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

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Date: Tue, 2 Jul 2024 12:19:02 +0200
From: Jan Kara <jack@...e.cz>
To: Jeff Layton <jlayton@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	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 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 :)

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).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR