[GIT PULL] pipe: General notification queue

From: David Howells
Date: Tue Nov 26 2019 - 18:18:38 EST

Hi Linus,

Can you consider pulling my general notification queue patchset after
you've pulled the preparatory pipework patchset? Or should it be deferred
to the next window?

The general notification queue is built on top a pipe, splicing kernel
events into the pipe from a variety of event sources. A 'notification'
mode pipe must be linked to the desired sources before it will start
getting any events delivered through it. Event sources include:

(1) Keys/keyrings, such as linking and unlinking keys and changing their

(2) General device events (single common queue) including:

- Block layer events, such as device errors

- USB subsystem events, such as device attach/remove, device reset,
device errors.

(3) I have patches for adding superblock and mount topology watches also,
but in a separate branch as there are other dependencies.

LSM hooks are included:

(1) A set of hooks are provided that allow an LSM to rule on whether or
not a watch may be set. Each of these hooks takes a different
"watched object" parameter, so they're not really shareable. The LSM
should use current's credentials. [Wanted by SELinux & Smack]

(2) A hook is provided to allow an LSM to rule on whether or not a
particular message may be posted to a particular queue. This is given
the credentials from the event generator (which may be the system) and
the watch setter. [Wanted by Smack]

I've provided SELinux and Smack with implementations of some of these hooks.


(1) Key/keyring notifications.

If you have your kerberos tickets in a file/directory, your gnome desktop
will monitor that using something like fanotify and tell you if your
credentials cache changes.

We also have the ability to cache your kerberos tickets in the session,
user or persistent keyring so that it isn't left around on disk across a
reboot or logout. Keyrings, however, cannot currently be monitored
asynchronously, so the desktop has to poll for it - not so good on a

This source will allow the desktop to avoid the need to poll.

(2) USB notifications.

GregKH was looking for a way to do USB notifications as I was looking to
find additional sources to implement. I'm not sure how he wants to use
them, but I'll let him speak to that himself.

(3) Block notifications.

This one I was thinking that I could make something like ddrescue better
by letting it get notifications this way. This was a target of
convenience since I had a dodgy disk I was trying to rescue.

It could also potentially be used help systemd, say, detect broken
devices and avoid trying to unmount them when trying to reboot the machine.

I can drop this for now if you prefer.

(4) Mount notifications.

This one is wanted to avoid repeated trawling of /proc/mounts or similar
to work out changes to the mount object attributes and mount topology.
I'm told that the proc file holding the namespace_sem is a point of
contention, especially as the process of generating the text descriptions
of the mounts/superblocks can be quite involved.

The notifications directly indicate the mounts involved in any particular
event and what the change was. You can poll /proc/mounts, but all you
know is that something changed; you don't know what and you don't know
how and reading that file may race with multiple changed being effected.

I pair this with a new fsinfo() system call that allows, amongst other
things, the ability to retrieve in one go an { id, change counter } tuple
from all the children of a specified mount, allowing buffer overruns to
be cleaned up quickly.

It's not just Red Hat that's potentially interested in this:


(5) Superblock notifications.

This one is provided to allow systemd or the desktop to more easily
detect events such as I/O errors and EDQUOT/ENOSPC.


(1) The notification queue is built on top of a standard pipe. Messages
are effectively spliced in. The pipe is opened with a special flag:


The special flag has the same value as O_EXCL (which doesn't seem like
it will ever be applicable in this context)[?]. It is given up front
to make it a lot easier to prohibit splice and co. from accessing the

[?] Should this be done some other way? I'd rather not use up a new
O_* flag if I can avoid it - should I add a pipe3() system call

The pipe is then configured::

ioctl(fds[1], IOC_WATCH_QUEUE_SET_SIZE, queue_depth);
ioctl(fds[1], IOC_WATCH_QUEUE_SET_FILTER, &filter);

Messages are then read out of the pipe using read().

[?] Should this use fcntl() rather than ioctl()?

(2) It should be possible to allow write() to insert data into the
notification pipes too, but this is currently disabled as the kernel
has to be able to insert messages into the pipe *without* holding
pipe->mutex and the code to make this work needs careful auditing.

(3) sendfile(), splice() and vmsplice() are disabled on notification pipes
because of the pipe->mutex issue and also because they sometimes want
to revert what they just did - but one or more notification messages
might've been interleaved in the ring.

(4) The kernel inserts messages with the wait queue spinlock held. This
means that pipe_read() and pipe_write() have to take the spinlock to
update the queue pointers.

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

This allows multiple heterogeneous sources to share a common buffer;
there are 16 million types available, of which I've used just a few,
so there is scope for others to be used. Tags may be specified when a
watchpoint is created to help distinguish the sources.

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

(7) Notification pipes don't interfere with each other; each may be bound
to a different set of event sources. Any particular notification will
be copied to all the queues that are currently watching for it - and
only those that are watching for it.

