Re: [PATCH bpf-next v1 1/3] tools/resolve_btfids: Simplify handling cross-endian compilation

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.

CC: Viktor Malik vmalik@redhat.com Signed-off-by: Tony Ambardar tony.ambardar@gmail.com

---

Acked-by: Viktor Malik vmalik@redhat.com

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);