linux-kernel - Re: [PATCH] Don't block writes to swap-files with ETXTBSY.

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Date:   Sat, 28 Aug 2021 09:10:15 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Christoph Hellwig" <hch@....de>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Christoph Hellwig" <hch@....de>,
        "David Howells" <dhowells@...hat.com>,
        torvalds@...ux-foundation.org, trond.myklebust@...marydata.com,
        linux-nfs@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Don't block writes to swap-files with ETXTBSY.

On Sat, 28 Aug 2021, Christoph Hellwig wrote:
> On Thu, Aug 26, 2021 at 09:57:39AM +1000, NeilBrown wrote:
> > 
> > Commit dc617f29dbe5 ("vfs: don't allow writes to swap files")
> > broke swap-over-NFS as it introduced an ETXTBSY error when NFS tries to
> > swap-out using ->direct_IO().
> > 
> > There is no sound justification for this error.  File permissions are
> > sufficient to stop non-root users from writing to a swap file, and root
> > must always be cautious not to do anything dangerous.
> > 
> > These checks effectively provide a mandatory write lock on swap, and
> > mandatory locks are not supported in Linux.
> > 
> > So remove all the checks that return ETXTBSY when attempts are made to
> > write to swap.
> 
> Swap files are not just any files and do need a mandatory write lock
> as they are part of the kernel VM and writing to them will mess up
> the kernel badly.  David Howells actually has sent various patches
> to fix swap over NFS in the last weeks.
> 
> 
There are lots of different things root can do which will mess up the
kernel badly.  The backing-store can still be changed through some other
means.
Do you have a particular threat or risk scenario other than "root might
get careless"?

Yes, I've seen David's patches.  I posted this one because I think the
original patch which broke swap-over-NFS was not just unfortunate, but
wrong.  Permissions are how we protect files, not ETXTBSY.

NeilBrown