Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP_IMMUTABLE

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Colin Walters <walters@verbum.org>
Cc: Jan Kara <jack@suse.cz>,
	linux-nvdimm@lists.01.org, Dave Chinner <david@fromorbit.com>,
	linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	linux-xfs@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP_IMMUTABLE
Date: Mon, 31 Jul 2017 11:23:52 -0700	[thread overview]
Message-ID: <20170731182352.GE4477@magnolia> (raw)
In-Reply-To: <1501522933.602272.1058529880.6C4A2D98@webmail.messagingengine.com>

On Mon, Jul 31, 2017 at 01:42:13PM -0400, Colin Walters wrote:
> 
> 
> On Mon, Jul 31, 2017, at 12:32 PM, Colin Walters wrote:
> > On Mon, Jul 31, 2017, at 12:29 PM, Dan Williams wrote:
> > > 
> > > How is S_CONTENTS_IMMUTABLE different than S_IMMUTABLE?
> > 
> > We still want the ability to make hardlinks.
> 
> Also of course, symmetrically, to unlink.   If we used S_IMMUTABLE for /etc/sudoers,
> it'd still be racy since one would have to transiently remove the flag in order
> to replace it with a new version.
> 
> Related to this topic is the fact that S_IMMUTABLE is itself mutable; I
> think once S_IMMUTABLE_CONTENTS is set, it would not be able to made
> mutable again.  
> 
> Also I just remembered that since then memfd_create() and more notably
> fcntl(F_ADD_SEALS) landed - in fact it already has flags for what we want
> here AFAICS.  Your S_IOMAP_IMMUTABLE is fcntl(F_ADD_SEALS, F_SEAL_SHRINK | F_SEAL_GROW)

I don't think F_SEAL_{SHRINK,GROW} prevents reflinking or CoW of file data,
which are two things that cannot happen under S_IOMAP_IMMUTABLE that
aren't size changes.  From the implementation it looks like shrink and
grow are only supposed to disallow changes to i_size, not i_blocks (or
the file block map).

Then again, I suppose F_SEAL_* only work on shmem, so maybe it simply
isn't defined for any other filesystem...?  e.g. it doesn't prohibit
reflink, but the only fs implementing seals doesn't support reflink.

<shrug>

Seals cannot be removed, which is too strict for the S_IOMAP_IMMUTABLE
user cases being presented.

> and mine just adds in F_SEAL_WRITE.  I think there was some discussion
> of the seals for persistent files when memfd_create() landed, but I can't
> find it offhand.

--D

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