linux-kernel - Re: [PATCH v3 05/13] generic/1226: Add atomic write test using fio crc check verifier

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aIhmG-l4nWOAzz2I@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Tue, 29 Jul 2025 11:41:39 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: John Garry <john.g.garry@...cle.com>
Cc: Zorro Lang <zlang@...hat.com>, fstests@...r.kernel.org,
        Ritesh Harjani <ritesh.list@...il.com>, djwong@...nel.org,
        tytso@....edu, linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v3 05/13] generic/1226: Add atomic write test using fio
 crc check verifier

On Mon, Jul 28, 2025 at 03:00:40PM +0100, John Garry wrote:
> On 28/07/2025 14:35, Ojaswin Mujoo wrote:
> > > We guarantee that the write is committed all-or-nothing, but do rely on
> > > userspace not issuing racing atomic writes or racing regular writes.
> > > 
> > > I can easily change this, as I mentioned, but I am not convinced that it is
> > > a must.
> > Purely from a design point of view, I feel we are breaking atomicity and
> > hence we should serialize or just stop userspace from doing this (which
> > is a bit extreme).
> 
> If you check the man page description of RWF_ATOMIC, it does not mention
> serialization. The user should conclude that usual direct IO rules apply,
> i.e. userspace is responsible for serializing.

My mental model of serialization in context of atomic writes is that if
user does 64k atomic write A followed by a parallel overlapping 64kb
atomic write B then the user might see complete A or complete B (we
don't guarantee) but not a mix of A and B.

> 
> > 
> > I know userspace should ideally not do overwriting atomic writes but if
> > it is something we are allowing (which we do) then it is
> > kernel's responsibility to ensure atomicity. Sure we can penalize them
> > by serializing the writes but not by tearing it.
> > 
> > With that reasoning, I don't think the test should accomodate for this
> > particular scenario.
> 
> I can send a patch to the community for xfs (to provide serialization), like
> I showed earlier, to get opinion.

Thanks, that would be great.

Regards,
John
> 
> Thanks,
> John
>