Re: [PATCH bpf v1] bpf: fix libelf endian handling in resolv_btfids

Tony Ambardar
2021-06-18T00:21:06Z

On Thu, 17 Jun 2021 at 04:22, Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 6/17/21 11:02 AM, Jiri Olsa wrote:
> > On Thu, Jun 17, 2021 at 12:28:00AM +0200, Mark Wielaard wrote:
> >> On Wed, Jun 16, 2021 at 06:38:33PM +0200, Jiri Olsa wrote:
> >>>>> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> >>>>> index d636643ddd35..f32c059fbfb4 100644
> >>>>> --- a/tools/bpf/resolve_btfids/main.c
> >>>>> +++ b/tools/bpf/resolve_btfids/main.c
> >>>>> @@ -649,6 +649,9 @@ static int symbols_patch(struct object *obj)
> >>>>> if (sets_patch(obj))
> >>>>> return -1;
> >>>>> + /* Set type to ensure endian translation occurs. */
> >>>>> + obj->efile.idlist->d_type = ELF_T_WORD;
> >>>>
> >>>> The change makes sense to me as .BTF_ids contains just a list of
> >>>> u32's.
> >>>>
> >>>> Jiri, could you double check on this?
> >>>
> >>> the comment in ELF_T_WORD declaration suggests the size depends on
> >>> elf's class?
> >>>
> >>> ELF_T_WORD, /* Elf32_Word, Elf64_Word, ... */
> >>>
> >>> data in .BTF_ids section are allways u32
> >>>
> >>> I have no idea how is this handled in libelf (perhaps it's ok),
> >>> but just that comment above suggests it could be also 64 bits,
> >>> cc-ing Frank and Mark for more insight
> >>
> >> It is correct to use ELF_T_WORD, which means a 32bit unsigned word.
> >>
> >> The comment is meant to explain that, but is really confusing if you
> >> don't know that Elf32_Word and Elf64_Word are the same thing (a 32bit
> >> unsigned word). This comes from being "too consistent" in defining all
> >> data types for both 32bit and 64bit ELF, even if those types are the
> >> same in both formats...
> >>
> >> Only Elf32_Addr/Elf64_Addr and Elf32_Off/Elf64_Off are different
> >> sizes. But Elf32/Elf_64_Half (16 bit), Elf32/Elf64_Word (32 bit),
> >> Elf32/Elf64_Xword (64 bit) and their Sword/Sxword (signed) variants
> >> are all identical data types in both the Elf32 and Elf64 formats.
> >>
> >> I don't really know why. It seems the original ELF spec was 32bit only
> >> and when introducing the ELF64 format "they" simply duplicated all
> >> data types whether or not those data type were actually different
> >> between the 32 and 64 bit format.
> >
> > nice, thanks for details
> >
> > Acked-by: Jiri Olsa <jolsa@redhat.com>
>
> Tony, could you do a v2 and summarize the remainder of the discussion in
> here for the commit message? Would be good to explicitly document the
> assumptions made and why they work.

Sure, Daniel, I'll update the commit details and resend.

Thanks,
Tony

> Thanks everyone,
> Daniel