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

Jiri Olsa
2021-06-17T09:02:29Z

On Thu, Jun 17, 2021 at 12:28:00AM +0200, Mark Wielaard wrote:
> Hoi,
>
> 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>

jirka