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

[Eric W. Biederman]

Ian Kent <raven@xxxxxxxxxx> writes:

> On Sat, 2016-09-17 at 22:10 +0200, Mateusz Guzik wrote:
>> On Wed, Sep 14, 2016 at 02:14:45PM +0800, Ian Kent wrote:
>> > 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.
>> > 
>> > @@ -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))
>> 
>> This change is within RCU lookup.
>> 
>> is_local_mountpoint may end up calling __is_local_mountpoint, which will
>> optionally take the namespace_sem lock, resulting in a splat:
>
> Yes, that's a serious problem I missed.
>
> snip ...
>
>> I don't know this code. Perhaps it will be perfectly fine performance wise to
>> just drop out of RCU lookup in this case.
>
> It's a bit worse than that.
>
> I think being able to continue the rcu-walk for an already mounted dentry that
> is not being expired is an important part of the performance improvement given
> by the series that added this.
>
> Can you confirm that Neil?
>
> But for the case here the existing test can allow rcu-walk to continue for a
> dentry that would attempt to trigger an automount so it's also a bug in the
> existing code.

I don't think the existing code is buggy.  As I read __follow_mount_rcu
if DCACHE_NEED_AUTOMOUNT is set on the dentry after return from d_manage
the code will break out of the rcu walk.

> Any thoughts on how we can handle this Neil, I'm having a bit of trouble working
> out how to resolve this one.

I believe in this case d_mountpoint is enough for the rcu walk case.  If
the mountpoint turns out not to be local __follow_mount_rcu will kick
out of the rcu walk as it will return false.  Because:
	return !mounted && !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);

I am not quite certain about the non-rcu case.  That can't be
is_local_mountpoint as that is inside a spinlock and is_local_mountpoint
can sleep.  Perhaps d_mountpoint is simply correct for autofs4_d_manage.

Eric