LLVM upstream commit https://reviews.llvm.org/D102712 made some changes to bpf relocations to make them llvm linker lld friendly. The scope of existing relocations R_BPF_64_{64,32} is narrowed and new relocations R_BPF_64_{ABS32,ABS64,NODYLD32} are introduced. Let us add some documentation about llvm bpf relocations so people can understand how to resolve them properly in their respective tools. Cc: John Fastabend <john.fastabend@xxxxxxxxx> Cc: Lorenz Bauer <lmb@xxxxxxxxxxxxxx> Signed-off-by: Yonghong Song <yhs@xxxxxx> --- Documentation/bpf/index.rst | 1 + Documentation/bpf/llvm_reloc.rst | 168 +++++++++++++++++++++++++++++++ 2 files changed, 169 insertions(+) create mode 100644 Documentation/bpf/llvm_reloc.rst diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst index a702f67dd45f..93e8cf12a6d4 100644 --- a/Documentation/bpf/index.rst +++ b/Documentation/bpf/index.rst @@ -84,6 +84,7 @@ Other :maxdepth: 1 ringbuf + llvm_reloc .. Links: .. _networking-filter: ../networking/filter.rst diff --git a/Documentation/bpf/llvm_reloc.rst b/Documentation/bpf/llvm_reloc.rst new file mode 100644 index 000000000000..bc62bce591b1 --- /dev/null +++ b/Documentation/bpf/llvm_reloc.rst @@ -0,0 +1,168 @@ +.. 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 static function/variable references, the symbol often refers to +the section itself which has a value of 0. To identify actual static +function/variable, its section offset or some computation result +based on section offset is written to the original insn/data buffer, +which is called ``IA`` (implicit addend) below. For global +function/variables, the symbol refers to actual global and the implicit +addend is 0. + +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 + IA + 2 R_BPF_64_ABS64 normal data 64 r_offset S + IA + 3 R_BPF_64_ABS32 normal data 32 r_offset S + IA + 4 R_BPF_64_NODYLD32 .BTF[.ext] data 32 r_offset S + IA + 10 R_BPF_64_32 call insn 32 r_offset + 4 (S + IA) / 8 - 1 + +For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction. +The actual to-be-relocated data 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. + +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 + IA) / 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 -d 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 + 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 + 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 + 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 + +Three relocations are generated with ``llvm-readelf -r test.o``:: + + Relocation section '.rel.text' at offset 0x188 contains 3 entries: + Offset Info Type Symbol's Value Symbol's Name + 0000000000000010 000000040000000a R_BPF_64_32 0000000000000000 gfunc + 0000000000000030 000000020000000a R_BPF_64_32 0000000000000000 sec1 + 0000000000000040 0000000600000001 R_BPF_64_64 0000000000000000 global + +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 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 -- 2.30.2
