[PATCH bpf v1] libbpf: fix build failure from uninitialized variable warning

While compiling libbpf, some GCC versions (at least 8.4.0) have difficulty
determining control flow and a emit warning for potentially uninitialized
usage of 'map', which results in a build error if using "-Werror":

In file included from libbpf.c:56:
libbpf.c: In function '__bpf_object__open':
libbpf_internal.h:59:2: warning: 'map' may be used uninitialized in this function [-Wmaybe-uninitialized]
  libbpf_print(level, "libbpf: " fmt, ##__VA_ARGS__); \
  ^~~~~~~~~~~~
libbpf.c:5032:18: note: 'map' was declared here
  struct bpf_map *map, *targ_map;
                  ^~~

The warning/error is false based on code inspection, so silence it with a
NULL initialization.

Fixes: 646f02ffdd49 ("libbpf: Add BTF-defined map-in-map support")
Ref: 063e68813391 ("libbpf: Fix false uninitialized variable warning")

Signed-off-by: Tony Ambardar <redacted>
---
 tools/lib/bpf/libbpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index b688aadf09c5..46d727b45c81 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -5804,8 +5804,8 @@ static int bpf_object__collect_map_relos(struct bpf_object *obj,
 	int i, j, nrels, new_sz;
 	const struct btf_var_secinfo *vi = NULL;
 	const struct btf_type *sec, *var, *def;
+	struct bpf_map *map = NULL, *targ_map;
 	const struct btf_member *member;
-	struct bpf_map *map, *targ_map;
 	const char *name, *mname;
 	Elf_Data *symbols;
 	unsigned int moff;

-- 
2.25.1

On 8/31/20 2:03 AM, Tony Ambardar wrote:

While compiling libbpf, some GCC versions (at least 8.4.0) have difficulty
determining control flow and a emit warning for potentially uninitialized
usage of 'map', which results in a build error if using "-Werror":

In file included from libbpf.c:56:
libbpf.c: In function '__bpf_object__open':
libbpf_internal.h:59:2: warning: 'map' may be used uninitialized in this function [-Wmaybe-uninitialized]
   libbpf_print(level, "libbpf: " fmt, ##__VA_ARGS__); \
   ^~~~~~~~~~~~
libbpf.c:5032:18: note: 'map' was declared here
   struct bpf_map *map, *targ_map;
                   ^~~

The warning/error is false based on code inspection, so silence it with a
NULL initialization.

Fixes: 646f02ffdd49 ("libbpf: Add BTF-defined map-in-map support")
Ref: 063e68813391 ("libbpf: Fix false uninitialized variable warning")

Signed-off-by: Tony Ambardar <redacted>

Applied, thanks!

On Mon, 31 Aug 2020 at 07:59, Daniel Borkmann [off-list ref] wrote:

On 8/31/20 2:03 AM, Tony Ambardar wrote:

quoted

While compiling libbpf, some GCC versions (at least 8.4.0) have difficulty
determining control flow and a emit warning for potentially uninitialized
usage of 'map', which results in a build error if using "-Werror":

In file included from libbpf.c:56:
libbpf.c: In function '__bpf_object__open':
libbpf_internal.h:59:2: warning: 'map' may be used uninitialized in this function [-Wmaybe-uninitialized]
   libbpf_print(level, "libbpf: " fmt, ##__VA_ARGS__); \
   ^~~~~~~~~~~~
libbpf.c:5032:18: note: 'map' was declared here
   struct bpf_map *map, *targ_map;
                   ^~~

The warning/error is false based on code inspection, so silence it with a
NULL initialization.

Fixes: 646f02ffdd49 ("libbpf: Add BTF-defined map-in-map support")
Ref: 063e68813391 ("libbpf: Fix false uninitialized variable warning")

Signed-off-by: Tony Ambardar <redacted>

Applied, thanks!

Thanks, Daniel. I forgot to ask/confirm whether this will get applied
to the 5.8.x stable branch, since it was first encountered there.