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Re: [PATCH] backends,bpf: add proper relocation support
- From: Mark Wielaard <mark at klomp dot org>
- To: Yonghong Song <yhs at fb dot com>, elfutils-devel at sourceware dot org, ast at fb dot com, kafai at fb dot com
- Cc: Richard Henderson <rth at twiddle dot net>
- Date: Sat, 16 Jun 2018 16:04:09 +0200
- Subject: Re: [PATCH] backends,bpf: add proper relocation support
- References: <20180615224030.2430236-1-yhs@fb.com>
Hi,
I added Richard to the CC, who added the original BPF support.
Who might remember where the R_BPF_MAP_FD comes from (see at the end).
On Fri, 2018-06-15 at 15:40 -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".
Yes. elfutils/libdwfl only does "simple relocations", but that is all
you need anyway.
Do you have a test file (binary for something simple/trivial generated
by llc -march=bpf that contains at least one reloc). I looked at your
implementation and I am sure it works correctly. But having a small
testfile is always a plus.
> 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.
>
> Tested locally with building pahole with this patch and
> pahole is able to display structures in bpf object file properly.
Patch looks good. Thanks. I'll add a ChangeLog entry because that is
what we still do.
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
> backends/Makefile.am | 2 +-
> backends/bpf_init.c | 1 +
> backends/bpf_reloc.def | 3 ++-
> backends/bpf_symbol.c | 54
> ++++++++++++++++++++++++++++++++++++++++++++++++++
> libelf/elf.h | 3 ++-
> 5 files changed, 60 insertions(+), 3 deletions(-)
> create mode 100644 backends/bpf_symbol.c
> [...]
> diff --git a/libelf/elf.h b/libelf/elf.h
> index f7748983..940e88dd 100644
> --- a/libelf/elf.h
> +++ b/libelf/elf.h
> @@ -3848,7 +3848,8 @@ enum
> /* BPF specific declarations. */
>
> #define R_BPF_NONE 0 /* No reloc */
> -#define R_BPF_MAP_FD 1 /* Map fd to pointer */
> +#define R_BPF_64_64 1
> +#define R_BPF_64_32 10
We should sync this with glibc. This file really is a copy of elf/elf.h
in glibc, which we periodically sync. It would be good if all projects
agree on the constants.
I would like to understand where the R_BPF_MAP_FD comes from. But I
assume it was a typo for BPF_PSEUDO_MAP_FD from bpf.h (which has the
same constant number 1).
I'll sent a patch to libc-alpha@sourceware.org unless you beat me to
it.
Thanks,
Mark