 4 OBJECT GLOBAL DEFAULT 4 g1
77 7: 0000000000000004 4 OBJECT GLOBAL DEFAULT 4 g2
78
79The 6th entry is global variable ``g1`` with value 0.
80
81Similarly, the second relocation is at ``.text`` offset ``0x18``, instruction 3,
82for global variable ``g2`` which has a symbol value 4, the offset
83from the start of ``.data`` section.
84
85The third and fourth relocations refers to static variables ``l1``
86and ``l2``. From ``.rel.text`` section above, it is not clear
87which symbols they really refers to as they both refers to
88symbol table entry 4, symbol ``sec``, which has ``STT_SECTION`` type
89and represents a section. So for static variable or function,
90the section offset is written to the original insn
91buffer, which is called ``A`` (addend). Looking at
92above insn ``7`` and ``11``, they have section offset ``8`` and ``12``.
93From symbol table, we can find that they correspond to entries ``2``
94and ``3`` for ``l1`` and ``l2``.
95
96In general, the ``A`` is 0 for global variables and functions,
97and is the section offset or some computation result based on
98section offset for static variables/functions. The non-section-offset
99case refers to function calls. See below for more details.
100
101Different Relocation Types
102==========================
103
104Six relocation types are supported. The following is an overview and
105``S`` represents the value of the symbol in the symbol table::
106
107 Enum ELF Reloc Type Description BitSize Offset Calculation
108 0 R_BPF_NONE None
109 1 R_BPF_64_64 ld_imm64 insn 32 r_offset + 4 S + A
110 2 R_BPF_64_ABS64 normal data 64 r_offset S + A
111 3 R_BPF_64_ABS32 normal data 32 r_offset S + A
112 4 R_BPF_64_NODYLD32 .BTF[.ext] data 32 r_offset S + A
113 10 R_BPF_64_32 call insn 32 r_offset + 4 (S + A) / 8 - 1
114
115For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction.
116The actual to-be-relocated data (0 or section offset)
117is stored at ``r_offset + 4`` and the read/write
118data bitsize is 32 (4 bytes). The relocation can be resolved with
119the symbol value plus implicit addend. Note that the ``BitSize`` is 32 which
120means the section offset must be less than or equal to ``UINT32_MAX`` and this
121is enforced by LLVM BPF backend.
122
123In another case, ``R_BPF_64_ABS64`` relocation type is used for normal 64-bit data.
124The actual to-be-relocated data is stored at ``r_offset`` and the read/write data
125bitsize is 64 (8 bytes). The relocation can be resolved with
126the symbol value plus implicit addend.
127
128Both ``R_BPF_64_ABS32`` and ``R_BPF_64_NODYLD32`` types are for 32-bit data.
129But ``R_BPF_64_NODYLD32`` specifically refers to relocations in ``.BTF`` and
130``.BTF.ext`` sections. For cases like bcc where llvm ``ExecutionEngine RuntimeDyld``
131is involved, ``R_BPF_64_NODYLD32`` types of relocations should not be resolved
132to actual function/variable address. Otherwise, ``.BTF`` and ``.BTF.ext``
133become unusable by bcc and kernel.
134
135Type ``R_BPF_64_32`` is used for call instruction. The call target section
136offset is stored at ``r_offset + 4`` (32bit) and calculated as
137``(S + A) / 8 - 1``.
