[PATCH bpf] bpf: Fix a crash when btf_parse_base() returns an error pointer

[Martin KaFai Lau <martin.lau@linux.dev>]

From: Martin KaFai Lau <martin.lau@kernel.org>

The pointer returned by btf_parse_base could be an error pointer.
IS_ERR() check is needed before calling btf_free(base_btf).

Cc: Alan Maguire <alan.maguire@oracle.com>
Fixes: 8646db238997 ("libbpf,bpf: Share BTF relocate-related code with kernel")
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
It was discovered in a bpf CI run that crashed in s390 due to
the wrong endian in the btf header. It caused the btf_parse_base()
to fail and triggered this crash.

This patch is tested in the bpf CI. It fails to load the bpf_testmod
but does not crash the kernel:
https://github.com/kernel-patches/bpf/actions/runs/10623574366/job/29450422150?pr=7630

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

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 520f49f422fe..e3377dd61f7e 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6283,7 +6283,7 @@ static struct btf *btf_parse_module(const char *module_name, const void *data,
 
 errout:
 	btf_verifier_env_free(env);
-	if (base_btf != vmlinux_btf)
+	if (!IS_ERR(base_btf) && base_btf != vmlinux_btf)
 		btf_free(base_btf);
 	if (btf) {
 		kvfree(btf->data);
-- 
2.43.5