1 .. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
7 This document describes LLVM BPF backend relocation types.
12 LLVM BPF backend records each relocation with the following 16-byte
17 Elf64_Addr r_offset; // Offset from the beginning of section.
18 Elf64_Xword r_info; // Relocation type and symbol index.
21 For example, for the following code::
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")));
28 return g1 + g2 + l1 + l2;
31 Compiled with ``clang -target bpf -O2 -c test.c``, the following is
32 the code with ``llvm-objdump -dr test.o``::
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
51 There are four relocations in the above for four ``LD_imm64`` instructions.
52 The following ``llvm-readelf -r test.o`` shows the binary values of the four
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
62 Each relocation is represented by ``Offset`` (8 bytes) and ``Info`` (8 bytes).
63 For example, the first relocation corresponds to the first instruction
64 (Offset 0x0) and the corresponding ``Info`` indicates the relocation type
65 of ``R_BPF_64_64`` (type 1) and the entry in the symbol table (entry 6).
66 The following is the symbol table with ``llvm-readelf -s test.o``::
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
79 The 6th entry is global variable ``g1`` with value 0.
81 Similarly, the second relocation is at ``.text`` offset ``0x18``, instruction 3,
82 has a type of ``R_BPF_64_64`` and refers to entry 7 in the symbol table.
83 The second relocation resolves to global variable ``g2`` which has a symbol
84 value 4. The symbol value represents the offset from the start of ``.data``
85 section where the initial value of the global variable ``g2`` is stored.
87 The third and fourth relocations refer to static variables ``l1``
88 and ``l2``. From the ``.rel.text`` section above, it is not clear
89 to which symbols they really refer as they both refer to
90 symbol table entry 4, symbol ``sec``, which has ``STT_SECTION`` type
91 and represents a section. So for a static variable or function,
92 the section offset is written to the original insn
93 buffer, which is called ``A`` (addend). Looking at
94 above insn ``7`` and ``11``, they have section offset ``8`` and ``12``.
95 From symbol table, we can find that they correspond to entries ``2``
96 and ``3`` for ``l1`` and ``l2``.
98 In general, the ``A`` is 0 for global variables and functions,
99 and is the section offset or some computation result based on
100 section offset for static variables/functions. The non-section-offset
101 case refers to function calls. See below for more details.
103 Different Relocation Types
104 ==========================
106 Six relocation types are supported. The following is an overview and
107 ``S`` represents the value of the symbol in the symbol table::
109 Enum ELF Reloc Type Description BitSize Offset Calculation
111 1 R_BPF_64_64 ld_imm64 insn 32 r_offset + 4 S + A
112 2 R_BPF_64_ABS64 normal data 64 r_offset S + A
113 3 R_BPF_64_ABS32 normal data 32 r_offset S + A
114 4 R_BPF_64_NODYLD32 .BTF[.ext] data 32 r_offset S + A
115 10 R_BPF_64_32 call insn 32 r_offset + 4 (S + A) / 8 - 1
117 For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction.
118 The actual to-be-relocated data (0 or section offset)
119 is stored at ``r_offset + 4`` and the read/write
120 data bitsize is 32 (4 bytes). The relocation can be resolved with
121 the symbol value plus implicit addend. Note that the ``BitSize`` is 32 which
122 means the section offset must be less than or equal to ``UINT32_MAX`` and this
123 is enforced by LLVM BPF backend.
125 In another case, ``R_BPF_64_ABS64`` relocation type is used for normal 64-bit data.
126 The actual to-be-relocated data is stored at ``r_offset`` and the read/write data
127 bitsize is 64 (8 bytes). The relocation can be resolved with
128 the symbol value plus implicit addend.
130 Both ``R_BPF_64_ABS32`` and ``R_BPF_64_NODYLD32`` types are for 32-bit data.
131 But ``R_BPF_64_NODYLD32`` specifically refers to relocations in ``.BTF`` and
132 ``.BTF.ext`` sections. For cases like bcc where llvm ``ExecutionEngine RuntimeDyld``
133 is involved, ``R_BPF_64_NODYLD32`` types of relocations should not be resolved
134 to actual function/variable address. Otherwise, ``.BTF`` and ``.BTF.ext``
135 become unusable by bcc and kernel.
