Yonghong Song wrote: > > > On 5/24/21 10:23 AM, Andrii Nakryiko wrote: > > On Sat, May 22, 2021 at 9:39 AM Yonghong Song <yhs@xxxxxx> wrote: > >> > >> 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 > >> Thanks Yonghong, I found this helpful. I still had to crack open llvm code though to follow along. A couple small suggestions below, may or may not be useful. Overall looks good. > >> .. 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. Above was too terse for me to follow without looking into some clang examples. Maybe an example right here would help not sure? Maybe expand the text a bit? I don't have a really good suggestion. > >> + > >> +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 > > > > There are cases where we set all 64-bits of ld_imm64 (e.g., extern > > ksym, global variables). Or those will be a different relocation now > > (R_BPF_64_ABS64?). If not, I think BitSize 64 is more correct here. > > It is still R_BPF_64_64. In llvm, we have restriction that section > offset must be <= UINT32_MAX, and that is why only 32bit is used > to find the actual symbol in symbol table. 32bit permits 4GB section > which should enough in practice for a bpf program. ^^^ maybe add this note in the doc somewhere? I had similar questions. > > libbpf or tools can write to full 64bits of imm values of ld_imm64 insn. > > The name is a little bit misleading, but it has become part of ABI > and lives in /usr/include/elf.h and we are not able to change it > any more. > > > > > Looking at LLVM diff I haven't found a test for global variables (at > > least I didn't realize it was there), so double-checking here (and it > > might be a good idea to have an explicit test for global variables?) > > We have llvm/test/CodeGen/BPF/reloc.ll and > llvm/test/CodeGen/BPF/reloc-btf.ll covering R_BPF_64_ABS64. But I think > I can enhance > llvm/test/CodeGen/BPF/reloc-2.ll to cover an explicit global variable case. ^^^ maybe cross-reference llvm tests from kernel docs side? I often look at these when I get something unexpected/unknown maybe others would find it helpful, but not know where to look? > > > > >> + 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 > >> +======== > >> + I liked the examples.
