linux-kernel - Re: [PATCH v4] statmount: accept fd as a parameter

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Message-Id: <DDUK7IZ56YJW.1UVWZS6GIOPUW@gmail.com>
Date: Wed, 29 Oct 2025 11:03:40 +0530
From: "Bhavik Sachdev" <b.sachdev1904@...il.com>
To: "Bhavik Sachdev" <b.sachdev1904@...il.com>, "Alexander Viro"
 <viro@...iv.linux.org.uk>, "Christian Brauner" <brauner@...nel.org>
Cc: <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>, "Aleksa
 Sarai" <cyphar@...har.com>, "Pavel Tikhomirov" <ptikhomirov@...tuozzo.com>,
 "Jan Kara" <jack@...e.cz>, "John Garry" <john.g.garry@...cle.com>, "Arnaldo
 Carvalho de Melo" <acme@...hat.com>, "Darrick J . Wong"
 <djwong@...nel.org>, "Namhyung Kim" <namhyung@...nel.org>, "Ingo Molnar"
 <mingo@...nel.org>, "Andrei Vagin" <avagin@...il.com>, "Alexander
 Mikhalitsyn" <alexander@...alicyn.com>
Subject: Re: [PATCH v4] statmount: accept fd as a parameter

On Wed Oct 29, 2025 at 10:47 AM IST, Bhavik Sachdev wrote:
> Changes from v3 [2] to v4:
> * Change the string returned when there is no mountpoint to be
> "[unmounted]" instead of "[detached]".
> * Remove the new DEFINE_FREE put_file and use the one already present in
> include/linux/file.h (fput) [3].
> * Inside listmount consistently pass 0 in flags to copy_mnt_id_req and
> prepare_klistmount()->grab_requested_mnt_ns() and remove flags from the
> prepare_klistmount prototype.
> * If STATMOUNT_BY_FD is set, check for mnt_ns_id && mnt_id to be 0.
What I meant to say is that, we check that mnt_ns_id == 0 and mnt_id ==
0, when STATMOUNT_BY_FD is specified.
>
> Changes from v2 [4] to v3:
> * Rename STATMOUNT_FD flag to STATMOUNT_BY_FD.
> * Fixed UAF bug caused by the reference to fd_mount being bound by scope
> of CLASS(fd_raw, f)(kreq.fd) by using fget_raw instead.
> * Reused @spare parameter in mnt_id_req instead of adding new fields to
> the struct.
>
> Changes from v1 [5] to v2:
> v1 of this patchset, took a different approach and introduced a new
> umount_mnt_ns, to which "unmounted" mounts would be moved to (instead of
> their namespace being NULL) thus allowing them to be still available via
> statmount.
>
> Introducing umount_mnt_ns complicated namespace locking and modified
> performance sensitive code [6] and it was agreed upon that fd-based
> statmount would be better.
>
> [1]: https://github.com/checkpoint-restore/criu/pull/2754
> [2]: https://lore.kernel.org/all/20251024181443.786363-1-b.sachdev1904@gmail.com/
> [3]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/file.h#n97
> [4]: https://lore.kernel.org/linux-fsdevel/20251011124753.1820802-1-b.sachdev1904@gmail.com/
> [5]: https://lore.kernel.org/linux-fsdevel/20251002125422.203598-1-b.sachdev1904@gmail.com/
> [6]: https://lore.kernel.org/linux-fsdevel/7e4d9eb5-6dde-4c59-8ee3-358233f082d0@virtuozzo.com/