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@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Cc: Daniel Xu <dxu@xxxxxxxxx>
> Cc: Jiri Olsa <olsajiri@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 | 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
>
