proc/mounts: add cursor

If mounts are deleted after a read(2) call on /proc/self/mounts (or its
kin), the subsequent read(2) could miss a mount that comes after the
deleted one in the list.  This is because the file position is interpreted
as the number mount entries from the start of the list.

E.g. first read gets entries #0 to #9; the seq file index will be 10.  Then
entry #5 is deleted, resulting in #10 becoming #9 and #11 becoming #10,
etc...  The next read will continue from entry #10, and #9 is missed.

Solve this by adding a cursor entry for each open instance.  Taking the
global namespace_sem for write seems excessive, since we are only dealing
with a per-namespace list.  Instead add a per-namespace spinlock and use
that together with namespace_sem taken for read to protect against
concurrent modification of the mount list.  This may reduce parallelism of
is_local_mountpoint(), but it's hardly a big contention point.  We could
also use RCU freeing of cursors to make traversal not need additional
locks, if that turns out to be neceesary.

Only move the cursor once for each read (cursor is not added on open) to
minimize cacheline invalidation.  When EOF is reached, the cursor is taken
off the list, in order to prevent an excessive number of cursors due to
inactive open file descriptors.

Reported-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/fs/mount.h b/fs/mount.h
index 711a4093e475..c7abb7b394d8 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -9,7 +9,13 @@ struct mnt_namespace {
 	atomic_t		count;
 	struct ns_common	ns;
 	struct mount *	root;
+	/*
+	 * Traversal and modification of .list is protected by either
+	 * - taking namespace_sem for write, OR
+	 * - taking namespace_sem for read AND taking .ns_lock.
+	 */
 	struct list_head	list;
+	spinlock_t		ns_lock;
 	struct user_namespace	*user_ns;
 	struct ucounts		*ucounts;
 	u64			seq;	/* Sequence number to prevent loops */
@@ -133,9 +139,7 @@ struct proc_mounts {
 	struct mnt_namespace *ns;
 	struct path root;
 	int (*show)(struct seq_file *, struct vfsmount *);
-	void *cached_mount;
-	u64 cached_event;
-	loff_t cached_index;
+	struct mount cursor;
 };
 
 extern const struct seq_operations mounts_op;
@@ -153,3 +157,5 @@ static inline bool is_anon_ns(struct mnt_namespace *ns)
 {
 	return ns->seq == 0;
 }
+
+extern void mnt_cursor_del(struct mnt_namespace *ns, struct mount *cursor);