Re: [PATCH dwarves v1] btf_encoder: handle .BTF_ids section endianness when cross-compiling

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 21/11/2024 23:02, Eduard Zingerman 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;
>    };
> 
> 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
> 

LGTM, Thanks!

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>