cmd/cgo, runtime, runtime/cgo: use cgo context function

Add support for the context function set by runtime.SetCgoTraceback.
The context function was added in CL 17761, without support.
This CL is the support.

This CL has not been tested for real C code, as a working context
function for C code requires unwind support that does not seem to exist.
I wanted to get the CL out before the freeze.

I apologize for the length of this CL.  It's mostly plumbing, but
unfortunately the plumbing is processor-specific.

Change-Id: I8ce11a0de9b3dafcc29efd2649d776e93bff0e90
Reviewed-on: https://go-review.googlesource.com/22508Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/cgo/doc.go

@@ -511,7 +511,6 @@ file compiled by gcc, the file x.cgo2.c:
 	void
 	_cgo_be59f0f25121_Cfunc_puts(void *v)
 	{
-		_cgo_wait_runtime_init_done();
 		struct {
 			char* p0;
 			int r;
@@ -520,8 +519,7 @@ file compiled by gcc, the file x.cgo2.c:
 		a->r = puts((void*)a->p0);
 	}
 
-It waits for Go runtime to be initialized (required for shared libraries),
-extracts the arguments from the pointer to _Cfunc_puts's argument
+It extracts the arguments from the pointer to _Cfunc_puts's argument
 frame, invokes the system C function (in this case, puts), stores the
 result in the frame, and returns.
 
@@ -539,8 +537,8 @@ linkage to the desired libraries. The main function is provided by
 _cgo_main.c:
 
 	int main() { return 0; }
-	void crosscall2(void(*fn)(void*, int), void *a, int c) { }
-	void _cgo_wait_runtime_init_done() { }
+	void crosscall2(void(*fn)(void*, int, uintptr_t), void *a, int c, uintptr_t ctxt) { }
+	uintptr_t _cgo_wait_runtime_init_done() { }
 	void _cgo_allocate(void *a, int c) { }
 	void _cgo_panic(void *a, int c) { }
 

src/cmd/cgo/out.go

@@ -50,14 +50,16 @@ func (p *Package) writeDefs() {
 	// Write C main file for using gcc to resolve imports.
 	fmt.Fprintf(fm, "int main() { return 0; }\n")
 	if *importRuntimeCgo {
-		fmt.Fprintf(fm, "void crosscall2(void(*fn)(void*, int), void *a, int c) { }\n")
-		fmt.Fprintf(fm, "void _cgo_wait_runtime_init_done() { }\n")
+		fmt.Fprintf(fm, "void crosscall2(void(*fn)(void*, int, __SIZE_TYPE__), void *a, int c, __SIZE_TYPE__ ctxt) { }\n")
+		fmt.Fprintf(fm, "__SIZE_TYPE__ _cgo_wait_runtime_init_done() { return 0; }\n")
+		fmt.Fprintf(fm, "void _cgo_release_context(__SIZE_TYPE__ ctxt) { }\n")
 		fmt.Fprintf(fm, "char* _cgo_topofstack(void) { return (char*)0; }\n")
 	} else {
 		// If we're not importing runtime/cgo, we *are* runtime/cgo,
 		// which provides these functions. We just need a prototype.
-		fmt.Fprintf(fm, "void crosscall2(void(*fn)(void*, int), void *a, int c);\n")
-		fmt.Fprintf(fm, "void _cgo_wait_runtime_init_done();\n")
+		fmt.Fprintf(fm, "void crosscall2(void(*fn)(void*, int, __SIZE_TYPE__), void *a, int c, __SIZE_TYPE__ ctxt);\n")
+		fmt.Fprintf(fm, "__SIZE_TYPE__ _cgo_wait_runtime_init_done();\n")
+		fmt.Fprintf(fm, "void _cgo_release_context(__SIZE_TYPE__);\n")
 	}
 	fmt.Fprintf(fm, "void _cgo_allocate(void *a, int c) { }\n")
 	fmt.Fprintf(fm, "void _cgo_panic(void *a, int c) { }\n")
@@ -700,8 +702,9 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 	fmt.Fprintf(fgcc, "/* Created by cgo - DO NOT EDIT. */\n")
 	fmt.Fprintf(fgcc, "#include \"_cgo_export.h\"\n\n")
 
