Re: [PATCH bpf-next] libbpf: fix internal USDT address translation logic for shared libraries

Alexei Starovoitov
2022-06-16T20:00:03Z

On Wed, Jun 15, 2022 at 10:56 PM Andrii Nakryiko <andrii@kernel.org> wrote:
>
> Perform the same virtual address to file offset translation that libbpf
> is doing for executable ELF binaries also for shared libraries.
> Currently libbpf is making a simplifying and sometimes wrong assumption
> that for shared libraries relative virtual addresses inside ELF are
> always equal to file offsets.
>
> Unfortunately, this is not always the case with LLVM's lld linker, which
> now by default generates quite more complicated ELF segments layout.
> E.g., for liburandom_read.so from selftests/bpf, here's an excerpt from
> readelf output listing ELF segments (a.k.a. program headers):
>
> Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
> PHDR 0x000040 0x0000000000000040 0x0000000000000040 0x0001f8 0x0001f8 R 0x8
> LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x0005e4 0x0005e4 R 0x1000
> LOAD 0x0005f0 0x00000000000015f0 0x00000000000015f0 0x000160 0x000160 R E 0x1000
> LOAD 0x000750 0x0000000000002750 0x0000000000002750 0x000210 0x000210 RW 0x1000
> LOAD 0x000960 0x0000000000003960 0x0000000000003960 0x000028 0x000029 RW 0x1000
>
> Compare that to what is generated by GNU ld (or LLVM lld's with extra
> -znoseparate-code argument which disables this cleverness in the name of
> file size reduction):
>
> Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
> LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000550 0x000550 R 0x1000
> LOAD 0x001000 0x0000000000001000 0x0000000000001000 0x000131 0x000131 R E 0x1000
> LOAD 0x002000 0x0000000000002000 0x0000000000002000 0x0000ac 0x0000ac R 0x1000
> LOAD 0x002dc0 0x0000000000003dc0 0x0000000000003dc0 0x000262 0x000268 RW 0x1000
>
> You can see from the first example above that for executable (Flg == "R E")
> PT_LOAD segment (LOAD #2), Offset doesn't match VirtAddr columns.
> And it does in the second case (GNU ld output).
>
> This is important because all the addresses, including USDT specs,
> operate in a virtual address space, while kernel is expecting file
> offsets when performing uprobe attach. So such mismatches have to be
> properly taken care of and compensated by libbpf, which is what this
> patch is fixing.
>
> Also patch clarifies few function and variable names, as well as updates
> comments to reflect this important distinction (virtaddr vs file offset)
> and to ephasize that shared libraries are not all that different from
> executables in this regard.
>
> This patch also changes selftests/bpf Makefile to force urand_read and
> liburand_read.so to be built with Clang and LLVM's lld (and explicitly
> request this ELF file size optimization through -znoseparate-code linker
> parameter) to validate libbpf logic and ensure regressions don't happen
> in the future. I've bundled these selftests changes together with libbpf
> changes to keep the above description tied with both libbpf and
> selftests changes.
>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
> tools/lib/bpf/usdt.c | 123 ++++++++++++++-------------

What should be the Fixes tag here?
Back to the beginning of this usdt.c file?