(8) When recording a notification, the kernel will not sleep, but will
rather mark a queue as having lost a message if there's insufficient
space. read() will fabricate a loss notification message at an
appropriate point later.

(9) The notification pipe is created and then watchpoints are attached to
it. Examples of this include:

keyctl_watch_key(KEY_SPEC_SESSION_KEYRING, fds[1], 0x01);
watch_devices(fds[1], 0x02, 0);

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

(10) Watches are removed if either the notification pipe is destroyed or
the watched object is destroyed. In the latter case, a message will
be generated indicating the enforced watch removal.

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 container.

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


(*) The keyutils tree has a pipe-watch branch that has keyctl commands for
making use of notifications. Proposed manual pages can also be found
on this branch, though a couple of them really need to go to the main
manpages repository instead.

If the kernel supports the watching of keys, then running "make test"
on that branch will cause the testing infrastructure to spawn a
monitoring process on the side that monitors a notifications pipe for
all the key/keyring changes induced by the tests and they'll all be
checked off to make sure they happened.



(*) Fix some bits found by kbuild.

ver #2:

(*) Declare O_NOTIFICATION_PIPE to use and switch it to be the same value
as O_EXCL rather then O_TMPFILE (the latter is a bit nasty in its

ver #1:

(*) Build on top of standard pipes instead of having a driver.

The following changes since commit 3c0edea9b29f9be6c093f236f762202b30ac9431:

pipe: Remove sync on wake_ups (2019-11-15 16:22:54 +0000)

are available in the Git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git tags/notifications-pipe-core-20191115

for you to fetch changes up to 72fb99936e50f6c17490aeb445c87406a1265d52:

smack: Implement the watch_key and post_notification hooks (2019-11-15 16:23:58 +0000)

General notification queue built on pipes

David Howells (14):
uapi: General notification queue definitions
security: Add hooks to rule on setting a watch
security: Add a hook for the point of notification insertion
pipe: Add general notification queue support
keys: Add a notification facility
Add sample notification program
pipe: Allow buffers to be marked read-whole-or-error for notifications
pipe: Add notification lossage handling
Add a general, global device notification watch list
block: Add block layer notifications
usb: Add USB subsystem notifications
selinux: Implement the watch_key security hook
smack: Implement the watch_key and post_notification hooks

Documentation/ioctl/ioctl-number.rst | 1 +
Documentation/security/keys/core.rst | 58 +++
Documentation/watch_queue.rst | 385 ++++++++++++++++
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 +
block/Kconfig | 9 +
block/blk-core.c | 29 ++
drivers/base/Kconfig | 9 +
drivers/base/Makefile | 1 +
drivers/base/watch.c | 90 ++++
drivers/usb/core/Kconfig | 9 +
drivers/usb/core/devio.c | 47 ++
drivers/usb/core/hub.c | 4 +
fs/pipe.c | 242 +++++++---
fs/splice.c | 12 +-
include/linux/blkdev.h | 15 +
include/linux/device.h | 7 +
include/linux/key.h | 3 +
include/linux/lsm_audit.h | 1 +
include/linux/lsm_hooks.h | 38 ++
include/linux/pipe_fs_i.h | 27 +-
include/linux/security.h | 32 +-
include/linux/syscalls.h | 1 +
include/linux/usb.h | 18 +
include/linux/watch_queue.h | 127 ++++++
include/uapi/asm-generic/unistd.h | 4 +-
include/uapi/linux/keyctl.h | 2 +
include/uapi/linux/watch_queue.h | 158 +++++++
init/Kconfig | 12 +
kernel/Makefile | 1 +
kernel/sys_ni.c | 1 +
kernel/watch_queue.c | 658 ++++++++++++++++++++++++++++
samples/Kconfig | 7 +
samples/Makefile | 1 +
samples/watch_queue/Makefile | 7 +
samples/watch_queue/watch_test.c | 251 +++++++++++
security/keys/Kconfig | 9 +
security/keys/compat.c | 3 +
security/keys/gc.c | 5 +
security/keys/internal.h | 30 +-
security/keys/key.c | 38 +-
security/keys/keyctl.c | 99 ++++-
security/keys/keyring.c | 20 +-
security/keys/request_key.c | 4 +-
security/security.c | 23 +
security/selinux/hooks.c | 14 +
security/smack/smack_lsm.c | 82 +++-
63 files changed, 2506 insertions(+), 108 deletions(-)
create mode 100644 Documentation/watch_queue.rst
create mode 100644 drivers/base/watch.c
create mode 100644 include/linux/watch_queue.h
create mode 100644 include/uapi/linux/watch_queue.h
create mode 100644 kernel/watch_queue.c
create mode 100644 samples/watch_queue/Makefile
create mode 100644 samples/watch_queue/watch_test.c