Signal implement SYS_rt_sigtimedwait syscall 137

[doanbaotrung]

Add rt_sigtimedwait so that sigwait(3), sigwaitinfo(3), and sigtimedwait(3) work inside the guest. All three are thin wrappers over this syscall in musl and glibc; returning -ENOSYS breaks any application that uses a dedicated signal-management thread.

abi.h:
Define SYS_rt_sigtimedwait = 137, which sits between
rt_sigpending (136) and rt_sigqueueinfo (138) in the aarch64
Linux syscall table.

signal.c:
sigtimedwait_try_dequeue() - acquires sig_lock and dequeues one
signal matching the caller's mask. Thread-directed (private)
set is drained before the shared set, mirroring Linux
dequeue_signal() priority. Reuses signal_rt_dequeue_locked()
and signal_standard_peek_locked() from signal_deliver().
signal_rt_sigtimedwait() - reads and validates the guest mask
and optional timeout, then polls in 1 ms sleep chunks (same
pattern as interruptible_sleep_ns in time.c) until:
- a matching signal is consumed -> return signum
- timeout expires -> return -EAGAIN
- exit_group requested -> return -EINTR
- unblocked non-waited signal -> return -EINTR
SIGKILL/SIGSTOP are silently removed from the wait mask,
matching Linux do_sigtimedwait behavior.
NULL timeout_gva blocks indefinitely; {0,0} polls once.
info_gva = 0 skips the siginfo_t write (sigwait(3) path).

signal.h: declare signal_rt_sigtimedwait.
dispatch.tbl: register sc_rt_sigtimedwait with guest-ptr flag.
syscall.c: SC_FORWARD x0..x3 -> set, info, timeout, sigsetsize.

Fix #196

---

Summary by cubic

Implements SYS_rt_sigtimedwait (137) so sigwait(3), sigwaitinfo(3), and sigtimedwait(3) work inside the guest, matching Linux behavior and unblocking apps with a signal-manager thread.

• New Features
  • Wires up SYS_rt_sigtimedwait: defines 137, registers sc_rt_sigtimedwait, forwards args, and updates coverage mapping (sigwait, sigwaitinfo, sigtimedwait).
  • Consumes one pending signal (thread-directed before process-directed); optionally writes siginfo_t; masks SIGKILL/SIGSTOP; NULL timeout blocks, {0,0} polls once, finite waits in 1 ms chunks; returns signum on success, -EAGAIN on timeout, -EINTR on exit-group or an unblocked deliverable unrelated signal.
  • Adds tests/test-sigtimedwait.c and a Makefile target (links -lpthread) covering sigwait, sigwaitinfo with siginfo_t, zero-time poll, short timeout, and cross-thread delivery.

^{Written for commit c749b7a. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found and verified against the latest diff

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/syscall/signal.c">

<violation number="1" location="src/syscall/signal.c:1419">
P2: The saturation check uses `>` instead of `>=`, so a guest-supplied timespec with tv_sec == INT64_MAX/1000000000 and a large tv_nsec can make `tv_sec * 1000000000LL + tv_nsec` exceed INT64_MAX, causing signed overflow (UB) rather than saturating to INT64_MAX.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P2: The saturation check uses > instead of >=, so a guest-supplied timespec with tv_sec == INT64_MAX/1000000000 and a large tv_nsec can make tv_sec * 1000000000LL + tv_nsec exceed INT64_MAX, causing signed overflow (UB) rather than saturating to INT64_MAX.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/syscall/signal.c, line 1419:

<comment>The saturation check uses `>` instead of `>=`, so a guest-supplied timespec with tv_sec == INT64_MAX/1000000000 and a large tv_nsec can make `tv_sec * 1000000000LL + tv_nsec` exceed INT64_MAX, causing signed overflow (UB) rather than saturating to INT64_MAX.</comment>

<file context>
@@ -1332,7 +1332,162 @@ int64_t signal_rt_sigpending(guest_t *g, uint64_t set_gva, uint64_t sigsetsize)
+        if (lts.tv_sec > (INT64_MAX / 1000000000LL))
+            remaining_ns = INT64_MAX;
+        else
+            remaining_ns = lts.tv_sec * 1000000000LL + lts.tv_nsec;
+    }
+
</file context>

[Max042004]

signal_rt_sigtimedwait's EINTR check doesn't filter by signal disposition, so signals that should silently discard (SIG_IGN, or default-ignore/continue/stop like SIGCHLD/SIGURG/SIGWINCH/SIGCONT) spuriously wake the wait when they're not in mask

signal_pending_interruption() (signal.c:611) already implements the correct disposition filter. Suggest reusing that filtering logic here

[doanbaotrung]

Thanks for the review.

Fixed. The old check used a raw bitmask self_pending_locked() & ~blocked & ~mask which would spuriously wake the wait for signals like SIGCHLD, SIGURG, SIGWINCH, and SIGCONT — all of which are silently discarded by signal_deliver with no guest-visible effect.

Replaced it with a disposition-aware filter loop that mirrors signal_pending_interruption() exactly: a candidate signal only causes -EINTR if it has a real user-visible effect — an explicit non-SIG_IGN handler, or a SIG_DFL disposition of TERM/CORE. Signals with SIG_IGN or default IGN/CONT/STOP dispositions now correctly fall through and do not interrupt the wait.