Re: dcache_readdir NULL inode oops

[Will Deacon <will.deacon@arm.com>]

On Fri, Nov 30, 2018 at 04:08:52PM +0000, Al Viro wrote:
> On Fri, Nov 30, 2018 at 09:16:49AM -0600, Eric W. Biederman wrote:
> > >> > +       inode_lock(parent->d_inode);
> > >> >         dentry->d_fsdata = NULL;
> > >> >         drop_nlink(dentry->d_inode);
> > >> >         d_delete(dentry);
> > >> > +       inode_unlock(parent->d_inode);
> > >> > +
> > >> >         dput(dentry);   /* d_alloc_name() in devpts_pty_new() */
> > >> >  }
> > >
> > > This feels right but getting some feedback from others would be good.
> > 
> > This is going to be special at least because we are not coming through
> > the normal unlink path and we are manipulating the dcache.
> > 
> > This looks plausible.  If this is whats going on then we have had this
> > bug for a very long time.  I will see if I can make some time.
> > 
> > It looks like in the general case everything is serialized by the
> > devpts_mutex.  I wonder if just changing the order of operations
> > here would be enough.
> > 
> > AKA: drop_nlink d_delete then dentry->d_fsdata.  Ugh d_fsdata is not
> > implicated so that won't help here.
> 
> It certainly won't.  The thing is, this
>                 if (!dir_emit(ctx, next->d_name.name, next->d_name.len,
>                               d_inode(next)->i_ino, dt_type(d_inode(next))))
> in dcache_readdir() obviously can block, so all we can hold over it is
> blocking locks.  Which we do - specifically, ->i_rwsem on our directory.
> 
> It's actually worse than missing inode_lock() - consider the effects
> of mount --bind /mnt/foo /dev/pts/42.  What happens when that thing
> goes away?  Right, a lost mount...

Ha, I hadn't even considered that scenario. Urgh!

> I'll resurrect the "kernel-internal rm -rf done right" series and
> post it; devpts is not the only place suffering such problem (binfmt_misc,
> etc.)

Thanks. I'm happy to test that it solves this issue if you throw me on cc.

Will