| .. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) |
| |
| ==================== |
| BPF LLVM Relocations |
| ==================== |
| |
| This document describes LLVM BPF backend relocation types. |
| |
| Relocation Record |
| ================= |
| |
| LLVM BPF backend records each relocation with the following 16-byte |
| ELF structure:: |
| |
| typedef struct |
| { |
| Elf64_Addr r_offset; // Offset from the beginning of section. |
| Elf64_Xword r_info; // Relocation type and symbol index. |
| } Elf64_Rel; |
| |
| For example, for the following code:: |
| |
| int g1 __attribute__((section("sec"))); |
| int g2 __attribute__((section("sec"))); |
| static volatile int l1 __attribute__((section("sec"))); |
| static volatile int l2 __attribute__((section("sec"))); |
| int test() { |
| return g1 + g2 + l1 + l2; |
| } |
| |
| Compiled with ``clang -target bpf -O2 -c test.c``, the following is |
| the code with ``llvm-objdump -dr test.o``:: |
| |
| 0: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll |
| 0000000000000000: R_BPF_64_64 g1 |
| 2: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) |
| 3: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll |
| 0000000000000018: R_BPF_64_64 g2 |
| 5: 61 20 00 00 00 00 00 00 r0 = *(u32 *)(r2 + 0) |
| 6: 0f 10 00 00 00 00 00 00 r0 += r1 |
| 7: 18 01 00 00 08 00 00 00 00 00 00 00 00 00 00 00 r1 = 8 ll |
| 0000000000000038: R_BPF_64_64 sec |
| 9: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) |
| 10: 0f 10 00 00 00 00 00 00 r0 += r1 |
| 11: 18 01 00 00 0c 00 00 00 00 00 00 00 00 00 00 00 r1 = 12 ll |
| 0000000000000058: R_BPF_64_64 sec |
| 13: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) |
| 14: 0f 10 00 00 00 00 00 00 r0 += r1 |
| 15: 95 00 00 00 00 00 00 00 exit |
| |
| There are four relations in the above for four ``LD_imm64`` instructions. |
| The following ``llvm-readelf -r test.o`` shows the binary values of the four |
| relocations:: |
| |
| Relocation section '.rel.text' at offset 0x190 contains 4 entries: |
| Offset Info Type Symbol's Value Symbol's Name |
| 0000000000000000 0000000600000001 R_BPF_64_64 0000000000000000 g1 |
| 0000000000000018 0000000700000001 R_BPF_64_64 0000000000000004 g2 |
| 0000000000000038 0000000400000001 R_BPF_64_64 0000000000000000 sec |
| 0000000000000058 0000000400000001 R_BPF_64_64 0000000000000000 sec |
| |
| Each relocation is represented by ``Offset`` (8 bytes) and ``Info`` (8 bytes). |
| For example, the first relocation corresponds to the first instruction |
| (Offset 0x0) and the corresponding ``Info`` indicates the relocation type |
| of ``R_BPF_64_64`` (type 1) and the entry in the symbol table (entry 6). |
| The following is the symbol table with ``llvm-readelf -s test.o``:: |
| |
| Symbol table '.symtab' contains 8 entries: |
| Num: Value Size Type Bind Vis Ndx Name |
| 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND |
| 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS test.c |
| 2: 0000000000000008 4 OBJECT LOCAL DEFAULT 4 l1 |
| 3: 000000000000000c 4 OBJECT LOCAL DEFAULT 4 l2 |
| 4: 0000000000000000 0 SECTION LOCAL DEFAULT 4 sec |
| 5: 0000000000000000 128 FUNC GLOBAL DEFAULT 2 test |
| 6: 0000000000000000 4 OBJECT GLOBAL DEFAULT 4 g1 |
| 7: 0000000000000004 4 OBJECT GLOBAL DEFAULT 4 g2 |
| |
| The 6th entry is global variable ``g1`` with value 0. |
| |
| Similarly, the second relocation is at ``.text`` offset ``0x18``, instruction 3, |
| for global variable ``g2`` which has a symbol value 4, the offset |
| from the start of ``.data`` section. |
| |
