From 0e5032237ee5530147fbdf33134297e1490d5ec3 Mon Sep 17 00:00:00 2001 From: Bhavik Sachdev Date: Sat, 29 Nov 2025 14:41:21 +0530 Subject: statmount: accept fd as a parameter 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 Signed-off-by: Pavel Tikhomirov Signed-off-by: Bhavik Sachdev Link: https://patch.msgid.link/20251129091455.757724-3-b.sachdev1904@gmail.com Acked-by: Andrei Vagin Signed-off-by: Christian Brauner --- include/uapi/linux/mount.h | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) (limited to 'include/uapi/linux') diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h index 5d3f8c9e3a62..18c624405268 100644 --- a/include/uapi/linux/mount.h +++ b/include/uapi/linux/mount.h @@ -197,7 +197,10 @@ struct statmount { */ struct mnt_id_req { __u32 size; - __u32 mnt_ns_fd; + union { + __u32 mnt_ns_fd; + __u32 mnt_fd; + }; __u64 mnt_id; __u64 param; __u64 mnt_ns_id; @@ -232,4 +235,9 @@ struct mnt_id_req { #define LSMT_ROOT 0xffffffffffffffff /* root mount */ #define LISTMOUNT_REVERSE (1 << 0) /* List later mounts first */ +/* + * @flag bits for statmount(2) + */ +#define STATMOUNT_BY_FD 0x00000001U /* want mountinfo for given fd */ + #endif /* _UAPI_LINUX_MOUNT_H */ -- cgit v1.2.3