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From:   Trond Myklebust <trondmy@hammerspace.com>
To:     Alexander Viro <viro@ZenIV.linux.org.uk>
CC:     Neil Brown <neilb@suse.de>, Jeffrey Layton <jlayton@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Dave Wysochanski <dwysocha@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nfs <linux-nfs@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: allowing for a completely cached umount(2) pathwalk
Thread-Topic: allowing for a completely cached umount(2) pathwalk
Thread-Index: AQHZblNqsf7f4dFF3kOaw1n+rBD0gK8p1WaAgABDqACAAAytAIAABkkAgAAEJIA=
Date:   Fri, 14 Apr 2023 04:06:03 +0000
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> On Apr 13, 2023, at 23:51, Al Viro <viro@ZenIV.linux.org.uk> wrote:
>=20
> On Fri, Apr 14, 2023 at 03:28:45AM +0000, Trond Myklebust wrote:
>=20
>> We already have support for directory file descriptors when mounting wit=
h move_mount(). Why not add a umountat() with similar support for the unmou=
nt side?
>> Then add a syscall to allow users with (e.g.) the CAP_DAC_OVERRIDE privi=
lege to convert the mount-id into an O_PATH file descriptor.
>=20
> You can already do umount -l /proc/self/fd/69 if you have a descriptor.
> Converting mount-id to O_PATH... might be an interesting idea.

A dedicated umountat() might avoid the need for the lazy flag, if it were a=
llowed to close the descriptor on success for the special case of an empty =
path.

Looking more closely, it would seem that CAP_DAC_READ_SEARCH might be a suf=
ficient privilege requirement for the mount-id -> O_PATH syscall.

_________________________________
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com