[4/5] watch_queue: Implement mount topology and attribute change notifications [ver #2]

Add a mount notification facility whereby notifications about changes in
mount topology and configuration can be received.  Note that this only
covers vfsmount topology changes and not superblock events.  A separate
facility will be added for that.

Firstly, a watch queue needs to be created:

	pipe2(fds, O_NOTIFICATION_PIPE);
	ioctl(fds[1], IOC_WATCH_QUEUE_SET_SIZE, 256);

then a notification can be set up to report notifications via that queue:

	struct watch_notification_filter filter = {
		.nr_filters = 1,
		.filters = {
			[0] = {
				.type = WATCH_TYPE_MOUNT_NOTIFY,
				.subtype_filter[0] = UINT_MAX,
			},
		},
	};
	ioctl(fds[1], IOC_WATCH_QUEUE_SET_FILTER, &filter);
	watch_mount(AT_FDCWD, "/", 0, fds[1], 0x02);

In this case, it would let me monitor the mount topology subtree rooted at
"/" for events.  Mount notifications propagate up the tree towards the
root, so a watch will catch all of the events happening in the subtree
rooted at the watch.

After setting the watch, records will be placed into the queue when, for
example, as superblock switches between read-write and read-only.  Records
are of the following format:

	struct mount_notification {
		struct watch_notification watch;
		__u64	triggered_on;
		__u64	auxiliary_mount;
	} *n;

Where:

	n->watch.type will be WATCH_TYPE_MOUNT_NOTIFY.

	n->watch.subtype will indicate the type of event, such as
	NOTIFY_MOUNT_NEW_MOUNT.

	n->watch.info & WATCH_INFO_LENGTH will indicate the length of the
	record.

	n->watch.info & WATCH_INFO_ID will be the fifth argument to
	watch_mount(), shifted.

	n->watch.info & NOTIFY_MOUNT_IN_SUBTREE if true indicates that the
	notification was generated in the mount subtree rooted at the
	watch, and not actually in the watch itself.

	n->watch.info & NOTIFY_MOUNT_IS_RECURSIVE if true indicates that
	the notification was generated by an event (eg. SETATTR) that was
	applied recursively.  The notification is only generated for the
	object that initially triggered it.

	n->watch.info & NOTIFY_MOUNT_IS_NOW_RO will be used for
	NOTIFY_MOUNT_READONLY, being set if the mount becomes R/O, and
	being cleared otherwise, and for NOTIFY_MOUNT_NEW_MOUNT, being set
	if the new mount is readonly.

	n->watch.info & NOTIFY_MOUNT_IS_SUBMOUNT if true indicates that the
	NOTIFY_MOUNT_NEW_MOUNT notification is in response to a mount
	performed by the kernel (e.g. an automount).

	n->triggered_on indicates the ID of the mount to which the change
	was accounted (e.g. the new parent of a new mount).

	n->axiliary_mount indicates the ID of an additional mount that was
	affected (e.g. a new mount itself) or 0.

Note that it is permissible for event records to be of variable length -
or, at least, the length may be dependent on the subtype.  Note also that
the queue can be shared between multiple notifications of various types.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 Documentation/watch_queue.rst               |   12 +
 arch/alpha/kernel/syscalls/syscall.tbl      |    1 
 arch/arm/tools/syscall.tbl                  |    1 
 arch/arm64/include/asm/unistd.h             |    2 
 arch/arm64/include/asm/unistd32.h           |    2 
 arch/ia64/kernel/syscalls/syscall.tbl       |    1 
 arch/m68k/kernel/syscalls/syscall.tbl       |    1 
 arch/microblaze/kernel/syscalls/syscall.tbl |    1 
 arch/mips/kernel/syscalls/syscall_n32.tbl   |    1 
 arch/mips/kernel/syscalls/syscall_n64.tbl   |    1 
 arch/mips/kernel/syscalls/syscall_o32.tbl   |    1 
 arch/parisc/kernel/syscalls/syscall.tbl     |    1 
 arch/powerpc/kernel/syscalls/syscall.tbl    |    1 
 arch/s390/kernel/syscalls/syscall.tbl       |    1 
 arch/sh/kernel/syscalls/syscall.tbl         |    1 
 arch/sparc/kernel/syscalls/syscall.tbl      |    1 
 arch/x86/entry/syscalls/syscall_32.tbl      |    1 
 arch/x86/entry/syscalls/syscall_64.tbl      |    1 
 arch/xtensa/kernel/syscalls/syscall.tbl     |    1 
 fs/Kconfig                                  |    9 +
 fs/Makefile                                 |    1 
 fs/mount.h                                  |   18 ++
 fs/mount_notify.c                           |  222 +++++++++++++++++++++++++++
 fs/namespace.c                              |   22 +++
 include/linux/dcache.h                      |    1 
 include/linux/syscalls.h                    |    2 
 include/uapi/asm-generic/unistd.h           |    4 
 include/uapi/linux/watch_queue.h            |   31 ++++
 kernel/sys_ni.c                             |    3 
 29 files changed, 341 insertions(+), 4 deletions(-)
 create mode 100644 fs/mount_notify.c

Message ID	159646001456.1779777.5833836537798006352.stgit@warthog.procyon.org.uk (mailing list archive)
State	New, archived
Headers	show Return-Path: <SRS0=LTup=BN=vger.kernel.org=linux-fsdevel-owner@kernel.org> Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 Subject: [PATCH 4/5] watch_queue: Implement mount topology and attribute change notifications [ver #2] From: David Howells <dhowells@redhat.com> To: viro@zeniv.linux.org.uk Cc: dhowells@redhat.com, torvalds@linux-foundation.org, casey@schaufler-ca.com, sds@tycho.nsa.gov, nicolas.dichtel@6wind.com, raven@themaw.net, christian@brauner.io, jlayton@redhat.com, kzak@redhat.com, mszeredi@redhat.com, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org Date: Mon, 03 Aug 2020 14:06:54 +0100 Message-ID: <159646001456.1779777.5833836537798006352.stgit@warthog.procyon.org.uk> In-Reply-To: <159645997768.1779777.8286723139418624756.stgit@warthog.procyon.org.uk> References: <159645997768.1779777.8286723139418624756.stgit@warthog.procyon.org.uk> User-Agent: StGit/0.23 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk
Series	Mount notifications [ver #2] \| expand [0/5] Mount notifications [ver #2] [1/5] watch_queue: Limit the number of watches a user can hold [ver #2] [2/5] watch_queue: Make watch_sizeof() check record size [ver #2] [3/5] watch_queue: Add security hooks to rule on setting mount watches [ver #2] [4/5] watch_queue: Implement mount topology and attribute change notifications [ver #2] [5/5] watch_queue: sample: Display mount tree change notifications [ver #2]

[4/5] watch_queue: Implement mount topology and attribute change notifications [ver #2]

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