-	fmt.Fprintf(fgcc, "extern void crosscall2(void (*fn)(void *, int), void *, int);\n")
-	fmt.Fprintf(fgcc, "extern void _cgo_wait_runtime_init_done();\n\n")
+	fmt.Fprintf(fgcc, "extern void crosscall2(void (*fn)(void *, int, __SIZE_TYPE__), void *, int, __SIZE_TYPE__);\n")
+	fmt.Fprintf(fgcc, "extern __SIZE_TYPE__ _cgo_wait_runtime_init_done();\n")
+	fmt.Fprintf(fgcc, "extern void _cgo_release_context(__SIZE_TYPE__);\n\n")
 	fmt.Fprintf(fgcc, "%s\n", tsanProlog)
 
 	for _, exp := range p.ExpFunc {
@@ -803,10 +806,10 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		}
 		fmt.Fprintf(fgcch, "\nextern %s;\n", s)
 
-		fmt.Fprintf(fgcc, "extern void _cgoexp%s_%s(void *, int);\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fgcc, "extern void _cgoexp%s_%s(void *, int, __SIZE_TYPE__);\n", cPrefix, exp.ExpName)
 		fmt.Fprintf(fgcc, "\n%s\n", s)
 		fmt.Fprintf(fgcc, "{\n")
-		fmt.Fprintf(fgcc, "\t_cgo_wait_runtime_init_done();\n")
+		fmt.Fprintf(fgcc, "\t__SIZE_TYPE__ _cgo_ctxt = _cgo_wait_runtime_init_done();\n")
 		fmt.Fprintf(fgcc, "\t%s %v a;\n", ctype, p.packedAttribute())
 		if gccResult != "void" && (len(fntype.Results.List) > 1 || len(fntype.Results.List[0].Names) > 1) {
 			fmt.Fprintf(fgcc, "\t%s r;\n", gccResult)
@@ -819,8 +822,9 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 				fmt.Fprintf(fgcc, "\ta.p%d = p%d;\n", i, i)
 			})
 		fmt.Fprintf(fgcc, "\t_cgo_tsan_release();\n")
-		fmt.Fprintf(fgcc, "\tcrosscall2(_cgoexp%s_%s, &a, %d);\n", cPrefix, exp.ExpName, off)
+		fmt.Fprintf(fgcc, "\tcrosscall2(_cgoexp%s_%s, &a, %d, _cgo_ctxt);\n", cPrefix, exp.ExpName, off)
 		fmt.Fprintf(fgcc, "\t_cgo_tsan_acquire();\n")
+		fmt.Fprintf(fgcc, "\t_cgo_release_context(_cgo_ctxt);\n")
 		if gccResult != "void" {
 			if len(fntype.Results.List) == 1 && len(fntype.Results.List[0].Names) <= 1 {
 				fmt.Fprintf(fgcc, "\treturn a.r0;\n")
@@ -845,10 +849,10 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		fmt.Fprintf(fgo2, "//go:cgo_export_static _cgoexp%s_%s\n", cPrefix, exp.ExpName)
 		fmt.Fprintf(fgo2, "//go:nosplit\n") // no split stack, so no use of m or g
 		fmt.Fprintf(fgo2, "//go:norace\n")  // must not have race detector calls inserted
-		fmt.Fprintf(fgo2, "func _cgoexp%s_%s(a unsafe.Pointer, n int32) {\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fgo2, "func _cgoexp%s_%s(a unsafe.Pointer, n int32, ctxt uintptr) {\n", cPrefix, exp.ExpName)
 		fmt.Fprintf(fgo2, "\tfn := %s\n", goname)
 		// The indirect here is converting from a Go function pointer to a C function pointer.
-		fmt.Fprintf(fgo2, "\t_cgo_runtime_cgocallback(**(**unsafe.Pointer)(unsafe.Pointer(&fn)), a, uintptr(n));\n")
+		fmt.Fprintf(fgo2, "\t_cgo_runtime_cgocallback(**(**unsafe.Pointer)(unsafe.Pointer(&fn)), a, uintptr(n), ctxt);\n")
 		fmt.Fprintf(fgo2, "}\n")
 
 		fmt.Fprintf(fm, "int _cgoexp%s_%s;\n", cPrefix, exp.ExpName)
@@ -1337,7 +1341,7 @@ func _cgo_runtime_cgocall(unsafe.Pointer, uintptr) int32
 func _cgo_runtime_cmalloc(uintptr) unsafe.Pointer
 
 //go:linkname _cgo_runtime_cgocallback runtime.cgocallback
-func _cgo_runtime_cgocallback(unsafe.Pointer, unsafe.Pointer, uintptr)
+func _cgo_runtime_cgocallback(unsafe.Pointer, unsafe.Pointer, uintptr, uintptr)
 
 //go:linkname _cgoCheckPointer runtime.cgoCheckPointer
 func _cgoCheckPointer(interface{}, ...interface{}) interface{}
@@ -1580,5 +1584,5 @@ static void GoInit(void) {
 		runtime_iscgo = 1;
 }
 
