linux-kernel - [PATCH 06/18] fsinfo: Add a uniquifier ID to struct mount [ver #21]

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Date:   Mon, 03 Aug 2020 14:37:16 +0100
From:   David Howells <dhowells@...hat.com>
To:     viro@...iv.linux.org.uk
Cc:     dhowells@...hat.com, torvalds@...ux-foundation.org,
        raven@...maw.net, mszeredi@...hat.com, christian@...uner.io,
        jannh@...gle.com, darrick.wong@...cle.com, kzak@...hat.com,
        jlayton@...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 06/18] fsinfo: Add a uniquifier ID to struct mount [ver #21]

Add a uniquifier ID to struct mount that is effectively unique over the
kernel lifetime to deal around mnt_id values being reused.  This can then
be exported through fsinfo() to allow detection of replacement mounts that
happen to end up with the same mount ID.

The normal mount handle is still used for referring to a particular mount.

The mount notification is then changed to convey these unique mount IDs
rather than the mount handle.

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

 fs/mount.h        |    3 +++
 fs/mount_notify.c |    4 ++--
 fs/namespace.c    |    3 +++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/mount.h b/fs/mount.h
index 85456a5f5a3a..1037781be055 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -79,6 +79,9 @@ struct mount {
 	int mnt_expiry_mark;		/* true if marked for expiry */
 	struct hlist_head mnt_pins;
 	struct hlist_head mnt_stuck_children;
+#ifdef CONFIG_FSINFO
+	u64	mnt_unique_id;		/* ID unique over lifetime of kernel */
+#endif
 #ifdef CONFIG_MOUNT_NOTIFICATIONS
 	struct watch_list *mnt_watchers; /* Watches on dentries within this mount */
 #endif
diff --git a/fs/mount_notify.c b/fs/mount_notify.c
index 44f570e4cebe..d8ba66ed5f77 100644
--- a/fs/mount_notify.c
+++ b/fs/mount_notify.c
@@ -90,7 +90,7 @@ void notify_mount(struct mount *trigger,
 	n.watch.type	= WATCH_TYPE_MOUNT_NOTIFY;
 	n.watch.subtype	= subtype;
 	n.watch.info	= info_flags | watch_sizeof(n);
-	n.triggered_on	= trigger->mnt_id;
+	n.triggered_on	= trigger->mnt_unique_id;
 
 	switch (subtype) {
 	case NOTIFY_MOUNT_EXPIRY:
@@ -102,7 +102,7 @@ void notify_mount(struct mount *trigger,
 	case NOTIFY_MOUNT_UNMOUNT:
 	case NOTIFY_MOUNT_MOVE_FROM:
 	case NOTIFY_MOUNT_MOVE_TO:
-		n.auxiliary_mount	= aux->mnt_id;
+		n.auxiliary_mount = aux->mnt_unique_id;
 		break;
 
 	default:
diff --git a/fs/namespace.c b/fs/namespace.c
index b2b9920ffd3c..1db8a64cd76f 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -115,6 +115,9 @@ static int mnt_alloc_id(struct mount *mnt)
 	if (res < 0)
 		return res;
 	mnt->mnt_id = res;
+#ifdef CONFIG_FSINFO
+	mnt->mnt_unique_id = atomic64_inc_return(&vfs_unique_counter);
+#endif
 	return 0;
 }