Re: [PATCH bpf-next v4 0/2] tools/resolve_btfids: fix cross-compilation to non-host endianness

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Feb 06, 2024 at 01:46:08PM +0100, Viktor Malik wrote:
> The .BTF_ids section is pre-filled with zeroed BTF ID entries during the
> build and afterwards patched by resolve_btfids with correct values.
> Since resolve_btfids always writes in host-native endianness, it relies
> on libelf to do the translation when the target ELF is cross-compiled to
> a different endianness (this was introduced in commit 61e8aeda9398
> ("bpf: Fix libelf endian handling in resolv_btfids")).
> 
> Unfortunately, the translation will corrupt the flags fields of SET8
> entries because these were written during vmlinux compilation and are in
> the correct endianness already. This will lead to numerous selftests
> failures such as:
> 
>     $ sudo ./test_verifier 502 502
>     #502/p sleepable fentry accept FAIL
>     Failed to load prog 'Invalid argument'!
>     bpf_fentry_test1 is not sleepable
>     verification time 34 usec
>     stack depth 0
>     processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
>     Summary: 0 PASSED, 0 SKIPPED, 1 FAILED
> 
> Since it's not possible to instruct libelf to translate just certain
> values, let's manually bswap the flags (both global and entry flags) in
> resolve_btfids when needed, so that libelf then translates everything
> correctly.
> 
> The first patch of the series refactors resolve_btfids by using types
> from btf_ids.h instead of accessing the BTF ID data using magic offsets.
> 
> ---
> Changes in v4:
> - remove unnecessary vars and pointer casts (suggested by Daniel Xu)

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> 
> Changes in v3:
> - add byte swap of global 'flags' field in btf_id_set8 (suggested by
>   Jiri Olsa)
> - cleaner refactoring of sets_patch (suggested by Jiri Olsa)
> - add compile-time assertion that IDs are at the beginning of pairs
>   struct in btf_id_set8 (suggested by Daniel Borkmann)
> 
> Changes in v2:
> - use type defs from btf_ids.h (suggested by Andrii Nakryiko)
> 
> Viktor Malik (2):
>   tools/resolve_btfids: Refactor set sorting with types from btf_ids.h
>   tools/resolve_btfids: fix cross-compilation to non-host endianness
> 
>  tools/bpf/resolve_btfids/main.c | 70 ++++++++++++++++++++++++++-------
>  tools/include/linux/btf_ids.h   |  9 +++++
>  2 files changed, 65 insertions(+), 14 deletions(-)
> 
> -- 
> 2.43.0
>