138
139Examples
140========
141
142Types ``R_BPF_64_64`` and ``R_BPF_64_32`` are used to resolve ``ld_imm64``
143and ``call`` instructions. For example::
144
145 __attribute__((noinline)) __attribute__((section("sec1")))
146 int gfunc(int a, int b) {
147 return a * b;
148 }
149 static __attribute__((noinline)) __attribute__((section("sec1")))
150 int lfunc(int a, int b) {
151 return a + b;
152 }
153 int global __attribute__((section("sec2")));
154 int test(int a, int b) {
155 return gfunc(a, b) + lfunc(a, b) + global;
156 }
157
158Compiled with ``clang -target bpf -O2 -c test.c``, we will have
159following code with `llvm-objdump -dr test.o``::
160
161 Disassembly of section .text:
162
163 0000000000000000 <test>:
164 0: bf 26 00 00 00 00 00 00 r6 = r2
165 1: bf 17 00 00 00 00 00 00 r7 = r1
166 2: 85 10 00 00 ff ff ff ff call -1
167 0000000000000010: R_BPF_64_32 gfunc
168 3: bf 08 00 00 00 00 00 00 r8 = r0
169 4: bf 71 00 00 00 00 00 00 r1 = r7
170 5: bf 62 00 00 00 00 00 00 r2 = r6
171 6: 85 10 00 00 02 00 00 00 call 2
172 0000000000000030: R_BPF_64_32 sec1
173 7: 0f 80 00 00 00 00 00 00 r0 += r8
174 8: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
175 0000000000000040: R_BPF_64_64 global
176 10: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
177 11: 0f 10 00 00 00 00 00 00 r0 += r1
178 12: 95 00 00 00 00 00 00 00 exit
179
180 Disassembly of section sec1:
181
182 0000000000000000 <gfunc>:
183 0: bf 20 00 00 00 00 00 00 r0 = r2
184 1: 2f 10 00 00 00 00 00 00 r0 *= r1
185 2: 95 00 00 00 00 00 00 00 exit
186
187 0000000000000018 <lfunc>:
188 3: bf 20 00 00 00 00 00 00 r0 = r2
189 4: 0f 10 00 00 00 00 00 00 r0 += r1
190 5: 95 00 00 00 00 00 00 00 exit
191
192The first relocation corresponds to ``gfunc(a, b)`` where ``gfunc`` has a value of 0,
193so the ``call`` instruction offset is ``(0 + 0)/8 - 1 = -1``.
194The second relocation corresponds to ``lfunc(a, b)`` where ``lfunc`` has a section
195offset ``0x18``, so the ``call`` instruction offset is ``(0 + 0x18)/8 - 1 = 2``.
196The third relocation corresponds to ld_imm64 of ``global``, which has a section
197offset ``0``.
198
199The following is an example to show how R_BPF_64_ABS64 could be generated::
200
201 int global() { return 0; }
202 struct t { void *g; } gbl = { global };
203
204Compiled with ``clang -target bpf -O2 -g -c test.c``, we will see a
205relocation below in ``.data`` section with command
206``llvm-readelf -r test.o``::
207
208 Relocation section '.rel.data' at offset 0x458 contains 1 entries:
209 Offset Info Type Symbol's Value Symbol's Name
210 0000000000000000 0000000700000002 R_BPF_64_ABS64 0000000000000000 global
211
212The relocation says the first 8-byte of ``.data`` section should be
213filled with address of ``global`` variable.
214
215With ``llvm-readelf`` output, we can see that dwarf sections have a bunch of
216``R_BPF_64_ABS32`` and ``R_BPF_64_ABS64`` relocations::
217
218 Relocation section '.rel.debug_info' at offset 0x468 contains 13 entries:
219 Offset Info Type Symbol's Value Symbol's Name
220 0000000000000006 0000000300000003 R_BPF_64_ABS32 0000000000000000 .debug_abbrev
221 000000000000000c 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
222 0000000000000012 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
223 0000000000000016 0000000600000003 R_BPF_64_ABS32 0000000000000000 .debug_line
224 000000000000001a 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
225 000000000000001e 0000000200000002 R_BPF_64_ABS64 0000000000000000 .text
226 000000000000002b 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
227 0000000000000037 0000000800000002 R_BPF_64_ABS64 0000000000000000 gbl
228 0000000000000040 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
229 ......
230
231The .BTF/.BTF.ext sections has R_BPF_64_NODYLD32 relocations::
232
233 Relocation section '.rel.BTF' at offset 0x538 contains 1 entries:
234 Offset Info Type Symbol's Value Symbol's Name
235 0000000000000084 0000000800000004 R_BPF_64_NODYLD32 0000000000000000 gbl
236
237 Relocation section '.rel.BTF.ext' at offset 0x548 contains 2 entries:
238 Offset Info Type Symbol's Value Symbol's Name
239 000000000000002c 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text
240 0000000000000040 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text