Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware

[Ian Kent <raven@themaw.net>]

On Wed, 2016-09-14 at 12:28 -0500, Eric W. Biederman wrote:
> Ian Kent <raven@themaw.net> writes:
> 
> > If an automount mount is clone(2)ed into a file system that is
> > propagation private, when it later expires in the originating
> > namespace subsequent calls to autofs ->d_automount() for that
> > dentry in the original namespace will return ELOOP until the
> > mount is manually umounted in the cloned namespace.
> > 
> > In the same way, if an autofs mount is triggered by automount(8)
> > running within a container the dentry will be seen as mounted in
> > the root init namespace and calls to ->d_automount() in that namespace
> > will return ELOOP until the mount is umounted within the container.
> > 
> > Also, have_submounts() can return an incorect result when a mount
> > exists in a namespace other than the one being checked.
> 
> Overall this appears to be a fairly reasonable set of changes.  It does
> increase the expense when an actual mount point is encountered, but if
> these are the desired some increase in cost when a dentry is a
> mountpoint is unavoidable.

The possibility of a significant increase in overhead with this change for
autofs is one reason I've held back on posting the change for a long time.

If there are many instances of a mount (ie. thousands) I think the mnt_namespace
mount list could become large enough to be a problem. So that list might
eventually need to be a hashed list instead of a linear list.

But this would likely also be a problem in areas other than just autofs.

