linux-kernel - Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP

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Date:   Mon, 31 Jul 2017 13:42:13 -0400
From:   Colin Walters <walters@...bum.org>
To:     Colin Walters <walters@...bum.org>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>, linux-nvdimm@...ts.01.org,
        Dave Chinner <david@...morbit.com>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        Jeff Moyer <jmoyer@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP_IMMUTABLE

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)
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.