-extern void _cgo_wait_runtime_init_done() __attribute__ ((weak));
+extern __SIZE_TYPE__ _cgo_wait_runtime_init_done() __attribute__ ((weak));
 `

src/runtime/asm_386.s

@@ -612,23 +612,25 @@ noswitch:
 	MOVL	AX, ret+8(FP)
 	RET
 
-// cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
+// cgocallback(void (*fn)(void*), void *frame, uintptr framesize, uintptr ctxt)
 // Turn the fn into a Go func (by taking its address) and call
 // cgocallback_gofunc.
-TEXT runtime·cgocallback(SB),NOSPLIT,$12-12
+TEXT runtime·cgocallback(SB),NOSPLIT,$16-16
 	LEAL	fn+0(FP), AX
 	MOVL	AX, 0(SP)
 	MOVL	frame+4(FP), AX
 	MOVL	AX, 4(SP)
 	MOVL	framesize+8(FP), AX
 	MOVL	AX, 8(SP)
+	MOVL	ctxt+12(FP), AX
+	MOVL	AX, 12(SP)
 	MOVL	$runtime·cgocallback_gofunc(SB), AX
 	CALL	AX
 	RET
 
-// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize)
+// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize, uintptr ctxt)
 // See cgocall.go for more details.
-TEXT ·cgocallback_gofunc(SB),NOSPLIT,$12-12
+TEXT ·cgocallback_gofunc(SB),NOSPLIT,$12-16
 	NO_LOCAL_POINTERS
 
 	// If g is nil, Go did not create the current thread.
@@ -696,17 +698,19 @@ havem:
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
 	//
-	// In the new goroutine, 0(SP) holds the saved oldm (DX) register.
-	// 4(SP) and 8(SP) are unused.
+	// In the new goroutine, 4(SP) holds the saved oldm (DX) register.
+	// 8(SP) is unused.
 	MOVL	m_curg(BP), SI
 	MOVL	SI, g(CX)
 	MOVL	(g_sched+gobuf_sp)(SI), DI // prepare stack as DI
 	MOVL	(g_sched+gobuf_pc)(SI), BP
 	MOVL	BP, -4(DI)
+	MOVL	ctxt+12(FP), CX
 	LEAL	-(4+12)(DI), SP
-	MOVL	DX, 0(SP)
+	MOVL	DX, 4(SP)
+	MOVL	CX, 0(SP)
 	CALL	runtime·cgocallbackg(SB)
-	MOVL	0(SP), DX
+	MOVL	4(SP), DX
 
 	// Restore g->sched (== m->curg->sched) from saved values.
 	get_tls(CX)

src/runtime/asm_amd64.s

@@ -622,23 +622,25 @@ nosave:
 	MOVL	AX, ret+16(FP)
 	RET
 
-// cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
+// cgocallback(void (*fn)(void*), void *frame, uintptr framesize, uintptr ctxt)
 // Turn the fn into a Go func (by taking its address) and call
 // cgocallback_gofunc.
-TEXT runtime·cgocallback(SB),NOSPLIT,$24-24
+TEXT runtime·cgocallback(SB),NOSPLIT,$32-32
 	LEAQ	fn+0(FP), AX
 	MOVQ	AX, 0(SP)
 	MOVQ	frame+8(FP), AX
 	MOVQ	AX, 8(SP)
 	MOVQ	framesize+16(FP), AX
 	MOVQ	AX, 16(SP)
+	MOVQ	ctxt+24(FP), AX
+	MOVQ	AX, 24(SP)
 	MOVQ	$runtime·cgocallback_gofunc(SB), AX
 	CALL	AX
 	RET
 
-// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize)
+// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize, uintptr ctxt)
 // See cgocall.go for more details.
-TEXT ·cgocallback_gofunc(SB),NOSPLIT,$8-24
+TEXT ·cgocallback_gofunc(SB),NOSPLIT,$16-32
 	NO_LOCAL_POINTERS
 
 	// If g is nil, Go did not create the current thread.
@@ -706,7 +708,7 @@ havem:
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
 	//
-	// In the new goroutine, 0(SP) holds the saved R8.
+	// In the new goroutine, 8(SP) holds the saved R8.
 	MOVQ	m_curg(BX), SI
 	MOVQ	SI, g(CX)
 	MOVQ	(g_sched+gobuf_sp)(SI), DI  // prepare stack as DI
@@ -714,14 +716,16 @@ havem:
 	MOVQ	BX, -8(DI)
 	// Compute the size of the frame, including return PC and, if
 	// GOEXPERIMENT=framepointer, the saved based pointer
+	MOVQ	ctxt+24(FP), BX
 	LEAQ	fv+0(FP), AX
 	SUBQ	SP, AX
 	SUBQ	AX, DI
 	MOVQ	DI, SP
 