> 
> May I ask the motiviation for this set of changes?  Reading through the
> changes I don't grasp why we want to change the behavior of autofs.
> What problem is being solved?  What are the benefits?
> 
> Eric
> 
> > Signed-off-by: Ian Kent <raven@themaw.net>
> > Cc: Al Viro <viro@ZenIV.linux.org.uk>
> > Cc: Eric W. Biederman <ebiederm@xmission.com>
> > Cc: Omar Sandoval <osandov@osandov.com>
> > ---
> >  fs/autofs4/dev-ioctl.c |    2 +-
> >  fs/autofs4/expire.c    |    4 ++--
> >  fs/autofs4/root.c      |   30 +++++++++++++++---------------
> >  fs/autofs4/waitq.c     |    2 +-
> >  4 files changed, 19 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
> > index c7fcc74..0024e25 100644
> > --- a/fs/autofs4/dev-ioctl.c
> > +++ b/fs/autofs4/dev-ioctl.c
> > @@ -564,7 +564,7 @@ static int autofs_dev_ioctl_ismountpoint(struct file
> > *fp,
> >  
> >  		devid = new_encode_dev(dev);
> >  
> > -		err = have_submounts(path.dentry);
> > +		err = have_local_submounts(path.dentry);
> >  
> >  		if (follow_down_one(&path))
> >  			magic = path.dentry->d_sb->s_magic;
> > diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
> > index d8e6d42..7cc34ef 100644
> > --- a/fs/autofs4/expire.c
> > +++ b/fs/autofs4/expire.c
> > @@ -236,7 +236,7 @@ static int autofs4_tree_busy(struct vfsmount *mnt,
> >  		 * count for the autofs dentry.
> >  		 * If the fs is busy update the expiry counter.
> >  		 */
> > -		if (d_mountpoint(p)) {
> > +		if (is_local_mountpoint(p)) {
> >  			if (autofs4_mount_busy(mnt, p)) {
> >  				top_ino->last_used = jiffies;
> >  				dput(p);
> > @@ -280,7 +280,7 @@ static struct dentry *autofs4_check_leaves(struct
> > vfsmount *mnt,
> >  	while ((p = get_next_positive_dentry(p, parent))) {
> >  		pr_debug("dentry %p %pd\n", p, p);
> >  
> > -		if (d_mountpoint(p)) {
> > +		if (is_local_mountpoint(p)) {
> >  			/* Can we umount this guy */
> >  			if (autofs4_mount_busy(mnt, p))
> >  				continue;
> > diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
> > index fa84bb8..4150ad6 100644
> > --- a/fs/autofs4/root.c
> > +++ b/fs/autofs4/root.c
> > @@ -123,7 +123,7 @@ static int autofs4_dir_open(struct inode *inode, struct
> > file *file)
> >  	 * it.
> >  	 */
> >  	spin_lock(&sbi->lookup_lock);
> > -	if (!d_mountpoint(dentry) && simple_empty(dentry)) {
> > +	if (!is_local_mountpoint(dentry) && simple_empty(dentry)) {
> >  		spin_unlock(&sbi->lookup_lock);
> >  		return -ENOENT;
> >  	}
> > @@ -370,28 +370,28 @@ static struct vfsmount *autofs4_d_automount(struct
> > path *path)
> >  
> >  	/*
> >  	 * If the dentry is a symlink it's equivalent to a directory
> > -	 * having d_mountpoint() true, so there's no need to call back
> > -	 * to the daemon.
> > +	 * having is_local_mountpoint() true, so there's no need to
> > +	 * call back to the daemon.
> >  	 */
> >  	if (d_really_is_positive(dentry) && d_is_symlink(dentry)) {
> >  		spin_unlock(&sbi->fs_lock);
> >  		goto done;
> >  	}
> >  
> > -	if (!d_mountpoint(dentry)) {
> > +	if (!is_local_mountpoint(dentry)) {
> >  		/*
> >  		 * It's possible that user space hasn't removed directories
> >  		 * after umounting a rootless multi-mount, although it
> > -		 * should. For v5 have_submounts() is sufficient to handle
> > -		 * this because the leaves of the directory tree under the
> > -		 * mount never trigger mounts themselves (they have an
> > autofs
> > -		 * trigger mount mounted on them). But v4 pseudo direct
> > mounts
> > -		 * do need the leaves to trigger mounts. In this case we
> > -		 * have no choice but to use the list_empty() check and
> > -		 * require user space behave.
> > +		 * should. For v5 have_local_submounts() is sufficient to
> > +		 * handle this because the leaves of the directory tree
> > under
> > +		 * the mount never trigger mounts themselves (they have an
> > +		 * autofs trigger mount mounted on them). But v4 pseudo
> > +		 * direct mounts do need the leaves to trigger mounts. In
> > +		 * this case we have no choice but to use the list_empty()
> > +		 * check and require user space behave.
> >  		 */
> >  		if (sbi->version > 4) {
> > -			if (have_submounts(dentry)) {
> > +			if (have_local_submounts(dentry)) {
> >  				spin_unlock(&sbi->fs_lock);
> >  				goto done;
> >  			}
> > @@ -431,7 +431,7 @@ static int autofs4_d_manage(struct dentry *dentry, bool
> > rcu_walk)
> >  
> >  	/* The daemon never waits. */
> >  	if (autofs4_oz_mode(sbi)) {
> > -		if (!d_mountpoint(dentry))
> > +		if (!is_local_mountpoint(dentry))
> >  			return -EISDIR;
> >  		return 0;
> >  	}
> > @@ -460,7 +460,7 @@ static int autofs4_d_manage(struct dentry *dentry, bool
> > rcu_walk)
> >  
> >  		if (ino->flags & AUTOFS_INF_WANT_EXPIRE)
> >  			return 0;
> > -		if (d_mountpoint(dentry))
> > +		if (is_local_mountpoint(dentry))
> >  			return 0;
> >  		inode = d_inode_rcu(dentry);
> >  		if (inode && S_ISLNK(inode->i_mode))
> > @@ -487,7 +487,7 @@ static int autofs4_d_manage(struct dentry *dentry, bool
> > rcu_walk)
> >  		 * we can avoid needless calls ->d_automount() and avoid
> >  		 * an incorrect ELOOP error return.
> >  		 */
> > -		if ((!d_mountpoint(dentry) && !simple_empty(dentry)) ||
> > +		if ((!is_local_mountpoint(dentry) && !simple_empty(dentry))
> > ||
> >  		    (d_really_is_positive(dentry) && d_is_symlink(dentry)))
> >  			status = -EISDIR;
> >  	}
> > diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
> > index 431fd7e..911f4d5 100644
> > --- a/fs/autofs4/waitq.c
> > +++ b/fs/autofs4/waitq.c
> > @@ -333,7 +333,7 @@ static int validate_request(struct autofs_wait_queue
> > **wait,
> >  					dentry = new;
> >  			}
> >  		}
> > -		if (have_submounts(dentry))
> > +		if (have_local_submounts(dentry))
> >  			valid = 0;
> >  
> >  		if (new)