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

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

On 27/11/2024 01:50, Eduard Zingerman wrote:
> btf_encoder__tag_kfuncs() reads .BTF_ids section to identify a set of
> kfuncs present in the ELF file being processed.
> This section consists of:
> - arrays of uint32_t elements;
> - arrays of records with the following structure:
>    struct btf_id_and_flag {
>        uint32_t id;
>        uint32_t flags;
>    };
> 
> When endianness of a binary operated by pahole differs from the host
> system's endianness, these fields require byte-swapping before use.
> Currently, this byte-swapping does not occur, resulting in kfuncs not
> being marked with declaration tags.
> 
> This commit resolves the issue by using elf_getdata_rawchunk()
> function to read .BTF_ids section data. When called with ELF_T_WORD as
> 'type' parameter it does necessary byte order conversion
> (only if host and elf endianness do not match).
> 
> 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>
> Fixes: 72e88f29c6f7 ("pahole: Inject kfunc decl tags into BTF")
> Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> ---
>   btf_encoder.c | 26 ++++++++++++++++++++------
>   1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index e1adddf..3754884 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -1904,18 +1904,32 @@ static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
>   			goto out;
>   		}
>   
> -		data = elf_getdata(scn, 0);
> -		if (!data) {
> -			elf_error("Failed to get ELF section(%d) data", i);
> -			goto out;
> -		}
> -
>   		if (shdr.sh_type == SHT_SYMTAB) {
> +			data = elf_getdata(scn, 0);
> +			if (!data) {
> +				elf_error("Failed to get ELF section(%d) data", i);
> +				goto out;
> +			}
> +
>   			symbols_shndx = i;
>   			symscn = scn;
>   			symbols = data;
>   			strtabidx = shdr.sh_link;
>   		} else if (!strcmp(secname, BTF_IDS_SECTION)) {
> +			/* .BTF_ids section consists of uint32_t elements,
> +			 * and thus might need byte order conversion.
> +			 * However, it has type PROGBITS, hence elf_getdata()
> +			 * won't automatically do the conversion.
> +			 * Use elf_getdata_rawchunk() instead,
> +			 * ELF_T_WORD tells it to do the necessary conversion.
> +			 */
> +			data = elf_getdata_rawchunk(elf, shdr.sh_offset, shdr.sh_size, ELF_T_WORD);
> +			if (!data) {
> +				elf_error("Failed to get %s ELF section(%d) data",
> +					  BTF_IDS_SECTION, i);
> +				goto out;
> +			}
> +
>   			idlist_shndx = i;
>   			idlist_addr = shdr.sh_addr;
>   			idlist = data;

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