On 9/20/24 09:49, Tony Ambardar wrote:
> Initially, the .BTF_ids section was created zero-filled and then patched
> with BTF IDs by resolve_btfids on the build host. Patching was done in
> native endianness and thus failed to work for cross-endian compile targets.
> This was fixed in [1] by using libelf-based translation to output patched
> data in target byte order.
>
> The addition of 8-byte BTF sets in [2] lead to .BTF_ids creation with both
> target-endian values and zero-filled data to be later patched. This again
> broke cross-endian compilation as the already-correct target-endian values
> were translated on output by libelf [1]. The problem was worked around [3]
> by manually converting BTF SET8 values to native endianness, so that final
> libelf output translation yields data in target byte order.
>
> Simplify and make the code more robust against future changes like [2] by
> employing libelf-based endian translation on both input and output, which
> is typical of libelf usage.
>
> [1]: 61e8aeda9398 ("bpf: Fix libelf endian handling in resolv_btfids")
> [2]: ef2c6f370a63 ("tools/resolve_btfids: Add support for 8-byte BTF sets")
> [3]: 903fad439466 ("tools/resolve_btfids: Fix cross-compilation to non-host
> endianness")
>
> CC: Viktor Malik <[email protected]>
> Signed-off-by: Tony Ambardar <[email protected]>
> ---
Acked-by: Viktor Malik <[email protected]>
> tools/bpf/resolve_btfids/main.c | 60 ++++++++++++---------------------
> 1 file changed, 22 insertions(+), 38 deletions(-)
>
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index d54aaa0619df..9f1ab23ed014 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -90,14 +90,6 @@
>
> #define ADDR_CNT 100
>
> -#if __BYTE_ORDER == __LITTLE_ENDIAN
> -# define ELFDATANATIVE ELFDATA2LSB
> -#elif __BYTE_ORDER == __BIG_ENDIAN
> -# define ELFDATANATIVE ELFDATA2MSB
> -#else
> -# error "Unknown machine endianness!"
> -#endif
> -
> struct btf_id {
> struct rb_node rb_node;
> char *name;
> @@ -125,7 +117,6 @@ struct object {
> int idlist_shndx;
> size_t strtabidx;
> unsigned long idlist_addr;
> - int encoding;
> } efile;
>
> struct rb_root sets;
> @@ -325,11 +316,30 @@ static int compressed_section_fix(Elf *elf, Elf_Scn
> *scn, GElf_Shdr *sh)
> return 0;
> }
>
> +static int btfids_endian_fix(struct object *obj)
> +{
> + Elf_Data *btfids = obj->efile.idlist;
> + Elf *elf = obj->efile.elf;
> + int file_byteorder;
> +
> + /* This should always succeed due to prior ELF checks */
> + file_byteorder = elf_getident(elf, NULL)[EI_DATA];
> +
> + /* Set type to ensure endian translation occurs, and manually invoke
> + * translation on input since .BTF_ids section as created disables it.
> + */
> + btfids->d_type = ELF_T_WORD;
> + if (gelf_xlatetom(elf, btfids, btfids, file_byteorder) == NULL) {
> + pr_err("FAILED xlatetom .BTF_ids data: %s\n", elf_errmsg(-1));
> + return -1;
> + }
> + return 0;
> +}
> +
> static int elf_collect(struct object *obj)
> {
> Elf_Scn *scn = NULL;
> size_t shdrstrndx;
> - GElf_Ehdr ehdr;
> int idx = 0;
> Elf *elf;
> int fd;
> @@ -361,13 +371,6 @@ static int elf_collect(struct object *obj)
> return -1;
> }
>
> - if (gelf_getehdr(obj->efile.elf, &ehdr) == NULL) {
> - pr_err("FAILED cannot get ELF header: %s\n",
> - elf_errmsg(-1));
> - return -1;
> - }
> - obj->efile.encoding = ehdr.e_ident[EI_DATA];
> -
> /*
> * Scan all the elf sections and look for save data
> * from .BTF_ids section and symbols.
> @@ -409,6 +412,8 @@ static int elf_collect(struct object *obj)
> obj->efile.idlist = data;
> obj->efile.idlist_shndx = idx;
> obj->efile.idlist_addr = sh.sh_addr;
> + if (btfids_endian_fix(obj))
> + return -1;
> } else if (!strcmp(name, BTF_BASE_ELF_SEC)) {
> /* If a .BTF.base section is found, do not resolve
> * BTF ids relative to vmlinux; resolve relative
> @@ -706,24 +711,6 @@ static int sets_patch(struct object *obj)
> */
> BUILD_BUG_ON((u32 *)set8->pairs != &set8->pairs[0].id);
> qsort(set8->pairs, set8->cnt, sizeof(set8->pairs[0]),
> cmp_id);
> -
> - /*
> - * When ELF endianness does not match endianness of the
> - * host, libelf will do the translation when updating
> - * the ELF. This, however, corrupts SET8 flags which are
> - * already in the target endianness. So, let's bswap
> - * them to the host endianness and libelf will then
> - * correctly translate everything.
> - */
> - if (obj->efile.encoding != ELFDATANATIVE) {
> - int i;
> -
> - set8->flags = bswap_32(set8->flags);
> - for (i = 0; i < set8->cnt; i++) {
> - set8->pairs[i].flags =
> - bswap_32(set8->pairs[i].flags);
> - }
> - }
> }
>
> pr_debug("sorting addr %5lu: cnt %6d [%s]\n",
> @@ -748,9 +735,6 @@ static int symbols_patch(struct object *obj)
> if (sets_patch(obj))
> return -1;
>
> - /* Set type to ensure endian translation occurs. */
> - obj->efile.idlist->d_type = ELF_T_WORD;
> -
> elf_flagdata(obj->efile.idlist, ELF_C_SET, ELF_F_DIRTY);
>
> err = elf_update(obj->efile.elf, ELF_C_WRITE);
