linux-kernel - Re: [PATCH 01/10] fs: turn inode ctime fields into a single ktime

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Message-ID: <958080f6de517cf9d0a1994e3ca500f23599ca33.camel@kernel.org>
Date: Tue, 02 Jul 2024 07:44:19 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Jan Kara <jack@...e.cz>
Cc: Christoph Hellwig <hch@...radead.org>, "Darrick J. Wong"
 <djwong@...nel.org>,  Alexander Viro <viro@...iv.linux.org.uk>, Christian
 Brauner <brauner@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Masami
 Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Chandan Babu R <chandan.babu@...cle.com>,
 Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>,
 Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>, David Sterba
 <dsterba@...e.com>,  Hugh Dickins <hughd@...gle.com>, Andrew Morton
 <akpm@...ux-foundation.org>, kernel-team@...com, 
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-trace-kernel@...r.kernel.org, linux-xfs@...r.kernel.org, 
 linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org,
 linux-mm@...ck.org,  linux-nfs@...r.kernel.org
Subject: Re: [PATCH 01/10] fs: turn inode ctime fields into a single ktime_t

On Tue, 2024-07-02 at 12:19 +0200, Jan Kara wrote:
> On Tue 02-07-24 05:56:37, Jeff Layton wrote:
> > On Tue, 2024-07-02 at 00:37 -0700, Christoph Hellwig wrote:
> > > On Mon, Jul 01, 2024 at 08:22:07PM -0400, Jeff Layton wrote:
> > > > 2) the filesystem has been altered (fuzzing? deliberate doctoring?).
> > > > 
> > > > None of these seem like legitimate use cases so I'm arguing that we
> > > > shouldn't worry about them.
> > > 
> > > Not worry seems like the wrong answer here.  Either we decide they
> > > are legitimate enough and we preserve them, or we decide they are
> > > bogus and refuse reading the inode.  But we'll need to consciously
> > > deal with the case.
> > > 
> > 
> > Is there a problem with consciously dealing with it by clamping the
> > time at KTIME_MAX? If I had a fs with corrupt timestamps, the last
> > thing I'd want is the filesystem refusing to let me at my data because
> > of them.
> 
> Well, you could also view it differently: If I have a fs that corrupts time
> stamps, the last thing I'd like is that the kernel silently accepts it
> without telling me about it :)
> 

Fair enough.

> But more seriously, my filesystem development experience shows that if the
> kernel silently tries to accept and fixup the breakage, it is nice in the
> short term (no complaining users) but it tends to get ugly in the long term
> (where tend people come up with nasty cases where it was wrong to fix it
> up). So I think Christoph's idea of refusing to load inodes with ctimes out
> of range makes sense. Or at least complain about it if nothing else (which
> has some precedens in the year 2038 problem).

Complaining about it is fairly simple. We could just throw a pr_warn in
inode_set_ctime_to_ts when the time comes back as KTIME_MAX. This might
also be what we need to do for filesystems like NFS, where a future
ctime on the server is not necessarily a problem for the client.

Refusing to load the inode on disk-based filesystems is harder, but is
probably possible. There are ~90 calls to inode_set_ctime_to_ts in the
kernel, so we'd need to vet the places that are loading times from disk
images or the like and fix them to return errors in this situation.

Is warning acceptable, or do we really need to reject inodes that have
corrupt timestamps like this?
-- 
Jeff Layton <jlayton@...nel.org>