From: Cong Wang <[email protected]> Document the new opt-in flag, its listener requirement and interaction with WAIT_KILLABLE_RECV. Explain syscall re-evaluation and the cancellation, timeout, fairness and argument-mutation tradeoffs.
Assisted-by: Codex:gpt-6 Signed-off-by: Cong Wang <[email protected]> --- .../userspace-api/seccomp_filter.rst | 29 +++++++++++++++++++ 1 file changed, 29 insertions(+) diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst index cff0fa7f3175..b6875ce54fe2 100644 --- a/Documentation/userspace-api/seccomp_filter.rst +++ b/Documentation/userspace-api/seccomp_filter.rst @@ -281,6 +281,35 @@ process will ignore non-fatal signals until the response is sent. Signals that are sent prior to the notification being received by userspace are handled normally. +``SECCOMP_FILTER_FLAG_RESTART_BEFORE_RECV`` can be set at filter installation +to restart a syscall interrupted while its notification is still awaiting +receipt, even if the signal handler was installed without ``SA_RESTART``. +The handler runs, and if it returns normally, syscall entry and the seccomp +filter are evaluated again. The abandoned notification is removed; a new +notification is queued if the filter again returns ``SECCOMP_RET_USER_NOTIF``. +This avoids returning ``EINTR`` before the syscall has executed, including +for calls such as ``close`` where callers do not retry on ``EINTR``. +A failed notification receive that resets the notification to its initial +state is also eligible for restart. + +The flag requires ``SECCOMP_FILTER_FLAG_NEW_LISTENER`` and can be used with +or without ``SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV``. Using both flags allows +handlers to run before receipt and defers non-fatal signals during supervisor +processing after receipt. Without ``SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV``, +interruptions after receipt retain the existing ``SA_RESTART`` behavior. +Fatal signals still terminate the notifying process. Neither flag changes +supervisor-supplied errors or the native syscall's restart behavior after a +``SECCOMP_USER_NOTIF_FLAG_CONTINUE`` response. + +Unconditional restart before receipt is opt-in: a normally returning signal +handler can no longer cancel that wait with ``EINTR``. Native syscall timeout +and signal-mask handling have not started during mediation. Repeated signals +can therefore extend elapsed time and, as restarted notifications join the +tail of the queue, delay receipt indefinitely. Handlers can also modify memory +referenced by syscall arguments, so the restarted call must be authorized +afresh; this flag does not provide an argument snapshot or prevent TOCTOU. +Existing behavior is unchanged when the flag is absent. + It is worth noting that ``struct seccomp_data`` contains the values of register arguments to the syscall, but does not contain pointers to memory. The task's memory is accessible to suitably privileged traces via ``ptrace()`` or -- 2.43.0
