tools resolve_btfids: Add size check to get_id function

To make sure we don't crash on malformed symbols.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-2-jolsa@kernel.org

tools/bpf/resolve_btfids/main.c

@@ -199,9 +199,16 @@ static char *get_id(const char *prefix_end)
 	/*
 	 * __BTF_ID__func__vfs_truncate__0
 	 * prefix_end =  ^
+	 * pos        =    ^
 	 */
-	char *p, *id = strdup(prefix_end + sizeof("__") - 1);
+	int len = strlen(prefix_end);
+	int pos = sizeof("__") - 1;
+	char *p, *id;
 
+	if (pos >= len)
+		return NULL;
+
+	id = strdup(prefix_end + pos);
 	if (id) {
 		/*
 		 * __BTF_ID__func__vfs_truncate__0