samples/seccomp: improve label helper

Fixes a potential corruption with uninitialized stack memory in the
seccomp BPF sample program.

[akpm@linux-foundation.org: coding-style fixlet]
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Robert Swiecki <swiecki@google.com>
Tested-by: Robert Swiecki <swiecki@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

samples/seccomp/bpf-fancy.c

@@ -25,7 +25,9 @@
 
 int main(int argc, char **argv)
 {
-	struct bpf_labels l;
+	struct bpf_labels l = {
+		.count = 0,
+	};
 	static const char msg1[] = "Please type something: ";
 	static const char msg2[] = "You typed: ";
 	char buf[256];

samples/seccomp/bpf-helper.c

@@ -10,6 +10,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include "bpf-helper.h"
@@ -63,6 +64,11 @@ __u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
 {
 	struct __bpf_label *begin = labels->labels, *end;
 	int id;
+
+	if (labels->count == BPF_LABELS_MAX) {
+		fprintf(stderr, "Too many labels\n");
+		exit(1);
+	}
 	if (labels->count == 0) {
 		begin->label = label;
 		begin->location = 0xffffffff;