runtime: fix errno sign for epollctl on mips, mips64 and ppc64

The caller of epollctl expects it to return a negative errno value,
but it returns a positive errno value on mips, mips64 and ppc64.
The change fixes this.

Updates #23446

Change-Id: Ie6372eca6c23de21964caaaa433c9a45ef93531e
Reviewed-on: https://go-review.googlesource.com/89235Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/export_linux_test.go

@@ -6,5 +6,11 @@
 
 package runtime
 
+import "unsafe"
+
 var NewOSProc0 = newosproc0
 var Mincore = mincore
+
+func Epollctl(epfd, op, fd int32, ev unsafe.Pointer) int32 {
+	return epollctl(epfd, op, fd, (*epollevent)(ev))
+}

src/runtime/runtime_linux_test.go

@@ -52,3 +52,12 @@ func TestMincoreErrorSign(t *testing.T) {
 		t.Errorf("mincore = %v, want %v", v, -EINVAL)
 	}
 }
+
+func TestEpollctlErrorSign(t *testing.T) {
+	v := Epollctl(-1, 1, -1, unsafe.Pointer(&struct{}{}))
+
+	const EBADF = 0x09
+	if v != -EBADF {
+		t.Errorf("epollctl = %v, want %v", v, -EBADF)
+	}
+}

src/runtime/sys_linux_mips64x.s

@@ -410,6 +410,7 @@ TEXT runtime·epollctl(SB),NOSPLIT|NOFRAME,$0
 	MOVV	ev+16(FP), R7
 	MOVV	$SYS_epoll_ctl, R2
 	SYSCALL
+	SUBVU	R2, R0, R2	// caller expects negative errno
 	MOVW	R2, ret+24(FP)
 	RET
 

src/runtime/sys_linux_mipsx.s

@@ -444,6 +444,7 @@ TEXT runtime·epollctl(SB),NOSPLIT,$0-20
 	MOVW	ev+12(FP), R7
 	MOVW	$SYS_epoll_ctl, R2
 	SYSCALL
+	SUBU	R2, R0, R2	// caller expects negative errno
 	MOVW	R2, ret+16(FP)
 	RET
 

src/runtime/sys_linux_ppc64x.s

@@ -496,6 +496,7 @@ TEXT runtime·epollctl(SB),NOSPLIT|NOFRAME,$0
 	MOVW	fd+8(FP), R5
 	MOVD	ev+16(FP), R6
 	SYSCALL	$SYS_epoll_ctl
+	NEG	R3	// caller expects negative errno
 	MOVW	R3, ret+24(FP)
 	RET