Re: [RFC PATCH v2 0/3] security: Add PR_CAPBSET_DROP_MASK

["Serge E. Hallyn" <serge@hallyn.com>]

On Tue, Sep 29, 2026 at 09:01:57PM +0800, Jinjie Ruan wrote:
> PR_CAPBSET_DROP only affects the calling thread, so dropping capabilities
> for a whole process means one call per capability per thread.  For a
> long-lived, multi-threaded process such as gVisor's sentry this is
> stop-the-world signal delivery and costs milliseconds per sandbox on a
> many-core host.
> 
> This series adds PR_CAPBSET_DROP_MASK, which removes a 64-bit mask of
> capabilities from the whole thread group in a single call.  The drop is
> recorded per thread group and folded into the bounding set wherever it
> gates gaining a capability, so already-running, concurrently-created and
> later-created threads -- as well as children forked by a sibling -- all
> observe it.
> 
> Trimming 41 capabilities in an arm64 KVM guest goes from ~8.5-23.6ms with
> the per-thread loop to ~11-14us, independent of the thread count.
> 
> Changes in RFC v2:
> - Solve concurrently clone and concurrently drop mask problem.
> - Solove sashiko problems in [1].
> - Link to RFC v1: https://lore.kernel.org/all/20260922095816.1191799-1-ruanjinjie@huawei.com/
> [1] https://sashiko.dev/#/patchset/20260922095816.1191799-1-ruanjinjie%40huawei.com

Sorry, somehow I didn't notice this version until today.  I will take
a close look later this week.  Thanks.

> Jinjie Ruan (3):
>   capability: Move mk_kernel_cap() to header
>   security: Add PR_CAPBSET_DROP_MASK for process-wide bounding-set drops
>   selftests: prctl: add process-wide bounding-set drop tests
> 
>  fs/proc/array.c                               |   3 +-
>  include/linux/capability.h                    |  10 +
>  include/linux/sched/signal.h                  |   8 +
>  include/uapi/linux/prctl.h                    |   1 +
>  kernel/capability.c                           |   5 -
>  kernel/fork.c                                 |   1 +
>  security/commoncap.c                          |  91 ++-
>  tools/testing/selftests/prctl/Makefile        |  12 +-
>  .../selftests/prctl/cap-bset-drop-test.c      | 641 ++++++++++++++++++
>  9 files changed, 759 insertions(+), 13 deletions(-)
>  create mode 100644 tools/testing/selftests/prctl/cap-bset-drop-test.c
> 
> -- 
> 2.34.1