| The third and fourth relocations refers to static variables ``l1`` |
| and ``l2``. From ``.rel.text`` section above, it is not clear |
| which symbols they really refers to as they both refers to |
| symbol table entry 4, symbol ``sec``, which has ``STT_SECTION`` type |
| and represents a section. So for static variable or function, |
| the section offset is written to the original insn |
| buffer, which is called ``A`` (addend). Looking at |
| above insn ``7`` and ``11``, they have section offset ``8`` and ``12``. |
| From symbol table, we can find that they correspond to entries ``2`` |
| and ``3`` for ``l1`` and ``l2``. |
| |
| In general, the ``A`` is 0 for global variables and functions, |
| and is the section offset or some computation result based on |
| section offset for static variables/functions. The non-section-offset |
| case refers to function calls. See below for more details. |
| |
| Different Relocation Types |
| ========================== |
| |
| Six relocation types are supported. The following is an overview and |
| ``S`` represents the value of the symbol in the symbol table:: |
| |
| Enum ELF Reloc Type Description BitSize Offset Calculation |
| 0 R_BPF_NONE None |
| 1 R_BPF_64_64 ld_imm64 insn 32 r_offset + 4 S + A |
| 2 R_BPF_64_ABS64 normal data 64 r_offset S + A |
| 3 R_BPF_64_ABS32 normal data 32 r_offset S + A |
| 4 R_BPF_64_NODYLD32 .BTF[.ext] data 32 r_offset S + A |
| 10 R_BPF_64_32 call insn 32 r_offset + 4 (S + A) / 8 - 1 |
| |
| For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction. |
| The actual to-be-relocated data (0 or section offset) |
| is stored at ``r_offset + 4`` and the read/write |
| data bitsize is 32 (4 bytes). The relocation can be resolved with |
| the symbol value plus implicit addend. Note that the ``BitSize`` is 32 which |
| means the section offset must be less than or equal to ``UINT32_MAX`` and this |
| is enforced by LLVM BPF backend. |
| |
| In another case, ``R_BPF_64_ABS64`` relocation type is used for normal 64-bit data. |
| The actual to-be-relocated data is stored at ``r_offset`` and the read/write data |
| bitsize is 64 (8 bytes). The relocation can be resolved with |
| the symbol value plus implicit addend. |
| |
| Both ``R_BPF_64_ABS32`` and ``R_BPF_64_NODYLD32`` types are for 32-bit data. |
| But ``R_BPF_64_NODYLD32`` specifically refers to relocations in ``.BTF`` and |
| ``.BTF.ext`` sections. For cases like bcc where llvm ``ExecutionEngine RuntimeDyld`` |
| is involved, ``R_BPF_64_NODYLD32`` types of relocations should not be resolved |
| to actual function/variable address. Otherwise, ``.BTF`` and ``.BTF.ext`` |
| become unusable by bcc and kernel. |
| |
| Type ``R_BPF_64_32`` is used for call instruction. The call target section |
| offset is stored at ``r_offset + 4`` (32bit) and calculated as |
| ``(S + A) / 8 - 1``. |
| |
| Examples |
| ======== |
| |
| Types ``R_BPF_64_64`` and ``R_BPF_64_32`` are used to resolve ``ld_imm64`` |
| and ``call`` instructions. For example:: |
| |
| __attribute__((noinline)) __attribute__((section("sec1"))) |
| int gfunc(int a, int b) { |
| return a * b; |
| } |
| static __attribute__((noinline)) __attribute__((section("sec1"))) |
| int lfunc(int a, int b) { |
| return a + b; |
| } |
| int global __attribute__((section("sec2"))); |
| int test(int a, int b) { |
| return gfunc(a, b) + lfunc(a, b) + global; |
| } |
| |
| Compiled with ``clang -target bpf -O2 -c test.c``, we will have |
| following code with `llvm-objdump -dr test.o``:: |
| |
| Disassembly of section .text: |
| |
| 0000000000000000 <test>: |
| 0: bf 26 00 00 00 00 00 00 r6 = r2 |
