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Date: Tue, 21 Apr 2026 23:35:38 -0700
From: "Darrick J. Wong" <djwong@kernel.org>
To: fuse-devel <fuse-devel@lists.linux.dev>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Bernd Schubert <bernd@bsbernd.com>
Subject: libfuse accepts subtypes with slashes in them??
Message-ID: <20260422063538.GD7739@frogsfrogsfrogs>
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Hey everyone,

So today I noticed that this happens on Debian 13:

$ mount -t fuse./bin/ls /root/ /etc/shadow
ls: cannot access 'rw,dev,suid': No such file or directory
-rw-r----- 1 root 1345 Nov 18 10:38 /etc/shadow

ls: cannot open directory '/root': Permission denied

Uh... is that intentional?  It looks as though mount does:

execve("/sbin/mount.fuse", ["/sbin/mount.fuse", "/root", "/etc/shadow", "-o", "rw", "-t", "fuse./bin/ls"]...)

and then mount.fuse just trims off everything after the "fuse." and
does:

execve("/bin/sh", ["/bin/sh", "-c", "'/bin/ls' '/root' '/etc/shadow' '-o' 'rw,dev,suid'"]...)

with the odd output that you see above.  Is that supposed to happen?
Or should libfuse reject subtypes with path separators?

--D