-	MOVQ	R8, 0(SP)
+	MOVQ	R8, 8(SP)
+	MOVQ	BX, 0(SP)
 	CALL	runtime·cgocallbackg(SB)
-	MOVQ	0(SP), R8
+	MOVQ	8(SP), R8
 
 	// Compute the size of the frame again. FP and SP have
 	// completely different values here than they did above,

src/runtime/asm_arm.s

@@ -530,23 +530,25 @@ g0:
 	MOVW	R0, ret+8(FP)
 	RET
 
-// cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
+// cgocallback(void (*fn)(void*), void *frame, uintptr framesize, uintptr ctxt)
 // Turn the fn into a Go func (by taking its address) and call
 // cgocallback_gofunc.
-TEXT runtime·cgocallback(SB),NOSPLIT,$12-12
+TEXT runtime·cgocallback(SB),NOSPLIT,$16-16
 	MOVW	$fn+0(FP), R0
 	MOVW	R0, 4(R13)
 	MOVW	frame+4(FP), R0
 	MOVW	R0, 8(R13)
 	MOVW	framesize+8(FP), R0
 	MOVW	R0, 12(R13)
+	MOVW	ctxt+12(FP), R0
+	MOVW	R0, 16(R13)
 	MOVW	$runtime·cgocallback_gofunc(SB), R0
 	BL	(R0)
 	RET
 
-// cgocallback_gofunc(void (*fn)(void*), void *frame, uintptr framesize)
+// cgocallback_gofunc(void (*fn)(void*), void *frame, uintptr framesize, uintptr ctxt)
 // See cgocall.go for more details.
-TEXT	·cgocallback_gofunc(SB),NOSPLIT,$8-12
+TEXT	·cgocallback_gofunc(SB),NOSPLIT,$8-16
 	NO_LOCAL_POINTERS
 	
 	// Load m and g from thread-local storage.
@@ -611,17 +613,20 @@ havem:
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
 	//
-	// In the new goroutine, -8(SP) and -4(SP) are unused.
+	// In the new goroutine, -4(SP) is unused (where SP refers to
+	// m->curg's SP while we're setting it up, before we've adjusted it).
 	MOVW	m_curg(R8), R0
 	BL	setg<>(SB)
 	MOVW	(g_sched+gobuf_sp)(g), R4 // prepare stack as R4
 	MOVW	(g_sched+gobuf_pc)(g), R5
 	MOVW	R5, -12(R4)
+	MOVW	ctxt+12(FP), R0
+	MOVW	R0, -8(R4)
 	MOVW	$-12(R4), R13
 	BL	runtime·cgocallbackg(SB)
 
 	// Restore g->sched (== m->curg->sched) from saved values.
-	MOVW	0(R13), R5
+	MOVW	4(R13), R5
 	MOVW	R5, (g_sched+gobuf_pc)(g)
 	MOVW	$12(R13), R4
 	MOVW	R4, (g_sched+gobuf_sp)(g)

src/runtime/asm_arm64.s

@@ -554,23 +554,25 @@ g0:
 	MOVW	R0, ret+16(FP)
 	RET
 
-// cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
+// cgocallback(void (*fn)(void*), void *frame, uintptr framesize, uintptr ctxt)
 // Turn the fn into a Go func (by taking its address) and call
 // cgocallback_gofunc.
-TEXT runtime·cgocallback(SB),NOSPLIT,$24-24
+TEXT runtime·cgocallback(SB),NOSPLIT,$40-32
 	MOVD	$fn+0(FP), R0
 	MOVD	R0, 8(RSP)
 	MOVD	frame+8(FP), R0
 	MOVD	R0, 16(RSP)
 	MOVD	framesize+16(FP), R0
 	MOVD	R0, 24(RSP)
+	MOVD	ctxt+24(FP), R0
+	MOVD	R0, 32(RSP)
 	MOVD	$runtime·cgocallback_gofunc(SB), R0
 	BL	(R0)
 	RET
 
-// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize)
+// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize, uintptr ctxt)
 // See cgocall.go for more details.
-TEXT ·cgocallback_gofunc(SB),NOSPLIT,$24-24
+TEXT ·cgocallback_gofunc(SB),NOSPLIT,$24-32
 	NO_LOCAL_POINTERS
 
