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

[Mark Wielaard <mark@klomp.org>]

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.

Cheers,

Mark