fix(sandbox): do not gate launch confirmation on seccomp cancellation

The confirmation evidence check required seccomp.cancellation
(WAIT_KILLABLE_RECV, Linux 5.19+). On kernels older than 5.19 the sandbox
falls back to a plain listener and reports cancellation=false, so requiring it
stopped the supervisor from confirming the sandbox there ("confirmation failed:
sandbox confirmation evidence is incomplete or mismatched"). Cancellation is a
robustness property, not a containment prerequisite: containment still holds via
the listener itself. Drop it from the launch gate and update the conformance
test to assert rejection on a containment-critical primitive instead.

Signed-off-by: Akram <akram.benaissi@gmail.com>

Author: akram

crates/openshell-isolation-interface/src/contract.rs

@@ -562,7 +562,11 @@ impl SandboxConfirmEvidence {
             && self.seccomp.proc_fd_identity
             && self.seccomp.task_memory_read
             && self.seccomp.task_memory_write
-            && self.seccomp.cancellation
+            // `cancellation` (seccomp WAIT_KILLABLE_RECV, Linux 5.19+) is a
+            // robustness property, not a containment prerequisite: on older
+            // kernels (e.g. RHEL 9.x / 5.14) the sandbox falls back to a plain
+            // listener and reports cancellation=false. Containment still holds
+            // via the listener itself, so launch is not gated on cancellation.
             && self.landlock_abi >= 3
             && self.landlock_allow_deny
             && self.udp_dns_round_trip

crates/openshell-isolation-interface/tests/backend_conformance.rs

@@ -538,7 +538,11 @@ async fn one_driver_runs_both_backends() {
 #[test]
 fn confirmation_constructor_rejects_incomplete_evidence() {
     let mut evidence = confirmation_evidence();
-    evidence.seccomp.cancellation = false;
+    // A missing containment-critical primitive (the notification listener
+    // itself) must be rejected. `cancellation` is intentionally NOT in this set:
+    // it reflects WAIT_KILLABLE_RECV (Linux 5.19+) and is unavailable on older
+    // kernels, where the sandbox degrades to a plain listener.
+    evidence.seccomp.new_listener = false;
     let result = ConfirmedBoundary::try_new(
         Box::new(MockReady::<Primary> { _k: PhantomData }),
         evidence,