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0e5032237e
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. Co-developed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com> Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com> Signed-off-by: Bhavik Sachdev <b.sachdev1904@gmail.com> Link: https://patch.msgid.link/20251129091455.757724-3-b.sachdev1904@gmail.com Acked-by: Andrei Vagin <avagin@gmail.com> Signed-off-by: Christian Brauner <brauner@kernel.org>