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Date:   Tue, 24 Sep 2019 15:51:04 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Josef Bacik <josef@toxicpanda.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        "zhengbin (A)" <zhengbin13@huawei.com>, Jan Kara <jack@suse.cz>,
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        "Yan, Zheng" <zyan@redhat.com>, linux-cifs@vger.kernel.org,
        Steve French <sfrench@us.ibm.com>
Subject: Re: [PATCH] Re: Possible FS race condition between iterate_dir and
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On Tue, Sep 24, 2019 at 09:30:26AM -0400, Josef Bacik wrote:

> > We pass next->d_name.name to dir_emit() (i.e. potentially to
> > copy_to_user()).  And we have no warranty that it's not a long
> > (== separately allocated) name, that will be freed while
> > copy_to_user() is in progress.  Sure, it'll get an RCU delay
> > before freeing, but that doesn't help us at all.
> > 
> > I'm not familiar with those areas in btrfs or cifs; could somebody
> > explain what's going on there and can we indeed end up finding aliases
> > to those suckers?
> 
> We can't for the btrfs case.  This is used for the case where we have a link to
> a subvolume but the root has disappeared already, so we add in that dummy inode.
> We completely drop the dcache from that root downards when we drop the
> subvolume, so we're not going to find aliases underneath those things.  Is that
> what you're asking?  Thanks,

Umm...  Completely drop, you say?  What happens if something had been opened
in there at that time?

Could you give a bit more background?  How much of that subvolume remains
and what does btrfs_lookup() have to work with?