btf_encoder: handle .BTF_ids section endianness

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).

Fixes: 72e88f2 ("pahole: Inject kfunc decl tags into BTF")
Reviewed-by: Vadim Fedorenko <[email protected]>
Signed-off-by: Eduard Zingerman <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Alan Maguire <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Daniel Xu <[email protected]>
Cc: [email protected]
Cc: Kumar Kartikeya Dwivedi <[email protected]>
Cc: Vadim Fedorenko <[email protected]>
Cc: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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;