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

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Nov 26, 2024 at 05:50:06PM -0800, 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);

looks good, I'm just curious about one thing..

so ELF_T_WORD enum has this comment: /* Elf32_Word, Elf64_Word, ... */

I did just quick check, ***so I might be easily wrong***, but I wonder the
code in __elf_xfctstom (which I assume is the one called for conversion)
chooses to swap 32/64 bits values based on elf->class .. so for 64bit ELF
class we swap 64bit values? ... while .BTF_ids has always 32 bit values

thanks,
jirka

> +			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;
> -- 
> 2.47.0
>