runtime: signal forwarding

Forward signals to signal handlers installed before Go installs its own,
under certain circumstances.  In particular, as iant@ suggests, signals are
forwarded iff:
   (1) a non-SIG_DFL signal handler existed before Go, and
   (2) signal is synchronous (i.e., one of SIGSEGV, SIGBUS, SIGFPE), and
   	(3a) signal occured on a non-Go thread, or
   	(3b) signal occurred on a Go thread but in CGo code.

Supported only on Linux, for now.

Change-Id: I403219ee47b26cf65da819fb86cf1ec04d3e25f5
Reviewed-on: https://go-review.googlesource.com/8712Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testsigfwd/main.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "fmt"
+
+/*
+#include <signal.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+int *p;
+static void sigsegv() {
+	*p = 1;
+	fprintf(stderr, "ERROR: C SIGSEGV not thrown on caught?.\n");
+	exit(2);
+}
+
+static void sighandler(int signum) {
+	if (signum == SIGSEGV) {
+		exit(0);  // success
+	}
+}
+
+static void __attribute__ ((constructor)) sigsetup(void) {
+	struct sigaction act;
+	act.sa_handler = &sighandler;
+	sigaction(SIGSEGV, &act, 0);
+}
+*/
+import "C"
+
+var p *byte
+
+func f() (ret bool) {
+	defer func() {
+		if recover() == nil {
+			fmt.Errorf("ERROR: couldn't raise SIGSEGV in Go.")
+			C.exit(2)
+		}
+		ret = true
+	}()
+	*p = 1
+	return false
+}
+
+func main() {
+	// Test that the signal originating in Go is handled (and recovered) by Go.
+	if !f() {
+		fmt.Errorf("couldn't recover from SIGSEGV in Go.")
+		C.exit(2)
+	}
+
+	// Test that the signal originating in C is handled by C.
+	C.sigsegv()
+}

src/cmd/dist/test.go

@@ -284,6 +284,9 @@ func (t *tester) registerTests() {
 		if t.hasBash() && t.goos != "android" && !iOS && t.gohostos != "windows" {
 			t.registerTest("cgo_errors", "../misc/cgo/errors", "./test.bash")
 		}
+		if t.gohostos == "linux" && t.extLink() {
+			t.registerTest("testsigfwd", "../misc/cgo/testsigfwd", "go", "run", "main.go")
+		}
 	}
 	if t.hasBash() && t.goos != "nacl" && t.goos != "android" && !iOS {
 		t.registerTest("doc_progs", "../doc/progs", "time", "go", "run", "run.go")

src/runtime/os_linux.go

@@ -18,6 +18,9 @@ func rt_sigaction(sig uintptr, new, old *sigactiont, size uintptr) int32
 //go:noescape
 func sigaltstack(new, old *sigaltstackt)
 
+//go:noescape
+func sigfwd(fn uintptr, sig uint32, info *siginfo, ctx unsafe.Pointer)
+
 //go:noescape
 func setitimer(mode int32, new, old *itimerval)
 

src/runtime/signal1_unix.go

@@ -11,6 +11,14 @@ const (
 	_SIG_IGN uintptr = 1
 )
 
+// Stores the signal handlers registered before Go installed its own.
+// These signal handlers will be invoked in cases where Go doesn't want to
+// handle a particular signal (e.g., signal occurred on a non-Go thread).
+// See sigfwdgo() for more information on when the signals are forwarded.
+//
+// Signal forwarding is currently available only on Linux.
+var fwdSig [_NSIG]uintptr
+
 func initsig() {
 	// _NSIG is the number of signals on this operating system.
 	// sigtable should describe what to do for all the possible signals.
@@ -25,7 +33,7 @@ func initsig() {
 		if t.flags == 0 || t.flags&_SigDefault != 0 {
 			continue
 		}
-
+		fwdSig[i] = getsig(i)
 		// For some signals, we respect an inherited SIG_IGN handler
 		// rather than insist on installing our own default handler.
 		// Even these signals can be fetched using the os/signal package.

