llvm_reloc.rst (11187B)
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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