linux-kernel - Re: [PATCH v7 2/3] statmount: accept fd as a parameter

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Message-ID: <CANaxB-zRh1=Azkx3fO4tmUAwzjQVE=-0=PeTN9aM1MxK1UzM+Q@mail.gmail.com>
Date: Tue, 2 Dec 2025 07:52:15 -0800
From: Andrei Vagin <avagin@...il.com>
To: Bhavik Sachdev <b.sachdev1904@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Shuah Khan <shuah@...nel.org>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	criu@...ts.linux.dev, Jan Kara <jack@...e.cz>, Jeff Layton <jlayton@...nel.org>, 
	Aleksa Sarai <cyphar@...har.com>, Miklos Szeredi <miklos@...redi.hu>, 
	Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, Alexander Mikhalitsyn <alexander@...alicyn.com>, 
	John Hubbard <jhubbard@...dia.com>, Amir Goldstein <amir73il@...il.com>, 
	"Martin K . Petersen" <martin.petersen@...cle.com>, Andrew Donnellan <ajd@...ux.ibm.com>
Subject: Re: [PATCH v7 2/3] statmount: accept fd as a parameter

On Sat, Nov 29, 2025 at 1:15 AM Bhavik Sachdev <b.sachdev1904@...il.com> wrote:
>
> Extend `struct mnt_id_req` to take in a fd and introduce STATMOUNT_BY_FD
> flag. When a valid fd is provided and STATMOUNT_BY_FD is set, statmount
> will return mountinfo about the mount the fd is on.
>
> This even works for "unmounted" mounts (mounts that have been umounted
> using umount2(mnt, MNT_DETACH)), if you have access to a file descriptor
> on that mount. These "umounted" mounts will have no mountpoint and no
> valid mount namespace. Hence, we unset the STATMOUNT_MNT_POINT and
> STATMOUNT_MNT_NS_ID in statmount.mask for "unmounted" mounts.
>
> In case of STATMOUNT_BY_FD, given that we already have access to an fd
> on the mount, accessing mount information without a capability check
> seems fine because of the following reasons:
>
> - All fs related information is available via fstatfs() without any
>   capability check.
> - Mount information is also available via /proc/pid/mountinfo (without
>   any capability check).
> - Given that we have access to a fd on the mount which tells us that we
>   had access to the mount at some point (or someone that had access gave
>   us the fd). So, we should be able to access mount info.
>

Acked-by: Andrei Vagin <avagin@...il.com>

> Co-developed-by: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
> Signed-off-by: Bhavik Sachdev <b.sachdev1904@...il.com>
> ---
>  fs/namespace.c             | 102 ++++++++++++++++++++++++-------------
>  include/uapi/linux/mount.h |  10 +++-
>  2 files changed, 76 insertions(+), 36 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ee36d67f1ac2..73ffa1fbdad7 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -5563,31 +5563,49 @@ static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
>
>  /* locks: namespace_shared */
>  static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
> -                       struct mnt_namespace *ns)
> +                        struct file *mnt_file, struct mnt_namespace *ns)

no spaces at the start of a line, code indent should use tabs.

./scripts/checkpatch.pl can help to avoid this sort of typos.

Thanks,
Andrei