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	Tue, 23 Dec 2025 18:59:26 +0000
Date: Tue, 23 Dec 2025 18:59:26 +0000
From: Al Viro <viro@zeniv.linux.org.uk>
To: Andrei Topala <topala.andrei@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] fs: allow rename across bind mounts on same superblock
Message-ID: <20251223185926.GD1712166@ZenIV>
References: <20251223173803.1623903-1-topala.andrei@gmail.com>
 <20251223175128.GC1712166@ZenIV>
 <CAF8SvsB0yQC7Meni=UQEehaT5YBQx2uEas8irhg3vWstdM_JVA@mail.gmail.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Tue, Dec 23, 2025 at 08:27:58PM +0200, Andrei Topala wrote:
> Rename would not work if the user doesn't have the right permissions. The
> patch calls `mnt_want_write()` on both mounts and uses each side's idmap
> for permission checks.
> 
> This would make renames faster in this case. Currently, users who have
> write permission on both mounts get EXDEV and must fall back to copying the
> data, which can be slow for large files.
> 
> I initially attempted to handle this in userspace (in coreutils mv), but
> it's more complex there. It requires parsing /proc/self/mountinfo to
> translate paths through bind mount roots, and it only benefits mv. Other
> tools that call rename() directly would hit the slow path.
> 
> Is there a specific isolation scenario where allowing rename between
> same-superblock mounts would be problematic? I'd like to understand the
> concern better.

Consider fun with moving a subdirectory of a mounted subtree to another
mounted subtree, while some process has been chdired into it, for
starters...