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Commit b37199e6 authored by Al Viro's avatar Al Viro
Browse files

rcuwalk: recheck mount_lock after mountpoint crossing attempts 



We can get false negative from __lookup_mnt() if an unrelated vfsmount
gets moved.  In that case legitimize_mnt() is guaranteed to fail,
and we will fall back to non-RCU walk... unless we end up running
into a hard error on a filesystem object we wouldn't have reached
if not for that false negative.  IOW, delaying that check until
the end of pathname resolution is wrong - we should recheck right
after we attempt to cross the mountpoint.  We don't need to recheck
unless we see d_mountpoint() being true - in that case even if
we have just raced with mount/umount, we can simply go on as if
we'd come at the moment when the sucker wasn't a mountpoint; if we
run into a hard error as the result, it was a legitimate outcome.
__lookup_mnt() returning NULL is different in that respect, since
it might've happened due to operation on completely unrelated
mountpoint.

Cc: stable@vger.kernel.org
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent e825196d

fs/namei.c

+13 −16
Original line number Diff line number Diff line
@@ -1109,7 +1109,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, 
			return false;



		if (!d_mountpoint(path->dentry))

			break;

			return true;



		mounted = __lookup_mnt(path->mnt, path->dentry);

		if (!mounted)
@@ -1125,20 +1125,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, 
		 */

		*inode = path->dentry->d_inode;

	}

	return true;

}



static void follow_mount_rcu(struct nameidata *nd)

{

	while (d_mountpoint(nd->path.dentry)) {

		struct mount *mounted;

		mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);

		if (!mounted)

			break;

		nd->path.mnt = &mounted->mnt;

		nd->path.dentry = mounted->mnt.mnt_root;

		nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);

	}

	return read_seqretry(&mount_lock, nd->m_seq);

}



static int follow_dotdot_rcu(struct nameidata *nd)
@@ -1166,7 +1153,17 @@ static int follow_dotdot_rcu(struct nameidata *nd) 
			break;

		nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);

	}

	follow_mount_rcu(nd);

	while (d_mountpoint(nd->path.dentry)) {

		struct mount *mounted;

		mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);

		if (!mounted)

			break;

		nd->path.mnt = &mounted->mnt;

		nd->path.dentry = mounted->mnt.mnt_root;

		nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);

		if (!read_seqretry(&mount_lock, nd->m_seq))

			goto failed;

	}

	nd->inode = nd->path.dentry->d_inode;

	return 0;