On Thu, Nov 21, 2024 at 11:02 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> btf_encoder__tag_kfuncs() reads .BTF_ids section to identify a set of
> kfuncs present in the ELF being processed. This section consists of
> records of the following shape:
>
> struct btf_id_and_flag {
> uint32_t id;
> uint32_t flags;
> };
>
Can we just set data->d_type to ELF_T_WORD and let libelf handle the byte swap?
> When endianness of binary operated by pahole differs from the
> host endianness these fields require byte swap before using.
>
> At the moment such byte swap does not happen and kfuncs are not marked
> with decl tags when e.g. s390 kernel is compiled on x86.
> To reproduces the bug:
> - follow instructions from [0] to build an s390 vmlinux;
> - execute:
> pahole --btf_features_strict=decl_tag_kfuncs,decl_tag \
> --btf_encode_detached=test.btf vmlinux
> - observe no kfuncs generated:
> bpftool btf dump test.btf format c | grep __ksym
>
> This commit fixes the issue by adding an endianness conversion step
> for .BTF_ids section data before main processing step, modifying the
> Elf_Data object in-place.
> The choice is such in order to:
> - minimize changes;
> - keep using Elf_Data, as it provides fields {d_size,d_off} used
> by kfunc processing routines;
> - avoid sprinkling bswap_32 at each 'struct btf_id_and_flag' field
> access in fear of forgetting to add new ones when code is modified.
>
> [0] https://docs.kernel.org/bpf/s390.html
>
> Cc: Alan Maguire <alan.maguire@xxxxxxxxxx>
> Cc: Daniel Xu <dxu@xxxxxxxxx>
> Cc: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> Cc: Vadim Fedorenko <vadfed@xxxxxxxx>
> Fixes: 72e88f29c6f7 ("pahole: Inject kfunc decl tags into BTF")
> Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> ---
> btf_encoder.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> lib/bpf | 2 +-
> 2 files changed, 43 insertions(+), 1 deletion(-)
>
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