Dear friends,

this patch series adds support for nested seccomp listeners. It allows container
runtimes and other sandboxing software to install seccomp listeners on top of
existing ones, which is useful for nested LXC containers and other similar use-cases.

I decided to go with conservative approach and limit the maximum number of nested listeners
to 8 per seccomp filter chain (MAX_LISTENERS_PER_PATH). This is done to avoid dynamic memory
allocations in the very hot __seccomp_filter() function, where we use a preallocated static
array on the stack to track matched listeners. 8 nested listeners should be enough for
almost any practical scenarios.

Expecting potential discussions around this patch series, I'm going to present a talk
at LPC 2025 about the design and implementation details of this feature [1].

Git tree (based on for-next/seccomp):
v1: https://github.com/mihalicyn/linux/commits/seccomp.mult.listeners.v1
current: https://github.com/mihalicyn/linux/commits/seccomp.mult.listeners

Link: https://lpc.events/event/19/contributions/2241/ [1]

Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: bpf@vger.kernel.org
Cc: Kees Cook <kees@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tycho Andersen <tycho@tycho.pizza>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Stéphane Graber <stgraber@stgraber.org>

Alexander Mikhalitsyn (6):
  seccomp: remove unused argument from seccomp_do_user_notification
  seccomp: prepare seccomp_run_filters() to support more than one
    listener
  seccomp: limit number of listeners in seccomp tree
  seccomp: handle multiple listeners case
  seccomp: relax has_duplicate_listeners check
  tools/testing/selftests/seccomp: test nested listeners

 .../userspace-api/seccomp_filter.rst          |   6 +
 include/linux/seccomp.h                       |   3 +-
 include/uapi/linux/seccomp.h                  |  13 +-
 kernel/seccomp.c                              |  99 +++++++++--
 tools/include/uapi/linux/seccomp.h            |  13 +-
 tools/testing/selftests/seccomp/seccomp_bpf.c | 162 ++++++++++++++++++
 6 files changed, 269 insertions(+), 27 deletions(-)

-- 
2.43.0