| 1: bf 17 00 00 00 00 00 00 r7 = r1 |
| 2: 85 10 00 00 ff ff ff ff call -1 |
| 0000000000000010: R_BPF_64_32 gfunc |
| 3: bf 08 00 00 00 00 00 00 r8 = r0 |
| 4: bf 71 00 00 00 00 00 00 r1 = r7 |
| 5: bf 62 00 00 00 00 00 00 r2 = r6 |
| 6: 85 10 00 00 02 00 00 00 call 2 |
| 0000000000000030: R_BPF_64_32 sec1 |
| 7: 0f 80 00 00 00 00 00 00 r0 += r8 |
| 8: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll |
| 0000000000000040: R_BPF_64_64 global |
| 10: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) |
| 11: 0f 10 00 00 00 00 00 00 r0 += r1 |
| 12: 95 00 00 00 00 00 00 00 exit |
| |
| Disassembly of section sec1: |
| |
| 0000000000000000 <gfunc>: |
| 0: bf 20 00 00 00 00 00 00 r0 = r2 |
| 1: 2f 10 00 00 00 00 00 00 r0 *= r1 |
| 2: 95 00 00 00 00 00 00 00 exit |
| |
| 0000000000000018 <lfunc>: |
| 3: bf 20 00 00 00 00 00 00 r0 = r2 |
| 4: 0f 10 00 00 00 00 00 00 r0 += r1 |
| 5: 95 00 00 00 00 00 00 00 exit |
| |
| The first relocation corresponds to ``gfunc(a, b)`` where ``gfunc`` has a value of 0, |
| so the ``call`` instruction offset is ``(0 + 0)/8 - 1 = -1``. |
| The second relocation corresponds to ``lfunc(a, b)`` where ``lfunc`` has a section |
| offset ``0x18``, so the ``call`` instruction offset is ``(0 + 0x18)/8 - 1 = 2``. |
| The third relocation corresponds to ld_imm64 of ``global``, which has a section |
| offset ``0``. |
| |
| The following is an example to show how R_BPF_64_ABS64 could be generated:: |
| |
| int global() { return 0; } |
| struct t { void *g; } gbl = { global }; |
| |
| Compiled with ``clang -target bpf -O2 -g -c test.c``, we will see a |
| relocation below in ``.data`` section with command |
| ``llvm-readelf -r test.o``:: |
| |
| Relocation section '.rel.data' at offset 0x458 contains 1 entries: |
| Offset Info Type Symbol's Value Symbol's Name |
| 0000000000000000 0000000700000002 R_BPF_64_ABS64 0000000000000000 global |
| |
| The relocation says the first 8-byte of ``.data`` section should be |
| filled with address of ``global`` variable. |
| |
| With ``llvm-readelf`` output, we can see that dwarf sections have a bunch of |
| ``R_BPF_64_ABS32`` and ``R_BPF_64_ABS64`` relocations:: |
| |
| Relocation section '.rel.debug_info' at offset 0x468 contains 13 entries: |
| Offset Info Type Symbol's Value Symbol's Name |
| 0000000000000006 0000000300000003 R_BPF_64_ABS32 0000000000000000 .debug_abbrev |
| 000000000000000c 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str |
| 0000000000000012 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str |
| 0000000000000016 0000000600000003 R_BPF_64_ABS32 0000000000000000 .debug_line |
| 000000000000001a 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str |
| 000000000000001e 0000000200000002 R_BPF_64_ABS64 0000000000000000 .text |
| 000000000000002b 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str |
| 0000000000000037 0000000800000002 R_BPF_64_ABS64 0000000000000000 gbl |
| 0000000000000040 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str |
| ...... |
| |
| The .BTF/.BTF.ext sections has R_BPF_64_NODYLD32 relocations:: |
| |
| Relocation section '.rel.BTF' at offset 0x538 contains 1 entries: |
| Offset Info Type Symbol's Value Symbol's Name |
| 0000000000000084 0000000800000004 R_BPF_64_NODYLD32 0000000000000000 gbl |
| |
| Relocation section '.rel.BTF.ext' at offset 0x548 contains 2 entries: |
| Offset Info Type Symbol's Value Symbol's Name |
| 000000000000002c 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text |
| 0000000000000040 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text |