src/runtime/signal_linux.go

@@ -4,6 +4,8 @@
 
 package runtime
 
+import "unsafe"
+
 type sigTabT struct {
 	flags int32
 	name  string
@@ -76,3 +78,53 @@ var sigtable = [...]sigTabT{
 	/* 63 */ {_SigNotify, "signal 63"},
 	/* 64 */ {_SigNotify, "signal 64"},
 }
+
+// Determines if the signal should be handled by Go and if not, forwards the
+// signal to the handler that was installed before Go's.  Returns whether the
+// signal was forwarded.
+//go:nosplit
+func sigfwdgo(sig uint32, info *siginfo, ctx unsafe.Pointer) bool {
+	g := getg()
+	c := &sigctxt{info, ctx}
+	if sig >= uint32(len(sigtable)) {
+		return false
+	}
+	fwdFn := fwdSig[sig]
+	flags := sigtable[sig].flags
+
+	// If there is no handler to forward to, no need to forward.
+	if fwdFn == _SIG_DFL {
+		return false
+	}
+	// Only forward synchronous signals.
+	if c.sigcode() == _SI_USER || flags&_SigPanic == 0 {
+		return false
+	}
+	// Determine if the signal occurred inside Go code.  We test that:
+	//   (1) we were in a goroutine (i.e., m.curg != nil), and
+	//   (2) we weren't in CGO (i.e., m.curg.syscallsp == 0).
+	if g != nil && g.m != nil && g.m.curg != nil && g.m.curg.syscallsp == 0 {
+		return false
+	}
+	// Signal not handled by Go, forward it.
+	if fwdFn != _SIG_IGN {
+		sigfwd(fwdFn, sig, info, ctx)
+	}
+	return true
+}
+
+// Continuation of the (assembly) sigtramp() logic.
+//go:nosplit
+func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
+	if sigfwdgo(sig, info, ctx) {
+		return
+	}
+	g := getg()
+	if g == nil {
+		badsignal(uintptr(sig))
+		return
+	}
+	setg(g.m.gsignal)
+	sighandler(sig, info, ctx, g)
+	setg(g)
+}

src/runtime/sys_linux_386.s

@@ -191,43 +191,25 @@ TEXT runtime·rt_sigaction(SB),NOSPLIT,$0
 	MOVL	AX, ret+16(FP)
 	RET
 
-TEXT runtime·sigtramp(SB),NOSPLIT,$44
-	get_tls(CX)
-
-	// check that g exists
-	MOVL	g(CX), DI
-	CMPL	DI, $0
-	JNE	6(PC)
-	MOVL	sig+0(FP), BX
-	MOVL	BX, 0(SP)
-	MOVL	$runtime·badsignal(SB), AX
+TEXT runtime·sigfwd(SB),NOSPLIT,$12-16
+	MOVL	sig+4(FP), AX
+	MOVL	AX, 0(SP)
+	MOVL	info+8(FP), AX
+	MOVL	AX, 4(SP)
+	MOVL	ctx+12(FP), AX
+	MOVL	AX, 8(SP)
+	MOVL	fn+0(FP), AX
 	CALL	AX
 	RET
 
-	// save g
-	MOVL	DI, 20(SP)
-
-	// g = m->gsignal
-	MOVL	g_m(DI), BX
-	MOVL	m_gsignal(BX), BX
-	MOVL	BX, g(CX)
-
-	// copy arguments for call to sighandler
+TEXT runtime·sigtramp(SB),NOSPLIT,$12
 	MOVL	sig+0(FP), BX
 	MOVL	BX, 0(SP)
 	MOVL	info+4(FP), BX
 	MOVL	BX, 4(SP)
 	MOVL	context+8(FP), BX
 	MOVL	BX, 8(SP)
-	MOVL	DI, 12(SP)
-
-	CALL	runtime·sighandler(SB)
-
-	// restore g
-	get_tls(CX)
-	MOVL	20(SP), BX
-	MOVL	BX, g(CX)
-
+	CALL	runtime·sigtrampgo(SB)
 	RET
 
