linux-kernel - Re: [REGRESSION] loopback block device on top of block devices don't work anymore

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Message-ID: <1182267c-d291-47bc-8e5f-2e11aa93421b@kernel.dk>
Date: Wed, 27 Aug 2025 13:55:28 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Lennart Poettering <mzxreary@...inter.de>, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, Rajeev Mishra <rajeevm@....com>
Subject: Re: [REGRESSION] loopback block device on top of block devices don't
 work anymore

On 8/27/25 1:30 PM, Lennart Poettering wrote:
> Heya!
> 
> Recent kernels fail if it is attempted to create a loopback block
> device on top of a block device (rather than a regular file), which is a
> feature long supported on Linux kernels, and that systemd relies on
> (specifically, systemd-repart does).
> 
> For example, this used to work:
> 
> losetup --find --show --offset=4096 --sizelimit=409600 /dev/nvme0n1
> 
> But now it doesn't anymore...
> 
> This is on 6.17rc3. My educated guess is that this is caused by
> Rajeev's 8aa5a3b68ad144da49a3d17f165e6561255e3529, which tightened the
> screws on validating the backing file's size, which now fails if the
> backing file isn't actually a file, but a block device.
> 
> (But I didn't spend more time tracking this down.)

Yeah sorry about that, the problem is the followup to the commit
you mentioned, and the fix for that is here:

https://web.git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/commit/?h=block-6.17&id=d14469ed7c00314fe8957b2841bda329e4eaf4ab

and will land in -rc4.

-- 
Jens Axboe