 	// Load g from thread-local storage.
@@ -640,12 +642,15 @@ havem:
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
 	//
-	// In the new goroutine, -16(SP) and -8(SP) are unused.
+	// In the new goroutine, -8(SP) is unused (where SP refers to
+	// m->curg's SP while we're setting it up, before we've adjusted it).
 	MOVD	m_curg(R8), g
 	BL	runtime·save_g(SB)
 	MOVD	(g_sched+gobuf_sp)(g), R4 // prepare stack as R4
 	MOVD	(g_sched+gobuf_pc)(g), R5
-	MOVD	R5, -(24+8)(R4)	// maintain 16-byte SP alignment
+	MOVD	R5, -(16+8)(R4)
+	MOVD	ctxt+24(FP), R0
+	MOVD	R0, -(24+8)(R4)	// maintain 16-byte SP alignment
 	MOVD	$-(24+8)(R4), R0
 	MOVD	R0, RSP
 	BL	runtime·cgocallbackg(SB)

src/runtime/asm_ppc64x.s

@@ -569,22 +569,24 @@ g0:
 	MOVW	R3, ret+16(FP)
 	RET
 
-// cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
+// cgocallback(void (*fn)(void*), void *frame, uintptr framesize, uintptr ctxt)
 // Turn the fn into a Go func (by taking its address) and call
 // cgocallback_gofunc.
-TEXT runtime·cgocallback(SB),NOSPLIT,$24-24
+TEXT runtime·cgocallback(SB),NOSPLIT,$32-32
 	MOVD	$fn+0(FP), R3
 	MOVD	R3, FIXED_FRAME+0(R1)
 	MOVD	frame+8(FP), R3
 	MOVD	R3, FIXED_FRAME+8(R1)
 	MOVD	framesize+16(FP), R3
 	MOVD	R3, FIXED_FRAME+16(R1)
+	MOVD	ctxt+24(FP), R3
+	MOVD	R3, FIXED_FRAME+24(R1)
 	MOVD	$runtime·cgocallback_gofunc(SB), R12
 	MOVD	R12, CTR
 	BL	(CTR)
 	RET
 
-// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize)
+// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize, uintptr ctxt)
 // See cgocall.go for more details.
 TEXT ·cgocallback_gofunc(SB),NOSPLIT,$16-24
 	NO_LOCAL_POINTERS
@@ -654,12 +656,15 @@ havem:
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
 	//
-	// In the new goroutine, -16(SP) and -8(SP) are unused.
+	// In the new goroutine, -8(SP) is unused (where SP refers to
+	// m->curg's SP while we're setting it up, before we've adjusted it).
 	MOVD	m_curg(R8), g
 	BL	runtime·save_g(SB)
 	MOVD	(g_sched+gobuf_sp)(g), R4 // prepare stack as R4
 	MOVD	(g_sched+gobuf_pc)(g), R5
-	MOVD	R5, -(FIXED_FRAME+16)(R4)
+	MOVD	R5, -(FIXED_FRAME+8)(R4)
+	MOVD	ctxt+24(FP), R1
+	MOVD	R1, -(FIXED_FRAME+16)(R4)
 	MOVD	$-(FIXED_FRAME+16)(R4), R1
 	BL	runtime·cgocallbackg(SB)
 

src/runtime/asm_s390x.s

@@ -541,23 +541,25 @@ g0:
 	MOVW	R2, ret+16(FP)
 	RET
 
-// cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
+// cgocallback(void (*fn)(void*), void *frame, uintptr framesize, uintptr ctxt)
 // Turn the fn into a Go func (by taking its address) and call
 // cgocallback_gofunc.
-TEXT runtime·cgocallback(SB),NOSPLIT,$24-24
+TEXT runtime·cgocallback(SB),NOSPLIT,$32-32
 	MOVD	$fn+0(FP), R3
 	MOVD	R3, 8(R15)
 	MOVD	frame+8(FP), R3
 	MOVD	R3, 16(R15)
 	MOVD	framesize+16(FP), R3
 	MOVD	R3, 24(R15)
+	MOVD	ctxt+24(FP), R3
+	MOVD	R3, 32(R15)
 	MOVD	$runtime·cgocallback_gofunc(SB), R3
 	BL	(R3)
 	RET
 
-// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize)
+// cgocallback_gofunc(FuncVal*, void *frame, uintptr framesize, uintptr ctxt)
 // See cgocall.go for more details.
-TEXT ·cgocallback_gofunc(SB),NOSPLIT,$16-24
+TEXT ·cgocallback_gofunc(SB),NOSPLIT,$16-32
 	NO_LOCAL_POINTERS
 
