[PATCH] bpf, libbpf: Smatch: Fix potential NULL pointer dereference

[Leo Yan]

Based on the following report from Smatch, fix the potential
NULL pointer dereference check.

  tools/lib/bpf/libbpf.c:3493
  bpf_prog_load_xattr() warn: variable dereferenced before check 'attr'
  (see line 3483)

3479 int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
3480                         struct bpf_object **pobj, int *prog_fd)
3481 {
3482         struct bpf_object_open_attr open_attr = {
3483                 .file           = attr->file,
3484                 .prog_type      = attr->prog_type,
                                       ^^^^^^
3485         };

At the head of function, it directly access 'attr' without checking if
it's NULL pointer.  This patch moves the values assignment after
validating 'attr' and 'attr->file'.

Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
---
 tools/lib/bpf/libbpf.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 197b574406b3..809b633fa3d9 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -3479,10 +3479,7 @@ int bpf_prog_load(const char *file, enum bpf_prog_type type,
 int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
 			struct bpf_object **pobj, int *prog_fd)
 {
-	struct bpf_object_open_attr open_attr = {
-		.file		= attr->file,
-		.prog_type	= attr->prog_type,
-	};
+	struct bpf_object_open_attr open_attr;
 	struct bpf_program *prog, *first_prog = NULL;
 	enum bpf_attach_type expected_attach_type;
 	enum bpf_prog_type prog_type;
@@ -3495,6 +3492,9 @@ int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
 	if (!attr->file)
 		return -EINVAL;
 
+	open_attr.file = attr->file;
+	open_attr.prog_type = attr->prog_type;
+
 	obj = bpf_object__open_xattr(&open_attr);
 	if (IS_ERR_OR_NULL(obj))
 		return -ENOENT;
-- 
2.17.1