[0/5] Mount notifications [ver #2]

Message

David Howells Aug. 3, 2020, 1:06 p.m. UTC

Here's a set of patches to add notifications for mount topology events,
such as mounting, unmounting, mount expiry, mount reconfiguration.

An LSM hook is included to an LSM to rule on whether or not a mount watch
may be set on a particular path.

Why do we want mount notifications?  Whilst /proc/mounts can be polled, it
only tells you that something changed in your namespace.  To find out, you
have to trawl /proc/mounts or similar to work out what changed in 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 notification generated here directly indicates the mounts involved in
any particular event and gives an idea of what the change was.

This is combined 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
dealt with quickly.

This is of use to systemd to improve efficiency:

	https://lore.kernel.org/linux-fsdevel/[e-mail]/

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

	https://lore.kernel.org/linux-fsdevel/[e-mail]/

The kernel patches can also be found here:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=notifications-pipe-core


===================
SIGNIFICANT CHANGES
===================

 ver #2:

 (*) Make the ID fields in the mount notification 64-bits.  They're left
     referring to the mount ID here, but switched to the mount unique ID in
     the patch in fsinfo that adds that. [Requested by Miklós Szeredi]

 (*) Dropped the event counters from the mount notification message.
     [Requested by Miklós].

     This can easily be added back later as the message length can be
     increased to show it.

 (*) Moved the mount event counters over to the fsinfo patchset.


David
---
David Howells (5):
      watch_queue: Limit the number of watches a user can hold
      watch_queue: Make watch_sizeof() check record size
      watch_queue: Add security hooks to rule on setting mount watches
      watch_queue: Implement mount topology and attribute change notifications
      watch_queue: sample: Display mount tree change notifications


 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/lsm_hook_defs.h               |   3 +
 include/linux/lsm_hooks.h                   |   6 +
 include/linux/security.h                    |   8 +
 include/linux/syscalls.h                    |   2 +
 include/linux/watch_queue.h                 |   7 +-
 include/uapi/asm-generic/unistd.h           |   4 +-
 include/uapi/linux/watch_queue.h            |  31 ++-
 kernel/sys_ni.c                             |   3 +
 samples/watch_queue/watch_test.c            |  41 +++-
 security/security.c                         |   7 +
 35 files changed, 411 insertions(+), 6 deletions(-)
 create mode 100644 fs/mount_notify.c

Comments

David Howells Aug. 3, 2020, 1:22 p.m. UTC | #1

David Howells <[e-mail]> wrote:

> 
> ===================
> SIGNIFICANT CHANGES
> ===================
> 
>  ver #2:
> 
>  (*) Make the ID fields in the mount notification 64-bits.  They're left
>      referring to the mount ID here, but switched to the mount unique ID in
>      the patch in fsinfo that adds that. [Requested by Miklós Szeredi]
> 
>  (*) Dropped the event counters from the mount notification message.
>      [Requested by Miklós].
> 
>      This can easily be added back later as the message length can be
>      increased to show it.
> 
>  (*) Moved the mount event counters over to the fsinfo patchset.

Also:

 (*) Added limitations on the number of concurrent watches that can be held
     by a user.  [Requested by Linus]

 (*) Removed the unused NOTIFY_MOUNT_IS_RECURSIVE flag.  [Requested by
     Miklós]

David