 	// Load m and g from thread-local storage.
@@ -622,12 +624,15 @@ havem:
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
 	//
-	// In the new goroutine, -16(SP) and -8(SP) are unused.
+	// In the new goroutine, -8(SP) is unused (where SP refers to
+	// m->curg's SP while we're setting it up, before we've adjusted it).
 	MOVD	m_curg(R8), g
 	BL	runtime·save_g(SB)
 	MOVD	(g_sched+gobuf_sp)(g), R4 // prepare stack as R4
 	MOVD	(g_sched+gobuf_pc)(g), R5
 	MOVD	R5, -24(R4)
+	MOVD	ctxt+24(FP), R5
+	MOVD	R5, -16(R4)
 	MOVD	$-24(R4), R15
 	BL	runtime·cgocallbackg(SB)
 

src/runtime/cgo.go

@@ -17,6 +17,7 @@ import "unsafe"
 //go:linkname _cgo_sys_thread_create _cgo_sys_thread_create
 //go:linkname _cgo_notify_runtime_init_done _cgo_notify_runtime_init_done
 //go:linkname _cgo_callers _cgo_callers
+//go:linkname _cgo_set_context_function _cgo_set_context_function
 
 var (
 	_cgo_init                     unsafe.Pointer
@@ -26,6 +27,7 @@ var (
 	_cgo_sys_thread_create        unsafe.Pointer
 	_cgo_notify_runtime_init_done unsafe.Pointer
 	_cgo_callers                  unsafe.Pointer
+	_cgo_set_context_function     unsafe.Pointer
 )
 
 // iscgo is set to true by the runtime/cgo package

src/runtime/cgo/asm_386.s

@@ -4,10 +4,9 @@
 
 #include "textflag.h"
 
-/*
- * void crosscall2(void (*fn)(void*, int32), void*, int32)
- * Save registers and call fn with two arguments.
- */
+// Called by C code generated by cmd/cgo.
+// func crosscall2(fn func(a unsafe.Pointer, n int32, ctxt uintptr), a unsafe.Pointer, n int32, ctxt uintptr)
+// Saves C callee-saved registers and calls fn with three arguments.
 TEXT crosscall2(SB),NOSPLIT,$0
 	PUSHL	BP
 	MOVL	SP, BP
@@ -15,14 +14,16 @@ TEXT crosscall2(SB),NOSPLIT,$0
 	PUSHL	SI
 	PUSHL	DI
 	
-	SUBL	$8, SP
+	SUBL	$12, SP
+	MOVL	20(BP), AX
+	MOVL	AX, 8(SP)
 	MOVL	16(BP), AX
 	MOVL	AX, 4(SP)
 	MOVL	12(BP), AX
 	MOVL	AX, 0(SP)
 	MOVL	8(BP), AX
 	CALL	AX
-	ADDL	$8, SP
+	ADDL	$12, SP
 	
 	POPL	DI
 	POPL	SI

src/runtime/cgo/asm_amd64.s

@@ -4,72 +4,73 @@
 
 #include "textflag.h"
 
-/*
- * void crosscall2(void (*fn)(void*, int32), void*, int32)
- * Save registers and call fn with two arguments.
- */
+// Called by C code generated by cmd/cgo.
+// func crosscall2(fn func(a unsafe.Pointer, n int32, ctxt uintptr), a unsafe.Pointer, n int32, ctxt uintptr)
+// Saves C callee-saved registers and calls fn with three arguments.
 TEXT crosscall2(SB),NOSPLIT,$0
 #ifndef GOOS_windows
 	SUBQ	$0x58, SP	/* keeps stack pointer 32-byte aligned */
 #else
-	SUBQ	$0xf8, SP	/* also need to save xmm6 - xmm15 */
+	SUBQ	$0x118, SP	/* also need to save xmm6 - xmm15 */
 #endif
-	MOVQ	BX, 0x10(SP)
-	MOVQ	BP, 0x18(SP)
-	MOVQ	R12, 0x20(SP)
-	MOVQ	R13, 0x28(SP)
-	MOVQ	R14, 0x30(SP)
-	MOVQ	R15, 0x38(SP)
+	MOVQ	BX, 0x18(SP)
+	MOVQ	BP, 0x20(SP)
+	MOVQ	R12, 0x28(SP)
+	MOVQ	R13, 0x30(SP)
+	MOVQ	R14, 0x38(SP)
+	MOVQ	R15, 0x40(SP)
 
