bpf: Change bpf_obj_name_cpy() to better ensure map's name is init by 0

During get_info_by_fd, the prog/map name is memcpy-ed.  It depends
on the prog->aux->name and map->name to be zero initialized.

bpf_prog_aux is easy to guarantee that aux->name is zero init.

The name in bpf_map may be harder to be guaranteed in the future when
new map type is added.

Hence, this patch makes bpf_obj_name_cpy() to always zero init
the prog/map name.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/syscall.c

@@ -322,6 +322,8 @@ static int bpf_obj_name_cpy(char *dst, const char *src)
 {
 	const char *end = src + BPF_OBJ_NAME_LEN;
 
+	memset(dst, 0, BPF_OBJ_NAME_LEN);
+
 	/* Copy all isalnum() and '_' char */
 	while (src < end && *src) {
 		if (!isalnum(*src) && *src != '_')
@@ -333,9 +335,6 @@ static int bpf_obj_name_cpy(char *dst, const char *src)
 	if (src == end)
 		return -EINVAL;
 
-	/* '\0' terminates dst */
-	*dst = 0;
-
 	return 0;
 }