 TEXT runtime·sigreturn(SB),NOSPLIT,$0

src/runtime/sys_linux_amd64.s

@@ -212,37 +212,20 @@ TEXT runtime·rt_sigaction(SB),NOSPLIT,$0-36
 	MOVL	AX, ret+32(FP)
 	RET
 
-TEXT runtime·sigtramp(SB),NOSPLIT,$64
-	get_tls(BX)
-
-	// check that g exists
-	MOVQ	g(BX), R10
-	CMPQ	R10, $0
-	JNE	5(PC)
-	MOVQ	DI, 0(SP)
-	MOVQ	$runtime·badsignal(SB), AX
+TEXT runtime·sigfwd(SB),NOSPLIT,$0-32
+	MOVQ	sig+8(FP), DI
+	MOVQ	info+16(FP), SI
+	MOVQ	ctx+24(FP), DX
+	MOVQ	fn+0(FP), AX
 	CALL	AX
 	RET
 
-	// save g
-	MOVQ	R10, 40(SP)
-
-	// g = m->gsignal
-	MOVQ	g_m(R10), AX
-	MOVQ	m_gsignal(AX), AX
-	MOVQ	AX, g(BX)
-
-	MOVQ	DI, 0(SP)
-	MOVQ	SI, 8(SP)
-	MOVQ	DX, 16(SP)
-	MOVQ	R10, 24(SP)
-
-	CALL	runtime·sighandler(SB)
-
-	// restore g
-	get_tls(BX)
-	MOVQ	40(SP), R10
-	MOVQ	R10, g(BX)
+TEXT runtime·sigtramp(SB),NOSPLIT,$24
+	MOVQ	DI, 0(SP)   // signum
+	MOVQ	SI, 8(SP)   // info
+	MOVQ	DX, 16(SP)  // ctx
+	MOVQ	$runtime·sigtrampgo(SB), AX
+	CALL AX
 	RET
 
 TEXT runtime·sigreturn(SB),NOSPLIT,$0

src/runtime/sys_linux_arm.s

@@ -327,7 +327,15 @@ TEXT runtime·sigaltstack(SB),NOSPLIT,$0
 	MOVW.HI	R8, (R8)
 	RET
 
-TEXT runtime·sigtramp(SB),NOSPLIT,$24
+TEXT runtime·sigfwd(SB),NOSPLIT,$0-16
+	MOVW	sig+4(FP), R0
+	MOVW	info+8(FP), R1
+	MOVW	ctx+12(FP), R2
+	MOVW	fn+0(FP), R11
+	BL	(R11)
+	RET
+
+TEXT runtime·sigtramp(SB),NOSPLIT,$12
 	// this might be called in external code context,
 	// where g is not set.
 	// first save R0, because runtime·load_g will clobber it
@@ -336,32 +344,10 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$24
 	CMP 	$0, R0
 	BL.NE	runtime·load_g(SB)
 
-	CMP 	$0, g
-	BNE 	4(PC)
-	// signal number is already prepared in 4(R13)
-	MOVW  	$runtime·badsignal(SB), R11
-	BL	(R11)
-	RET
-
-	// save g
-	MOVW	g, R3
-	MOVW	g, 20(R13)
-
-	// g = m->gsignal
-	MOVW	g_m(g), R8
-	MOVW	m_gsignal(R8), g
-
-	// copy arguments for call to sighandler
-	// R0 is already saved above
 	MOVW	R1, 8(R13)
 	MOVW	R2, 12(R13)
-	MOVW	R3, 16(R13)
-
-	BL	runtime·sighandler(SB)
-
-	// restore g
-	MOVW	20(R13), g
-
+	MOVW  	$runtime·sigtrampgo(SB), R11
+	BL	(R11)
 	RET
 