137 Type ``R_BPF_64_32`` is used for call instruction. The call target section
138 offset is stored at ``r_offset + 4`` (32bit) and calculated as
144 Types ``R_BPF_64_64`` and ``R_BPF_64_32`` are used to resolve ``ld_imm64``
145 and ``call`` instructions. For example::
147 __attribute__((noinline)) __attribute__((section("sec1")))
148 int gfunc(int a, int b) {
151 static __attribute__((noinline)) __attribute__((section("sec1")))
152 int lfunc(int a, int b) {
155 int global __attribute__((section("sec2")));
156 int test(int a, int b) {
157 return gfunc(a, b) + lfunc(a, b) + global;
160 Compiled with ``clang -target bpf -O2 -c test.c``, we will have
161 following code with `llvm-objdump -dr test.o``::
163 Disassembly of section .text:
165 0000000000000000 <test>:
166 0: bf 26 00 00 00 00 00 00 r6 = r2
167 1: bf 17 00 00 00 00 00 00 r7 = r1
168 2: 85 10 00 00 ff ff ff ff call -1
169 0000000000000010: R_BPF_64_32 gfunc
170 3: bf 08 00 00 00 00 00 00 r8 = r0
171 4: bf 71 00 00 00 00 00 00 r1 = r7
172 5: bf 62 00 00 00 00 00 00 r2 = r6
173 6: 85 10 00 00 02 00 00 00 call 2
174 0000000000000030: R_BPF_64_32 sec1
175 7: 0f 80 00 00 00 00 00 00 r0 += r8
176 8: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
177 0000000000000040: R_BPF_64_64 global
178 10: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
179 11: 0f 10 00 00 00 00 00 00 r0 += r1
180 12: 95 00 00 00 00 00 00 00 exit
182 Disassembly of section sec1:
184 0000000000000000 <gfunc>:
185 0: bf 20 00 00 00 00 00 00 r0 = r2
186 1: 2f 10 00 00 00 00 00 00 r0 *= r1
187 2: 95 00 00 00 00 00 00 00 exit
189 0000000000000018 <lfunc>:
190 3: bf 20 00 00 00 00 00 00 r0 = r2
191 4: 0f 10 00 00 00 00 00 00 r0 += r1
192 5: 95 00 00 00 00 00 00 00 exit
194 The first relocation corresponds to ``gfunc(a, b)`` where ``gfunc`` has a value of 0,
195 so the ``call`` instruction offset is ``(0 + 0)/8 - 1 = -1``.
196 The second relocation corresponds to ``lfunc(a, b)`` where ``lfunc`` has a section
197 offset ``0x18``, so the ``call`` instruction offset is ``(0 + 0x18)/8 - 1 = 2``.
198 The third relocation corresponds to ld_imm64 of ``global``, which has a section
201 The following is an example to show how R_BPF_64_ABS64 could be generated::
203 int global() { return 0; }
204 struct t { void *g; } gbl = { global };
206 Compiled with ``clang -target bpf -O2 -g -c test.c``, we will see a
207 relocation below in ``.data`` section with command
208 ``llvm-readelf -r test.o``::
210 Relocation section '.rel.data' at offset 0x458 contains 1 entries:
211 Offset Info Type Symbol's Value Symbol's Name
212 0000000000000000 0000000700000002 R_BPF_64_ABS64 0000000000000000 global
214 The relocation says the first 8-byte of ``.data`` section should be
215 filled with address of ``global`` variable.
217 With ``llvm-readelf`` output, we can see that dwarf sections have a bunch of
218 ``R_BPF_64_ABS32`` and ``R_BPF_64_ABS64`` relocations::
220 Relocation section '.rel.debug_info' at offset 0x468 contains 13 entries:
221 Offset Info Type Symbol's Value Symbol's Name
222 0000000000000006 0000000300000003 R_BPF_64_ABS32 0000000000000000 .debug_abbrev
223 000000000000000c 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
224 0000000000000012 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
225 0000000000000016 0000000600000003 R_BPF_64_ABS32 0000000000000000 .debug_line
226 000000000000001a 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
227 000000000000001e 0000000200000002 R_BPF_64_ABS64 0000000000000000 .text
228 000000000000002b 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
229 0000000000000037 0000000800000002 R_BPF_64_ABS64 0000000000000000 gbl
230 0000000000000040 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
233 The .BTF/.BTF.ext sections has R_BPF_64_NODYLD32 relocations::
235 Relocation section '.rel.BTF' at offset 0x538 contains 1 entries:
236 Offset Info Type Symbol's Value Symbol's Name
237 0000000000000084 0000000800000004 R_BPF_64_NODYLD32 0000000000000000 gbl
239 Relocation section '.rel.BTF.ext' at offset 0x548 contains 2 entries:
240 Offset Info Type Symbol's Value Symbol's Name
241 000000000000002c 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text
242 0000000000000040 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text