Re: [PATCH bpf-next v4 3/8] libbpf: Fix output .symtab byte-order during linking

[Tony Ambardar <tony.ambardar@gmail.com>]

On Fri, Aug 30, 2024 at 03:15:10PM -0700, Eduard Zingerman wrote:
> On Fri, 2024-08-30 at 00:29 -0700, Tony Ambardar wrote:
> > Object linking output data uses the default ELF_T_BYTE type for '.symtab'
> > section data, which disables any libelf-based translation. Explicitly set
> > the ELF_T_SYM type for output to restore libelf's byte-order conversion,
> > noting that input '.symtab' data is already correctly translated.
> > 
> > Fixes: faf6ed321cf6 ("libbpf: Add BPF static linker APIs")
> > Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
> > ---
> >  tools/lib/bpf/linker.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
> > index 9cd3d4109788..7489306cd6f7 100644
> > --- a/tools/lib/bpf/linker.c
> > +++ b/tools/lib/bpf/linker.c
> > @@ -396,6 +396,8 @@ static int init_output_elf(struct bpf_linker *linker, const char *file)
> >  		pr_warn_elf("failed to create SYMTAB data");
> >  		return -EINVAL;
> >  	}
> > +	/* Ensure libelf translates byte-order of symbol records */
> > +	sec->data->d_type = ELF_T_SYM;
> 
> I tried grepping through libelf to find out how this affects things,
> and identified that it is primarily used by elfutils/libelf/gelf_xlatetof.c:gelf_xlatetof(),
> which is an interface function and we don't seem to use it.
> It is also used by dwfl_* functions while applying relocations,
> but we don't use that either.
> 

Right, gelf_xlatetof() is exposed for _explicit_ user conversions, but
libelf still does translations implicitly for known section record types,
based on the ELF file's byte-order metadata. The idea is that ELF data
loaded in memory will be native-endianness for accessibility, but output
in the original endianness at rest/in a file, all transparently.

We try to follow the same idea in libbpf when opening and writing .BTF
and .BTF.ext data (e.g. see the *_raw_data() funcs).

> Could you please elaborate a bit on effects of this change?
> 

When linking objects of either endianness, libelf can translate the input
files based on ELF headers (endianness and type ELF_T_SYM) and allows us to
process .symtab data. When writing out the linked file however, we create a
new .symtab section in init_output_elf() but leave it as default ELT_T_BYTE
type, which undergoes no translation and leaves .symtab always in native
byte-order regardless of target endianness.

See also 61e8aeda9398 ("bpf: Fix libelf endian handling in resolv_btfids")
and related links for a similar example and explanations. Hope that helps.

Cheers,
Tony

> >  
> >  	str_off = strset__add_str(linker->strtab_strs, sec->sec_name);
> >  	if (str_off < 0)
>