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Subject[RFC][PATCH 0/5] Mount, Filesystem and Keyrings notifications

Hi Al,

Here's a set of patches to add a general variable-length notification queue
concept and to add sources of events for:

(1) Mount topology and reconfiguration change events.

(2) Superblocks EIO, ENOSPC and EDQUOT events (not complete yet).

(3) Key/keyring changes events

One of the reasons for this is so that we can remove the issue of processes
having to repeatedly and regularly scan /proc/mounts, which has proven to be a
system performance problem.

Design decisions:

(1) A misc chardev is used to create and open a ring buffer:

fd = open("/dev/watch_queue", O_RDWR);

which is then configured and mmap'd into userspace:

ioctl(fd, IOC_WATCH_QUEUE_SET_FILTER, &filter);
buf = mmap(NULL, BUF_SIZE * page_size, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);

The fd cannot be read or written (though there is a facility to use write
to inject records for debugging) and userspace just pulls data directly
out of the buffer.

(2) The ring index pointers are stored inside the ring and are thus
accessible to userspace. Userspace should only update the tail pointer
and never the head pointer or risk breaking the buffer. The kernel
checks that the pointers appear valid before trying to use them. A
'skip' record is maintained around the pointers.

(3) poll() can be used to wait for data to appear in the buffer.

(4) Records in the buffer are binary, typed and have a length so that they
can be of varying size.

This means that multiple heterogeneous sources can share a common
buffer. Tags may be specified when a watchpoint is created to help
distinguish the sources.

(5) The queue is reusable as there are 16 million types available, of which
I've used 4, so there is scope for others to be used.

(6) Records are filterable as types have up to 256 subtypes that can be
individually filtered. Other filtration is also available.

(7) Each time the buffer is opened, a new buffer is created - this means that
there's no interference between watchers.

(8) When recording a notification, the kernel will not sleep, but will rather
mark a queue as overrun if there's insufficient space, thereby avoiding
userspace causing the kernel to hang.

(9) The 'watchpoint' should be specific where possible, meaning that you
specify the object that you want to watch.

(10) The buffer is created and then watchpoints are attached to it, using one

keyctl_watch_key(KEY_SPEC_SESSION_KEYRING, fd, 0x01);
mount_notify(AT_FDCWD, "/", 0, fd, 0x02);
sb_notify(AT_FDCWD, "/mnt", 0, fd, 0x03);

where in all three cases, fd indicates the queue and the number after is
a tag between 0 and 255.

(11) The watch must be removed if either the watch buffer is destroyed or the
watched object is destroyed.

Things I want to avoid:

(1) Introducing features that make the core VFS dependent on the network
stack or networking namespaces (ie. usage of netlink).

(2) Dumping all this stuff into dmesg and having a daemon that sits there
parsing the output and distributing it as this then puts the
responsibility for security into userspace and makes handling namespaces
tricky. Further, dmesg might not exist or might be inaccessible inside a

(3) Letting users see events they shouldn't be able to see.

Further things that need to be done:

(1) fsinfo() syscall needs to find superblocks by ID as well as by path so
that it can query a superblock for information without the need to try
and work out how to reach it - if the calling process even can.

(2) A mount_info() syscall is needed that can enumerate all the children of a
mount. This is necessary because mountpoints can hide each other by
stacking, so paths are not unique keys. This will require the ability to
look up a mount by ID. This avoids the need to parse /proc/mounts.

(3) A keyctl call is needed to allow a watch on a keyring to be extended to
"children" of that keyring, such that the watch is removed from the child
if it is unlinked from the keyring.

(4) A global superblock event queue maybe?

(5) Propagating watches to child superblock over automounts?

The patches can be found here also:

David Howells (5):
General notification queue with user mmap()'able ring buffer
KEYS: Add a notification facility
vfs: Add a mount-notification facility
vfs: Add superblock notifications
Add sample notification program

Documentation/security/keys/core.rst | 59 ++
Documentation/watch_queue.rst | 305 ++++++++++++
arch/x86/entry/syscalls/syscall_32.tbl | 2
arch/x86/entry/syscalls/syscall_64.tbl | 2
drivers/misc/Kconfig | 9
drivers/misc/Makefile | 1
drivers/misc/watch_queue.c | 835 ++++++++++++++++++++++++++++++++
fs/Kconfig | 21 +
fs/Makefile | 1
fs/fs_context.c | 1
fs/mount.h | 26 +
fs/mount_notify.c | 178 +++++++
fs/namespace.c | 18 +
fs/super.c | 116 ++++
include/linux/dcache.h | 1
include/linux/fs.h | 77 +++
include/linux/key.h | 4
include/linux/syscalls.h | 4
include/linux/watch_queue.h | 87 +++
include/uapi/linux/keyctl.h | 1
include/uapi/linux/watch_queue.h | 156 ++++++
kernel/sys_ni.c | 6
mm/interval_tree.c | 2
mm/memory.c | 1
samples/Kconfig | 6
samples/Makefile | 2
samples/watch_queue/Makefile | 9
samples/watch_queue/watch_test.c | 232 +++++++++
security/keys/Kconfig | 10
security/keys/compat.c | 3
security/keys/gc.c | 5
security/keys/internal.h | 29 +
security/keys/key.c | 37 +
security/keys/keyctl.c | 90 +++
security/keys/keyring.c | 17 -
security/keys/request_key.c | 4
36 files changed, 2332 insertions(+), 25 deletions(-)
create mode 100644 Documentation/watch_queue.rst
create mode 100644 drivers/misc/watch_queue.c
create mode 100644 fs/mount_notify.c
create mode 100644 include/linux/watch_queue.h
create mode 100644 include/uapi/linux/watch_queue.h
create mode 100644 samples/watch_queue/Makefile
create mode 100644 samples/watch_queue/watch_test.c

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