linux-kernel - Re: [RFC PATCH 1/1] ovl: Use fsid as unique identifier for trusted origin

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Message-ID: <20251014174032.GC13776@suse.cz>
Date: Tue, 14 Oct 2025 19:40:32 +0200
From: David Sterba <dsterba@...e.cz>
To: Qu Wenruo <wqu@...e.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	André Almeida <andrealmeid@...lia.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kernel-dev@...lia.com, Miklos Szeredi <miklos@...redi.hu>,
	Amir Goldstein <amir73il@...il.com>, Chris Mason <clm@...com>,
	David Sterba <dsterba@...e.com>, Anand Jain <anand.jain@...cle.com>,
	"Guilherme G . Piccoli" <gpiccoli@...lia.com>
Subject: Re: [RFC PATCH 1/1] ovl: Use fsid as unique identifier for trusted
 origin

On Tue, Oct 14, 2025 at 03:43:54PM +1030, Qu Wenruo wrote:
> 在 2025/10/14 15:09, Christoph Hellwig 写道:
> > On Mon, Oct 13, 2025 at 10:57:07PM -0300, André Almeida wrote:
> >> Some filesystem have non-persistent UUIDs, that can change between
> >> mounting, even if the filesystem is not modified. To prevent
> >> false-positives when mounting overlayfs with index enabled, use the fsid
> >> reported from statfs that is persistent across mounts.
> > 
> > Please fix btrfs to not change uuids, as that completely defeats the
> > point of uuids.
> > 
> 
> That is the temp-fsid feature from Anand, introduced by commit 
> a5b8a5f9f835 ("btrfs: support cloned-device mount capability").
> 
> I'm not 100% sure if it's really that important to support mounting 
> cloned devices in the first place, as LVM will reject activating any LVs 
> if there is even conflicting VGs names, not to mention conflicting UUIDs.
> 
> If temp-fsid is causing problems with overlayfs, I'm happy to remove it, 
> as this really looks like a niche that no one is asking.

What do you mean no one asking?  This was specifically asked for by
Steam to do A/B root partition mounts for recovery. It is a niche use
case but it has its users.