linux-kernel - Re: [PATCH 2/3] xfs: disallow broken ioctls without compat-32-bit-time

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Date:   Wed, 8 Jan 2020 00:57:43 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Brian Foster <bfoster@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        Allison Collins <allison.henderson@...cle.com>,
        Jan Kara <jack@...e.cz>, Eric Sandeen <sandeen@...deen.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] xfs: disallow broken ioctls without
 compat-32-bit-time

On Tue, Jan 07, 2020 at 10:16:14AM -0800, Darrick J. Wong wrote:
> Yeah.  Fixing that (and maybe adding an ioctl to set the FS UUID online)
> were on my list for 5.6 but clearly I have to defer everything until 5.7
> because we've just run out of time.
> 
> Uh... I started looking into unifying the ext4 and xfs defrag ioctl, but
> gagged when I realized that the ext4 ioctl also handles the data copy
> inside the kernel.  I think that's the sort of behavior we should /not/
> allow into the new ioctl, though that also means that the required
> changes for ext4/e4defrag will be non-trivial.

Well, we should eventually end up with a common defrag tool (e.g. in
util-linux).  We might as well start of with the xfs_fsr codebase
for that or whatever suits us best.

> The btrfs defrag ioctl also contains thresholding information and
> optional knobs to enable compression, which makes me wonder if we should
> focus narrowly on swapext being "swap these extents but only if the
> source file hasn't changed" and not necessarily defrag?

That sounds like the most useful common API.

> ...in which case I wonder, can people (ab)use this interface for atomic
> file updates?  Create an O_TMPFILE, reflink the source file into the
> temp file, make your updates to the tempfile, and then swapext the
> donor's file data back into the source file, but only if the source file
> hasn't changed?

Sure.