[PATCH bpf] bpf: Do not close un-owned FD 0 on errors

[Daniel Xu <dxu@dxuuu.xyz>]

Before this patch, btf_new() was liable to close an arbitrary FD 0 if
BTF parsing failed. This was because:

* btf->fd was initialized to 0 through the calloc()
* btf__free() (in the `done` label) closed any FDs >= 0
* btf->fd is left at 0 if parsing fails

This issue was discovered on a system using libbpf v0.3 (without
BTF_KIND_FLOAT support) but with a kernel that had BTF_KIND_FLOAT types
in BTF. Thus, parsing fails.

While this patch technically doesn't fix any issues b/c upstream libbpf
has BTF_KIND_FLOAT support, it'll help prevent issues in the future if
more BTF types are added. It also allow the fix to be backported to
older libbpf's.

Fixes: 3289959b97ca ("libbpf: Support BTF loading and raw data output in both endianness")
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 tools/lib/bpf/btf.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index b46760b93bb4..7ff3d5ce44f9 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -804,6 +804,7 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 	btf->nr_types = 0;
 	btf->start_id = 1;
 	btf->start_str_off = 0;
+	btf->fd = -1;
 
 	if (base_btf) {
 		btf->base_btf = base_btf;
@@ -832,8 +833,6 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 	if (err)
 		goto done;
 
-	btf->fd = -1;
-
 done:
 	if (err) {
 		btf__free(btf);
-- 
2.31.1