linux-kernel - Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

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Date:   Fri, 6 Mar 2020 21:11:43 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Ian Kent <raven@...maw.net>, David Howells <dhowells@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

On Fri, Mar 6, 2020 at 9:05 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Fri, Mar 06, 2020 at 07:58:23PM +0000, Al Viro wrote:
> > On Fri, Mar 06, 2020 at 07:43:22PM +0000, Al Viro wrote:
> > > On Fri, Mar 06, 2020 at 05:25:49PM +0100, Miklos Szeredi wrote:
> > > > On Tue, Mar 03, 2020 at 08:46:09AM +0100, Miklos Szeredi wrote:
> > > > >
> > > > > I'm doing a patch.   Let's see how it fares in the face of all these
> > > > > preconceptions.
> > > >
> > > > Here's a first cut.  Doesn't yet have superblock info, just mount info.
> > > > Probably has rough edges, but appears to work.
> > >
> > > For starters, you have just made namespace_sem held over copy_to_user().
> > > This is not going to fly.
> >
> > In case if the above is too terse: you grab your mutex while under
> > namespace_sem (see attach_recursive_mnt()); the same mutex is held
> > while calling dir_emit().  Which can (and normally does) copy data
> > to userland-supplied buffer.
> >
> > NAK for that reason alone, and to be honest I had been too busy
> > suppressing the gag reflex to read and comment any deeper.
> >
> > I really hate that approach, in case it's not clear from the above.
> > To the degree that I don't trust myself to filter out the obscenities
> > if I try to comment on it right now.
> >
> > The only blocking thing we can afford under namespace_sem is GFP_KERNEL
> > allocation.
>
> Incidentally, attach_recursive_mnt() only gets you the root(s) of
> attached tree(s); try mount --rbind and see how much you've missed.

Okay.  Both trivially fixable:

 - the dir_emit() can be taken out from under the mutex and the rb
tree search repeated for every entry; possibly not as efficient, but I
guess at this point that's irrelevant

 - addition of the mountfs entry moved to the right places

Thanks,
Miklos