samples: bpf: Fix a couple of warnings

cookie_uid_helper_example.c: In function ‘main’:
cookie_uid_helper_example.c:178:69: warning: ‘ -j ACCEPT’ directive
	writing 10 bytes into a region of size between 8 and 58
	[-Wformat-overflow=]
  178 |  sprintf(rules, "iptables -A OUTPUT -m bpf --object-pinned %s -j ACCEPT",
      |								       ^~~~~~~~~~
/home/kkd/src/linux/samples/bpf/cookie_uid_helper_example.c:178:9: note:
	‘sprintf’ output between 53 and 103 bytes into a destination of size 100
  178 |  sprintf(rules, "iptables -A OUTPUT -m bpf --object-pinned %s -j ACCEPT",
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  179 |         file);
      |         ~~~~~

Fix by using snprintf and a sufficiently sized buffer.

tracex4_user.c:35:15: warning: ‘write’ reading 12 bytes from a region of
	size 11 [-Wstringop-overread]
   35 |         key = write(1, "\e[1;1H\e[2J", 12); /* clear screen */
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use size as 11.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-2-memxor@gmail.com

samples/bpf/cookie_uid_helper_example.c

@@ -167,7 +167,7 @@ static void prog_load(void)
 static void prog_attach_iptables(char *file)
 {
 	int ret;
-	char rules[100];
+	char rules[256];
 
 	if (bpf_obj_pin(prog_fd, file))
 		error(1, errno, "bpf_obj_pin");
@@ -175,8 +175,13 @@ static void prog_attach_iptables(char *file)
 		printf("file path too long: %s\n", file);
 		exit(1);
 	}
-	sprintf(rules, "iptables -A OUTPUT -m bpf --object-pinned %s -j ACCEPT",
-		file);
+	ret = snprintf(rules, sizeof(rules),
+		       "iptables -A OUTPUT -m bpf --object-pinned %s -j ACCEPT",
+		       file);
+	if (ret < 0 || ret >= sizeof(rules)) {
+		printf("error constructing iptables command\n");
+		exit(1);
+	}
 	ret = system(rules);
 	if (ret < 0) {
 		printf("iptables rule update failed: %d/n", WEXITSTATUS(ret));

samples/bpf/tracex4_user.c

@@ -32,7 +32,7 @@ static void print_old_objects(int fd)
 	__u64 key, next_key;
 	struct pair v;
 
-	key = write(1, "\e[1;1H\e[2J", 12); /* clear screen */
+	key = write(1, "\e[1;1H\e[2J", 11); /* clear screen */
 
 	key = -1;
 	while (bpf_map_get_next_key(fd, &key, &next_key) == 0) {