samples: user-trap: fix strict-aliasing warning

I started getting warnings for this one file, though I can't see what changed
since it was originally introduced in commit fec7b669 ("samples: add an
example of seccomp user trap").

samples/seccomp/user-trap.c: In function 'send_fd':
samples/seccomp/user-trap.c:50:11: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   50 |         *((int *)CMSG_DATA(cmsg)) = fd;
      |          ~^~~~~~~~~~~~~~~~~~~~~~~
samples/seccomp/user-trap.c: In function 'recv_fd':
samples/seccomp/user-trap.c:83:18: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   83 |         return *((int *)CMSG_DATA(cmsg));
      |                 ~^~~~~~~~~~~~~~~~~~~~~~~

Using a temporary pointer variable avoids the warning.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240212111737.917428-1-arnd@kernel.orgAcked-by: Tycho Andersen <tandersen@netflix.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

samples/seccomp/user-trap.c

@@ -33,6 +33,7 @@ static int send_fd(int sock, int fd)
 {
 	struct msghdr msg = {};
 	struct cmsghdr *cmsg;
+	int *fd_ptr;
 	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
 	struct iovec io = {
 		.iov_base = &c,
@@ -47,7 +48,8 @@ static int send_fd(int sock, int fd)
 	cmsg->cmsg_level = SOL_SOCKET;
 	cmsg->cmsg_type = SCM_RIGHTS;
 	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
-	*((int *)CMSG_DATA(cmsg)) = fd;
+	fd_ptr = (int *)CMSG_DATA(cmsg);
+	*fd_ptr = fd;
 	msg.msg_controllen = cmsg->cmsg_len;
 
 	if (sendmsg(sock, &msg, 0) < 0) {
@@ -62,6 +64,7 @@ static int recv_fd(int sock)
 {
 	struct msghdr msg = {};
 	struct cmsghdr *cmsg;
+	int *fd_ptr;
 	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
 	struct iovec io = {
 		.iov_base = &c,
@@ -79,8 +82,9 @@ static int recv_fd(int sock)
 	}
 
 	cmsg = CMSG_FIRSTHDR(&msg);
+	fd_ptr = (int *)CMSG_DATA(cmsg);
 
-	return *((int *)CMSG_DATA(cmsg));
+	return *fd_ptr;
 }
 
 static int user_trap_syscall(int nr, unsigned int flags)