futex: wake one waiter if futex_wake is called with a non-positive value

This change is needed to be compatible with the Linux kernel.

There is no glibc wrapper for the futex system call, so it is easy to
make a mistake and call syscall(__NR_futex, FUTEX_WAKE, addr) without
the fourth argument. This works on Linux, because it wakes one waiter
even if val is nonpositive.

PiperOrigin-RevId: 286494396

pkg/sentry/syscalls/linux/sys_futex.go

@@ -226,6 +226,11 @@ func Futex(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 		if mask == 0 {
 			return 0, nil, syserror.EINVAL
 		}
+		if val <= 0 {
+			// The Linux kernel wakes one waiter even if val is
+			// non-positive.
+			val = 1
+		}
 		n, err := t.Futex().Wake(t, addr, private, mask, val)
 		return uintptr(n), nil, err
 
@@ -242,6 +247,11 @@ func Futex(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 
 	case linux.FUTEX_WAKE_OP:
 		op := uint32(val3)
+		if val <= 0 {
+			// The Linux kernel wakes one waiter even if val is
+			// non-positive.
+			val = 1
+		}
 		n, err := t.Futex().WakeOp(t, addr, naddr, private, val, nreq, op)
 		return uintptr(n), nil, err
 

test/syscalls/linux/futex.cc

@@ -239,6 +239,27 @@ TEST_P(PrivateAndSharedFutexTest, Wake1_NoRandomSave) {
   EXPECT_THAT(futex_wake(IsPrivate(), &a, 1), SyscallSucceedsWithValue(1));
 }
 
+TEST_P(PrivateAndSharedFutexTest, Wake0_NoRandomSave) {
+  constexpr int kInitialValue = 1;
+  std::atomic<int> a = ATOMIC_VAR_INIT(kInitialValue);
+
+  // Prevent save/restore from interrupting futex_wait, which will cause it to
+  // return EAGAIN instead of the expected result if futex_wait is restarted
+  // after we change the value of a below.
+  DisableSave ds;
+  ScopedThread thread([&] {
+    EXPECT_THAT(futex_wait(IsPrivate(), &a, kInitialValue),
+                SyscallSucceedsWithValue(0));
+  });
+  absl::SleepFor(kWaiterStartupDelay);
+
+  // Change a so that if futex_wake happens before futex_wait, the latter
+  // returns EAGAIN instead of hanging the test.
+  a.fetch_add(1);
+  // The Linux kernel wakes one waiter even if val is 0 or negative.
+  EXPECT_THAT(futex_wake(IsPrivate(), &a, 0), SyscallSucceedsWithValue(1));
+}
+
 TEST_P(PrivateAndSharedFutexTest, WakeAll_NoRandomSave) {
   constexpr int kInitialValue = 1;
   std::atomic<int> a = ATOMIC_VAR_INIT(kInitialValue);