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

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

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);
+			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