bpf: Fix libelf endian handling in resolv_btfids

The vmlinux ".BTF_ids" ELF section is declared in btf_ids.h to hold a list
of zero-filled BTF IDs, which is then patched at link-time with correct
values by resolv_btfids. The section is flagged as "allocable" to preclude
compression, but notably the section contents (BTF IDs) are untyped.

When patching the BTF IDs, resolve_btfids writes in host-native endianness
and relies on libelf for any required translation on reading and updating
vmlinux. However, since the type of the .BTF_ids section content defaults
to ELF_T_BYTE (i.e. unsigned char), no translation occurs. This results in
incorrect patched values when cross-compiling to non-native endianness,
and can manifest as kernel Oops and test failures which are difficult to
troubleshoot [1].

Explicitly set the type of patched data to ELF_T_WORD, the architecture-
neutral ELF type corresponding to the u32 BTF IDs. This enables libelf to
transparently perform any needed endian conversions.

Fixes: fbbb68de ("bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object")
Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Frank Eigler <fche@redhat.com>
Cc: Mark Wielaard <mark@klomp.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/CAPGftE_eY-Zdi3wBcgDfkz_iOr1KF10n=9mJHm1_a_PykcsoeA@mail.gmail.com [1]
Link: https://lore.kernel.org/bpf/20210618061404.818569-1-Tony.Ambardar@gmail.com

tools/bpf/resolve_btfids/main.c

@@ -655,6 +655,9 @@ 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);