linux-kernel - Re: [RFC] xfs: fake fallocate success for always CoW inodes

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] [day] [month] [year] [list]

Message-ID: <lhu1pm6yzjv.fsf@oldenburg.str.redhat.com>
Date: Mon, 10 Nov 2025 11:03:48 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Christoph Hellwig <hch@....de>
Cc: Dave Chinner <david@...morbit.com>,  Matthew Wilcox
 <willy@...radead.org>,  Hans Holmberg <hans.holmberg@....com>,
  linux-xfs@...r.kernel.org,  Carlos Maiolino <cem@...nel.org>,  "Darrick J
 . Wong" <djwong@...nel.org>,  linux-fsdevel@...r.kernel.org,
  linux-kernel@...r.kernel.org,  libc-alpha@...rceware.org
Subject: Re: [RFC] xfs: fake fallocate success for always CoW inodes

* Christoph Hellwig:

> On Mon, Nov 10, 2025 at 06:27:41AM +0100, Florian Weimer wrote:
>> Sorry, I made the example confusing.
>> 
>> How would the application deal with failure due to lack of fallocate
>> support?  It would have to do a pwrite, like posix_fallocate does to
>> today, or maybe ftruncate.  This is way I think removing the fallback
>> from posix_fallocate completely is mostly pointless.
>
> In general it would ftruncate.  If it thinks it can't work without
> preallocation at all the application will fail, as again the lack
> of posix_fallocate means that space can't be preallocated.

Hmm.  It's not a 1:1 replacement: someone really needs to understand the
code and see what the appropriate way to deal with the situation is.  Of
course the posix_fallocate fallback path (or an application-level
equivalent) has the potential for data loss, too.  It's just a different
trade-off.

Thanks,
Florian