 #ifdef GOOS_windows
 	// Win64 save RBX, RBP, RDI, RSI, RSP, R12, R13, R14, R15 and XMM6 -- XMM15.
-	MOVQ	DI, 0x40(SP)
-	MOVQ	SI, 0x48(SP)
-	MOVUPS	X6, 0x50(SP)
-	MOVUPS	X7, 0x60(SP)
-	MOVUPS	X8, 0x70(SP)
-	MOVUPS	X9, 0x80(SP)
-	MOVUPS	X10, 0x90(SP)
-	MOVUPS	X11, 0xa0(SP)
-	MOVUPS	X12, 0xb0(SP)
-	MOVUPS	X13, 0xc0(SP)
-	MOVUPS	X14, 0xd0(SP)
-	MOVUPS	X15, 0xe0(SP)
+	MOVQ	DI, 0x48(SP)
+	MOVQ	SI, 0x50(SP)
+	MOVUPS	X6, 0x60(SP)
+	MOVUPS	X7, 0x70(SP)
+	MOVUPS	X8, 0x80(SP)
+	MOVUPS	X9, 0x90(SP)
+	MOVUPS	X10, 0xa0(SP)
+	MOVUPS	X11, 0xb0(SP)
+	MOVUPS	X12, 0xc0(SP)
+	MOVUPS	X13, 0xd0(SP)
+	MOVUPS	X14, 0xe0(SP)
+	MOVUPS	X15, 0xf0(SP)
 
-	MOVQ	DX, 0(SP)	/* arg */
-	MOVQ	R8, 8(SP)	/* argsize (includes padding) */
+	MOVQ	DX, 0x0(SP)	/* arg */
+	MOVQ	R8, 0x8(SP)	/* argsize (includes padding) */
+	MOVQ	R9, 0x10(SP)	/* ctxt */
 	
 	CALL	CX	/* fn */
 	
-	MOVQ	0x40(SP), DI
-	MOVQ	0x48(SP), SI
-	MOVUPS	0x50(SP), X6
-	MOVUPS	0x60(SP), X7
-	MOVUPS	0x70(SP), X8
-	MOVUPS	0x80(SP), X9
-	MOVUPS	0x90(SP), X10
-	MOVUPS	0xa0(SP), X11
-	MOVUPS	0xb0(SP), X12
-	MOVUPS	0xc0(SP), X13
-	MOVUPS	0xd0(SP), X14
-	MOVUPS	0xe0(SP), X15
+	MOVQ	0x48(SP), DI
+	MOVQ	0x50(SP), SI
+	MOVUPS	0x60(SP), X6
+	MOVUPS	0x70(SP), X7
+	MOVUPS	0x80(SP), X8
+	MOVUPS	0x90(SP), X9
+	MOVUPS	0xa0(SP), X10
+	MOVUPS	0xb0(SP), X11
+	MOVUPS	0xc0(SP), X12
+	MOVUPS	0xd0(SP), X13
+	MOVUPS	0xe0(SP), X14
+	MOVUPS	0xf0(SP), X15
 #else
-	MOVQ	SI, 0(SP)	/* arg */
-	MOVQ	DX, 8(SP)	/* argsize (includes padding) */
+	MOVQ	SI, 0x0(SP)	/* arg */
+	MOVQ	DX, 0x8(SP)	/* argsize (includes padding) */
+	MOVQ	CX, 0x10(SP)	/* ctxt */
 
 	CALL	DI	/* fn */
 #endif
 
-	MOVQ	0x10(SP), BX
-	MOVQ	0x18(SP), BP
-	MOVQ	0x20(SP), R12
-	MOVQ	0x28(SP), R13
-	MOVQ	0x30(SP), R14
-	MOVQ	0x38(SP), R15
+	MOVQ	0x18(SP), BX
+	MOVQ	0x20(SP), BP
+	MOVQ	0x28(SP), R12
+	MOVQ	0x30(SP), R13
+	MOVQ	0x38(SP), R14
+	MOVQ	0x40(SP), R15
 	
 #ifndef GOOS_windows
 	ADDQ	$0x58, SP
 #else
-	ADDQ	$0xf8, SP
+	ADDQ	$0x118, SP
 #endif
 	RET

src/runtime/cgo/asm_arm.s

@@ -4,21 +4,20 @@
 
 #include "textflag.h"
 
