With Yama ptrace_scope=2 and no CAP_SYS_PTRACE, process_vm_readv and
/proc/<pid>/mem both fail, so sc_decide cannot read path strings and every
workspace write gets EACCES. The old probe only checked the USER_NOTIF
round trip, so CST_SANDBOX=seccomp and the forced-seccomp test pass looked
available and then failed on the first write.
sc_can_peek() forks a child, places a marker on its stack, and retries the
same sc_peek path. sc_probe() now needs that too, so those hosts fall back
to none (or keep landlock) instead of a silent deny-all sandbox.
WHC
daa92a6cfb