 TEXT runtime·rtsigprocmask(SB),NOSPLIT,$0

src/runtime/sys_linux_arm64.s

@@ -215,7 +215,15 @@ TEXT runtime·rt_sigaction(SB),NOSPLIT,$-8-36
 	MOVW	R0, ret+32(FP)
 	RET
 
-TEXT runtime·sigtramp(SB),NOSPLIT,$64
+TEXT runtime·sigfwd(SB),NOSPLIT,$0-32
+	MOVW	sig+8(FP), R0
+	MOVD	info+16(FP), R1
+	MOVD	ctx+24(FP), R2
+	MOVD	fn+0(FP), R11
+	BL	(R11)
+	RET
+
+TEXT runtime·sigtramp(SB),NOSPLIT,$24
 	// this might be called in external code context,
 	// where g is not set.
 	// first save R0, because runtime·load_g will clobber it
@@ -225,31 +233,10 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$64
 	BEQ	2(PC)
 	BL	runtime·load_g(SB)
 
-	// check that g exists
-	CMP	g, ZR
-	BNE	ok
-	MOVD	$runtime·badsignal(SB), R0
-	BL	(R0)
-	RET
-
-ok:
-	// save g
-	MOVD	g, 40(RSP)
-	MOVD	g, R6
-
-	// g = m->gsignal
-	MOVD	g_m(g), R7
-	MOVD	m_gsignal(R7), g
-
-	// R0 is already saved above
 	MOVD	R1, 16(RSP)
 	MOVD	R2, 24(RSP)
-	MOVD	R6, 32(RSP)
-
-	BL	runtime·sighandler(SB)
-
-	// restore g
-	MOVD	40(RSP), g
+	MOVD	$runtime·sigtrampgo(SB), R0
+	BL	(R0)
 	RET
 
 TEXT runtime·mmap(SB),NOSPLIT,$-8

src/runtime/sys_linux_ppc64x.s

@@ -196,6 +196,15 @@ TEXT runtime·rt_sigaction(SB),NOSPLIT,$-8-36
 	MOVW	R3, ret+32(FP)
 	RETURN
 
+TEXT runtime·sigfwd(SB),NOSPLIT,$0-32
+	MOVW	sig+8(FP), R3
+	MOVD	info+16(FP), R4
+	MOVD	ctx+24(FP), R5
+	MOVD	fn+0(FP), R31
+	MOVD	R31, CTR
+	BL	(CTR)
+	RETURN
+
 #ifdef GOARCH_ppc64le
 // ppc64le doesn't need function descriptors
 TEXT runtime·sigtramp(SB),NOSPLIT,$64
@@ -217,33 +226,12 @@ TEXT runtime·_sigtramp(SB),NOSPLIT,$64
 	BEQ	2(PC)
 	BL	runtime·load_g(SB)
 
-	// check that g exists
-	CMP	g, $0
-	BNE	6(PC)
-	MOVD	R3, 8(R1)
-	MOVD	$runtime·badsignal(SB), R31
-	MOVD	R31, CTR
-	BL	(CTR)
-	RETURN
-
-	// save g
-	MOVD	g, 40(R1)
-	MOVD	g, R6
-
-	// g = m->gsignal
-	MOVD	g_m(g), R7
-	MOVD	m_gsignal(R7), g
-
 	MOVW	R3, 8(R1)
 	MOVD	R4, 16(R1)
 	MOVD	R5, 24(R1)
-	MOVD	R6, 32(R1)
-
-	BL	runtime·sighandler(SB)
-
-	// restore g
-	MOVD	40(R1), g
-
+	MOVD	$runtime·sigtrampgo(SB), R31
+	MOVD	R31, CTR
+	BL	(CTR)
 	RETURN
 
 TEXT runtime·mmap(SB),NOSPLIT,$-8