signal: serialize signalfd mask update against signal-dispatch read

set_mask() wrote _mask without any lock, while the signal-dispatch path reads
it via watches() while holding registry_lock. A concurrent signalfd() mask
update racing a signal delivery could read a torn/stale sigset_t and mis-route
a single signal. Take registry_lock in set_mask() so the update is serialized
against the dispatch-time reads (matching the lock the read side already holds).

Author: gburd

libc/signalfd.cc

@@ -66,7 +66,12 @@ class signal_fd final : public special_file {
         poll_wake(this, POLLIN);
     }
 
-    void set_mask(const sigset_t& mask) { _mask = mask; }
+    // Update the watched-signal set.  Taken under registry_lock (the same lock
+    // held while the signal-dispatch path calls watches()), so a concurrent
+    // signalfd() mask update cannot race a delivery's read of _mask.
+    void set_mask(const sigset_t& mask) {
+        WITH_LOCK(registry_lock) { _mask = mask; }
+    }
 
 private:
     mutable mutex _mutex;