[PATCH dwarves v3 0/1] btf_encoder: handle .BTF_ids section endianness

[Eduard Zingerman <eddyz87@gmail.com>]

Currently, pahole does not generate kfunc declaration tags when
generating BTF for ELF files with an endianness different from the
host system. For example, this issue occurs when processing a vmlinux
built for s390 on an x86 host.

To reproduce the bug:
- follow the instructions in [0] to build an s390 vmlinux;
- generate BTF requesting declaration tags for kfuncs:
  $ pahole --btf_features_strict=decl_tag_kfuncs,decl_tag \
           --btf_encode_detached=test.btf vmlinux
- observe that no kfuncs are generated:
  $ bpftool btf dump file test.btf format c | grep __ksym

This patch resolves the issue by adding the necessary byte-swapping
operations.

Changelog:
- v1 [1] -> v2:
  - avoid modifying the 'idlist' Elf_Data object directly.
    Instead, use struct local_elf_data (suggested by Jiri);
  - update the description of the .BTF_ids section (suggested by Jiri).
- v2 [2] -> v3:
  - use elf_getdata_rawchunk() instead of direct conversion loop
    (suggested by Andrii);
  - removed Jiri's acked-by and  Vadim's reviewed-by,
    as patch had changed significantly.

[0] https://docs.kernel.org/bpf/s390.html
[1] https://lore.kernel.org/dwarves/20241122070218.3832680-1-eddyz87@gmail.com/
[2] https://lore.kernel.org/dwarves/20241122214431.292196-1-eddyz87@gmail.com/

Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Xu <dxu@dxuuu.xyz>
Cc: Jiri Olsa <olsajiri@gmail.com>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Vadim Fedorenko <vadfed@meta.com>

Eduard Zingerman (1):
  btf_encoder: handle .BTF_ids section endianness

 btf_encoder.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

-- 
2.47.0