tee: try opening as a UNIX socket if open(2) fails

If we get EOPNOTSUPP from open() failing, then it may just be that we're
looking at a unix(4) socket instead of a normal file/device or whatnot.
Fallback to trying to open it as a unix(4) socket, which is a useful
feature that doesn't add much complexity.

Reviewed by:	des, emaste, markj
Differential Revision:	https://reviews.freebsd.org/D48197

usr.bin/tee/tee.1

@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 30, 2022
+.Dd December 25, 2024
 .Dt TEE 1
 .Os
 .Sh NAME
@@ -69,6 +69,12 @@ utility takes the default action for all signals,
 except in the event of the
 .Fl i
 option.
+.Pp
+This implementation of the
+.Nm
+utility may also write to
+.Xr unix 4
+sockets.
 .Sh EXIT STATUS
 .Ex -std
 .Sh EXAMPLES

usr.bin/tee/tee.c

@@ -29,10 +29,12 @@
  * SUCH DAMAGE.
  */
 
+#include <sys/types.h>
 #include <sys/capsicum.h>
 #include <sys/queue.h>
+#include <sys/socket.h>
 #include <sys/stat.h>
-#include <sys/types.h>
+#include <sys/un.h>
 
 #include <capsicum_helpers.h>
 #include <err.h>
@@ -52,6 +54,7 @@ struct entry {
 static STAILQ_HEAD(, entry) head = STAILQ_HEAD_INITIALIZER(head);
 
 static void add(int, const char *);
+static int tee_open(const char *, int);
 static void usage(void) __dead2;
 
 int
@@ -93,7 +96,7 @@ main(int argc, char *argv[])
 		oflags |= O_TRUNC;
 
 	for (exitval = 0; *argv; ++argv) {
-		if ((fd = open(*argv, oflags, DEFFILEMODE)) < 0) {
+		if ((fd = tee_open(*argv, oflags)) < 0) {
 			warn("%s", *argv);
 			exitval = 1;
 		} else {
@@ -149,3 +152,42 @@ add(int fd, const char *name)
 	p->name = name;
 	STAILQ_INSERT_HEAD(&head, p, entries);
 }
+
+static int
+tee_open(const char *path, int oflags)
+{
+	struct sockaddr_un sun = { .sun_family = AF_UNIX };
+	size_t pathlen;
+	int fd;
+
+	if ((fd = open(path, oflags, DEFFILEMODE)) >= 0)
+		return (fd);
+
+	if (errno != EOPNOTSUPP)
+		return (-1);
+
+	pathlen = strnlen(path, sizeof(sun.sun_path));
+	if (pathlen >= sizeof(sun.sun_path))
+		goto failed;
+
+	/*
+	 * For EOPNOTSUPP, we'll try again as a unix(4) socket.  Any errors here
+	 * we'll just surface as the original EOPNOTSUPP since they may not have
+	 * intended for this.
+	 */
+	fd = socket(PF_UNIX, SOCK_STREAM, 0);
+	if (fd < 0)
+		goto failed;
+
+	(void)strlcpy(&sun.sun_path[0], path, sizeof(sun.sun_path));
+	sun.sun_len = SUN_LEN(&sun);
+
+	if (connect(fd, (const struct sockaddr *)&sun, sun.sun_len) == 0)
+		return (fd);
+
+failed:
+	if (fd >= 0)
+		close(fd);
+	errno = EOPNOTSUPP;
+	return (-1);
+}

usr.bin/tee/tests/tee_test.sh

@@ -64,6 +64,37 @@ sigint_ignored_body()
 	atf_check -o inline:"text\ntext\n" cat file
 }
 
+atf_test_case unixsock "cleanup"
+unixsock_pidfile="nc.pid"
+
+unixsock_body()
+{
+	outfile=out.log
+
+	nc -lU logger.sock > "$outfile" &
+	npid=$!
+
+	atf_check -o save:"$unixsock_pidfile" echo "$npid"
+
+	# Wait for the socket to come online, just in case.
+	while [ ! -S logger.sock ]; do
+		sleep 0.1
+	done
+
+	atf_check -o inline:"text over socket\n" -x \
+	    'echo "text over socket" | tee logger.sock'
+
+	atf_check rm "$unixsock_pidfile"
+	atf_check -o inline:"text over socket\n" cat "$outfile"
+}
+unixsock_cleanup()
+{
+	if [ -s "$unixsock_pidfile" ]; then
+		read npid < "$unixsock_pidfile"
+		kill "$npid"
+	fi
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case single_file
@@ -71,4 +102,5 @@ atf_init_test_cases()
 	atf_add_test_case multiple_file
 	atf_add_test_case append
 	atf_add_test_case sigint_ignored
+	atf_add_test_case unixsock
 }