[PATCH 5.10-6.18] libbpf: fix cross-endian build with newer libbpf

[Dominique Martinet <asmadeus@codewreck.org>]

commit 087f3964f4b3 ("libbpf: Support layout section handling in BTF")
added extra data to the BTF header.

This extra data is ignored in the native case, but when building with
different endianness the bswap code in old libbpf would check for strict
size equality, leading to build failures such as this when BTF data is
generated with a newer version of libbpf (through pahole)
    BTFIDS  vmlinux.unstripped
  libbpf: Can't load BTF with non-native endianness due to unsupported header length 32
  FAILED: load BTF from vmlinux.unstripped: Operation not supported
  make[2]: *** [../scripts/Makefile.vmlinux:72: vmlinux.unstripped] Error 255

Newer libbpf versions also properly ignore trailing data in the bswap
case, but the patch is too intrusive to backport, just fix the header
length comparison.

Link: https://github.com/NixOS/nixpkgs/pull/570639
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
---
Hey,

we've got a cross build failure report for ppc64 in nixpkgs (link in
commit message), that turned out as described above.
(build broke for all kernels before 7.1)

In this case I believe it's easier to just fix the check than backport
the full layout section handling support, is that ok?
I've confirmed the build succeeds with this, I assume the reporter will
eventually also boot check but from my understanding of the code this
should be fine.

Feel free to throw this away if there's a better way, just let me know
if I can help.

Thanks

 tools/lib/bpf/btf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 9f141395c074..ae344cf58e85 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -236,7 +236,7 @@ static int btf_parse_hdr(struct btf *btf)
 
 	if (hdr->magic == bswap_16(BTF_MAGIC)) {
 		btf->swapped_endian = true;
-		if (bswap_32(hdr->hdr_len) != sizeof(struct btf_header)) {
+		if (bswap_32(hdr->hdr_len) < sizeof(struct btf_header)) {
 			pr_warn("Can't load BTF with non-native endianness due to unsupported header length %u\n",
 				bswap_32(hdr->hdr_len));
 			return -ENOTSUP;
-- 
2.56.1