net: Fix TOCTOU race condition in unix_conf_op

The unix_conf_op function reads the size of the sysctl entry array
twice. gcc thinks that it can lead to a time-of-check to time-of-use
(TOCTOU) race condition if the array size changes between the two reads.

Fixes #2398

criu/net.c

@@ -359,22 +359,23 @@ static int ipv6_conf_op(char *tgt, SysctlEntry **conf, int n, int op, SysctlEntr
 	return net_conf_op(tgt, conf, n, op, "ipv6", req, path, ARRAY_SIZE(devconfs6), devconfs6, def_conf);
 }
 
-static int unix_conf_op(SysctlEntry ***rconf, size_t *n, int op)
+static int unix_conf_op(SysctlEntry ***rconf, size_t *pn, int op)
 {
 	int i, ret = -1, flags = 0;
 	char path[ARRAY_SIZE(unix_conf_entries)][MAX_CONF_UNIX_PATH] = {};
 	struct sysctl_req req[ARRAY_SIZE(unix_conf_entries)] = {};
 	SysctlEntry **conf = *rconf;
+	size_t n = *pn;
 
-	if (*n != ARRAY_SIZE(unix_conf_entries)) {
-		pr_err("unix: Unexpected entries in config (%zu %zu)\n", *n, ARRAY_SIZE(unix_conf_entries));
+	if (n != ARRAY_SIZE(unix_conf_entries)) {
+		pr_err("unix: Unexpected entries in config (%zu %zu)\n", n, ARRAY_SIZE(unix_conf_entries));
 		return -EINVAL;
 	}
 
 	if (opts.weak_sysctls || op == CTL_READ)
 		flags = CTL_FLAGS_OPTIONAL;
 
-	for (i = 0; i < *n; i++) {
+	for (i = 0; i < n; i++) {
 		snprintf(path[i], MAX_CONF_UNIX_PATH, CONF_UNIX_FMT, unix_conf_entries[i]);
 		req[i].name = path[i];
 		req[i].flags = flags;
@@ -390,7 +391,7 @@ static int unix_conf_op(SysctlEntry ***rconf, size_t *n, int op)
 		}
 	}
 
-	ret = sysctl_op(req, *n, op, CLONE_NEWNET);
+	ret = sysctl_op(req, n, op, CLONE_NEWNET);
 	if (ret < 0) {
 		pr_err("unix: Failed to %s %s/<confs>\n", (op == CTL_READ) ? "read" : "write", CONF_UNIX_BASE);
 		return -1;
@@ -399,7 +400,7 @@ static int unix_conf_op(SysctlEntry ***rconf, size_t *n, int op)
 	if (op == CTL_READ) {
 		bool has_entries = false;
 
-		for (i = 0; i < *n; i++) {
+		for (i = 0; i < n; i++) {
 			if (req[i].flags & CTL_FLAGS_HAS) {
 				conf[i]->has_iarg = true;
 				if (!has_entries)
@@ -412,7 +413,7 @@ static int unix_conf_op(SysctlEntry ***rconf, size_t *n, int op)
 		 * Unix conf is optional.
 		 */
 		if (!has_entries) {
-			*n = 0;
+			*pn = 0;
 			*rconf = NULL;
 		}
 	}

scripts/build/Dockerfile.x86_64.hdr

@@ -1,4 +1,4 @@
-FROM ubuntu:focal
+FROM ubuntu:24.04
 
 COPY scripts/ci/apt-install /bin/apt-install