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Subject: [PATCH 03/17] watch_queue: sample: Display mount tree change
 notifications [ver #17]
From:   David Howells <dhowells@redhat.com>
To:     viro@zeniv.linux.org.uk
Cc:     dhowells@redhat.com, raven@themaw.net, mszeredi@redhat.com,
        christian@brauner.io, jannh@google.com, darrick.wong@oracle.com,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Date:   Fri, 21 Feb 2020 18:02:10 +0000
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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@redhat.com>
---

 samples/watch_queue/watch_test.c |   39 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/samples/watch_queue/watch_test.c b/samples/watch_queue/watch_test.c
index 0eaff5dc04c3..49d185150506 100644
--- a/samples/watch_queue/watch_test.c
+++ b/samples/watch_queue/watch_test.c
@@ -26,6 +26,9 @@
 #ifndef __NR_watch_devices
 #define __NR_watch_devices -1
 #endif
+#ifndef __NR_watch_mount
+#define __NR_watch_mount -1
+#endif
 
 #define BUF_SIZE 256
 
@@ -58,6 +61,27 @@ 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\n",
+	       m->triggered_on, n->subtype, mount_subtypes[n->subtype], m->changed_mount);
+}
+
 /*
  * Consume and display events.
  */
@@ -134,6 +158,9 @@ 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 +169,17 @@ 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 +213,11 @@ int main(int argc, char **argv)
 		exit(1);
 	}
 
+	if (syscall(__NR_watch_mount, AT_FDCWD, "/", 0, fd, 0x02) == -1) {
+		perror("watch_mount");
+		exit(1);
+	}
+
 	consumer(fd);
 	exit(0);
 }