linux-kernel - [PATCH 4/4] watch_queue: sample: Display mount tree change notifications

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Message-ID: <159559631836.2141315.12757117418181245451.stgit@warthog.procyon.org.uk>
Date:   Fri, 24 Jul 2020 14:11:58 +0100
From:   David Howells <dhowells@...hat.com>
To:     viro@...iv.linux.org.uk
Cc:     dhowells@...hat.com, torvalds@...ux-foundation.org,
        casey@...aufler-ca.com, sds@...ho.nsa.gov,
        nicolas.dichtel@...nd.com, raven@...maw.net, christian@...uner.io,
        jlayton@...hat.com, kzak@...hat.com, mszeredi@...hat.com,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 4/4] watch_queue: sample: Display mount tree change
 notifications

This is run like:

	./watch_test

and watches "/" for changes to the mount topology and the attributes of
individual mount objects.

	# mount -t tmpfs none /mnt
	# mount -o remount,ro /mnt
	# mount -o remount,rw /mnt

producing:

	# ./watch_test
	read() = 16
	NOTIFY[000]: ty=000002 sy=00 i=02000010
	MOUNT 00000060 change=0[new_mount] aux=416
	read() = 16
	NOTIFY[000]: ty=000002 sy=04 i=02010010
	MOUNT 000001a0 change=4[setattr] aux=0
	read() = 16
	NOTIFY[000]: ty=000002 sy=04 i=02010010
	MOUNT 000001a0 change=4[setattr] aux=0

Signed-off-by: David Howells <dhowells@...hat.com>
---

 samples/watch_queue/watch_test.c |   44 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/samples/watch_queue/watch_test.c b/samples/watch_queue/watch_test.c
index 46e618a897fe..b526de016de4 100644
--- a/samples/watch_queue/watch_test.c
+++ b/samples/watch_queue/watch_test.c
@@ -26,6 +26,9 @@
 #ifndef __NR_keyctl
 #define __NR_keyctl -1
 #endif
+#ifndef __NR_watch_mount
+#define __NR_watch_mount -1
+#endif
 
 #define BUF_SIZE 256
 
@@ -58,6 +61,32 @@ static void saw_key_change(struct watch_notification *n, size_t len)
 	       k->key_id, n->subtype, key_subtypes[n->subtype], k->aux);
 }
 
+static const char *mount_subtypes[256] = {
+	[NOTIFY_MOUNT_NEW_MOUNT]	= "new_mount",
+	[NOTIFY_MOUNT_UNMOUNT]		= "unmount",
+	[NOTIFY_MOUNT_EXPIRY]		= "expiry",
+	[NOTIFY_MOUNT_READONLY]		= "readonly",
+	[NOTIFY_MOUNT_SETATTR]		= "setattr",
+	[NOTIFY_MOUNT_MOVE_FROM]	= "move_from",
+	[NOTIFY_MOUNT_MOVE_TO]		= "move_to",
+};
+
+static void saw_mount_change(struct watch_notification *n, size_t len)
+{
+	struct mount_notification *m = (struct mount_notification *)n;
+
+	if (len != sizeof(struct mount_notification))
+		return;
+
+	printf("MOUNT %08x change=%u[%s] aux=%u ctr=%x,%x actr=%x\n",
+	       m->triggered_on, n->subtype, mount_subtypes[n->subtype],
+	       m->auxiliary_mount,
+	       m->topology_changes,
+	       m->attr_changes,
+	       m->aux_topology_changes);
+
+}
+
 /*
  * Consume and display events.
  */
@@ -134,6 +163,9 @@ static void consumer(int fd)
 			default:
 				printf("other type\n");
 				break;
+			case WATCH_TYPE_MOUNT_NOTIFY:
+				saw_mount_change(&n.n, len);
+				break;
 			}
 
 			p += len;
@@ -142,12 +174,17 @@ static void consumer(int fd)
 }
 
 static struct watch_notification_filter filter = {
-	.nr_filters	= 1,
+	.nr_filters	= 2,
 	.filters = {
 		[0]	= {
 			.type			= WATCH_TYPE_KEY_NOTIFY,
 			.subtype_filter[0]	= UINT_MAX,
 		},
+		[1] = {
+			.type			= WATCH_TYPE_MOUNT_NOTIFY,
+			// Reject move-from notifications
+			.subtype_filter[0]	= UINT_MAX & ~(1 << NOTIFY_MOUNT_MOVE_FROM),
+		},
 	},
 };
 
@@ -181,6 +218,11 @@ int main(int argc, char **argv)
 		exit(1);
 	}
 
+	if (syscall(__NR_watch_mount, AT_FDCWD, "/", 0, fd, 0xde) == -1) {
+		perror("watch_mount");
+		exit(1);
+	}
+
 	consumer(fd);
 	exit(0);
 }