Fix signalfd polling.

The signalfd descriptors otherwise always show as available. This can lead
programs to spin, assuming they are looking to see what signals are pending.

Updates #139

PiperOrigin-RevId: 274017890

2 files changed, 21 insertions, 1 deletions

diff --git a/pkg/sentry/kernel/signalfd/signalfd.go b/pkg/sentry/kernel/signalfd/signalfd.go
index 06fd5ec88..4b08d7d72 100644
--- a/pkg/sentry/kernel/signalfd/signalfd.go
+++ b/pkg/sentry/kernel/signalfd/signalfd.go
@@ -121,7 +121,10 @@ func (s *SignalOperations) Read(ctx context.Context, _ *fs.File, dst usermem.IOS
 
 // Readiness implements waiter.Waitable.Readiness.
 func (s *SignalOperations) Readiness(mask waiter.EventMask) waiter.EventMask {
-	return mask & waiter.EventIn
+	if mask&waiter.EventIn != 0 && s.target.PendingSignals()&s.Mask() != 0 {
+		return waiter.EventIn // Pending signals.
+	}
+	return 0
 }
 
 // EventRegister implements waiter.Waitable.EventRegister.
diff --git a/test/syscalls/linux/signalfd.cc b/test/syscalls/linux/signalfd.cc
index 54c598627..9379d5878 100644
--- a/test/syscalls/linux/signalfd.cc
+++ b/test/syscalls/linux/signalfd.cc
@@ -312,6 +312,23 @@ TEST(Signalfd, KillStillKills) {
   EXPECT_EXIT(tgkill(getpid(), gettid(), SIGKILL), KilledBySignal(SIGKILL), "");
 }
 
+TEST(Signalfd, Ppoll) {
+  sigset_t mask;
+  sigemptyset(&mask);
+  sigaddset(&mask, SIGKILL);
+  FileDescriptor fd =
+      ASSERT_NO_ERRNO_AND_VALUE(NewSignalFD(&mask, SFD_CLOEXEC));
+
+  // Ensure that the given ppoll blocks.
+  struct pollfd pfd = {};
+  pfd.fd = fd.get();
+  pfd.events = POLLIN;
+  struct timespec timeout = {};
+  timeout.tv_sec = 1;
+  EXPECT_THAT(RetryEINTR(ppoll)(&pfd, 1, &timeout, &mask),
+              SyscallSucceedsWithValue(0));
+}
+
 }  // namespace
 
 }  // namespace testing