Re: [PATCH v2] backends,bpf: add proper relocation support

[Mark Wielaard <mark@klomp.org>]

On Sat, 2018-06-16 at 13:02 -0700, Yonghong Song wrote:
> Due to libdw does not have proper BPF relocation support,
> the pahole cannot display filenames correctly for objects
> with default llvm options. So we have to invent
> a special option "llc -march=bpf -mattr=dwarfris" to
> prevent llvm from generating cross-section dwarf relocation
> records (https://reviews.llvm.org/rL326505).
> The pahole related discussion is in linux netdev
> mailing list (http://lists.openwall.net/netdev/2018/06/15/38, etc.)
> 
> We would like to add proper BPF relocation support
> to libdw so eventually we could retire the special llc bpf
> flag "-mattr=dwarfris".
> 
> The bpf relocations are defined in
> llvm_repo:include/llvm/BinaryFormat/ELFRelocs/BPF.def:
>   ELF_RELOC(R_BPF_NONE,        0)
>   ELF_RELOC(R_BPF_64_64,       1)
>   ELF_RELOC(R_BPF_64_32,      10)
> 
> Removed the relocation type R_BPF_MAP_FD whoes name does not
> confirm to llvm definition and replaced it with R_BPF_64_64.
> The BPF object is just a relocatible object, not an executable or
> a shared library, so assign ELF type to REL only in bpf_reloc.def.
> 
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  backends/ChangeLog                  |   7 +++++
>  backends/Makefile.am                |   2 +-
>  backends/bpf_init.c                 |   1 +
>  backends/bpf_reloc.def              |   3 +-
>  backends/bpf_symbol.c               |  54
> ++++++++++++++++++++++++++++++++++++
>  libelf/elf.h                        |   3 +-
>  tests/ChangeLog                     |   9 ++++++
>  tests/Makefile.am                   |   5 +++-
>  tests/run-reloc-bpf.sh              |  33 ++++++++++++++++++++++
>  tests/testfile-bpf-reloc.expect.bz2 | Bin 0 -> 300 bytes
>  tests/testfile-bpf-reloc.o.bz2      | Bin 0 -> 933 bytes
>  11 files changed, 113 insertions(+), 4 deletions(-)
>  create mode 100644 backends/bpf_symbol.c
>  create mode 100755 tests/run-reloc-bpf.sh
>  create mode 100644 tests/testfile-bpf-reloc.expect.bz2
>  create mode 100644 tests/testfile-bpf-reloc.o.bz2
> 
> Note:
>  I didn't add the Changelog to libelf/elf.h as I anticipate the
>  change will come from sync'ing with glibc.
>  If this patch version looks good, I can send another revision
>  once the libelf/elf.h is synced.

The patch looks perfect. And the new testcase is good. I used the
testfile to quickly test eu-readelf --debug-dump also got the
relocations right, and it did of course.

Just waiting for the glibc elf.h update/sync and then I'll push this to
master.

Thanks,

Mark