-/*
- * void crosscall2(void (*fn)(void*, int32), void*, int32)
- * Save registers and call fn with two arguments.
- */
+// Called by C code generated by cmd/cgo.
+// func crosscall2(fn func(a unsafe.Pointer, n int32, ctxt uintptr), a unsafe.Pointer, n int32, ctxt uintptr)
+// Saves C callee-saved registers and calls fn with three arguments.
 TEXT crosscall2(SB),NOSPLIT,$-4
 	/* 
 	 * We still need to save all callee save register as before, and then
-	 *  push 2 args for fn (R1 and R2).
+	 *  push 3 args for fn (R1, R2, R3).
 	 * Also note that at procedure entry in gc world, 4(R13) will be the
 	 *  first arg, so we must push another dummy reg (R0) for 0(R13).
 	 *  Additionally, runtime·load_g will clobber R0, so we need to save R0
 	 *  nevertheless.
 	 */
-	MOVM.WP	[R0, R1, R2, R4, R5, R6, R7, R8, R9, g, R11, R12, R14], (R13)
+	MOVM.WP	[R0, R1, R2, R3, R4, R5, R6, R7, R8, R9, g, R11, R12, R14], (R13)
 	BL	runtime·load_g(SB)
 	MOVW	R15, R14 // R15 is PC.
 	MOVW	0(R13), R15
-	MOVM.IAW	(R13), [R0, R1, R2, R4, R5, R6, R7, R8, R9, g, R11, R12, R15]
+	MOVM.IAW	(R13), [R0, R1, R2, R3, R4, R5, R6, R7, R8, R9, g, R11, R12, R15]

src/runtime/cgo/asm_arm64.s

@@ -4,14 +4,13 @@
 
 #include "textflag.h"
 
-/*
- * void crosscall2(void (*fn)(void*, int32), void*, int32)
- * Save registers and call fn with two arguments.
- */
+// Called by C code generated by cmd/cgo.
+// func crosscall2(fn func(a unsafe.Pointer, n int32, ctxt uintptr), a unsafe.Pointer, n int32, ctxt uintptr)
+// Saves C callee-saved registers and calls fn with three arguments.
 TEXT crosscall2(SB),NOSPLIT,$-8
 	/*
 	 * We still need to save all callee save register as before, and then
-	 *  push 2 args for fn (R1 and R2).
+	 *  push 3 args for fn (R1, R2, R3).
 	 * Also note that at procedure entry in gc world, 8(RSP) will be the
 	 *  first arg.
 	 * TODO(minux): use LDP/STP here if it matters.
@@ -19,26 +18,27 @@ TEXT crosscall2(SB),NOSPLIT,$-8
 	SUB	$(8*24), RSP
 	MOVD	R1, (8*1)(RSP)
 	MOVD	R2, (8*2)(RSP)
-	MOVD	R19, (8*3)(RSP)
-	MOVD	R20, (8*4)(RSP)
-	MOVD	R21, (8*5)(RSP)
-	MOVD	R22, (8*6)(RSP)
-	MOVD	R23, (8*7)(RSP)
-	MOVD	R24, (8*8)(RSP)
-	MOVD	R25, (8*9)(RSP)
-	MOVD	R26, (8*10)(RSP)
-	MOVD	R27, (8*11)(RSP)
-	MOVD	g, (8*12)(RSP)
-	MOVD	R29, (8*13)(RSP)
-	MOVD	R30, (8*14)(RSP)
-	FMOVD	F8, (8*15)(RSP)
-	FMOVD	F9, (8*16)(RSP)
-	FMOVD	F10, (8*17)(RSP)
-	FMOVD	F11, (8*18)(RSP)
-	FMOVD	F12, (8*19)(RSP)
-	FMOVD	F13, (8*20)(RSP)
-	FMOVD	F14, (8*21)(RSP)
-	FMOVD	F15, (8*22)(RSP)
+	MOVD	R3, (8*3)(RSP)
+	MOVD	R19, (8*4)(RSP)
+	MOVD	R20, (8*5)(RSP)
+	MOVD	R21, (8*6)(RSP)
+	MOVD	R22, (8*7)(RSP)
+	MOVD	R23, (8*8)(RSP)
+	MOVD	R24, (8*9)(RSP)
+	MOVD	R25, (8*10)(RSP)
+	MOVD	R26, (8*11)(RSP)
+	MOVD	R27, (8*12)(RSP)
+	MOVD	g, (8*13)(RSP)
+	MOVD	R29, (8*14)(RSP)
+	MOVD	R30, (8*15)(RSP)
+	FMOVD	F8, (8*16)(RSP)
+	FMOVD	F9, (8*17)(RSP)
+	FMOVD	F10, (8*18)(RSP)
+	FMOVD	F11, (8*19)(RSP)
+	FMOVD	F12, (8*20)(RSP)
+	FMOVD	F13, (8*21)(RSP)
+	FMOVD	F14, (8*22)(RSP)
+	FMOVD	F15, (8*23)(RSP)
 
 	MOVD	R0, R19
 
@@ -49,25 +49,26 @@ TEXT crosscall2(SB),NOSPLIT,$-8
 
 	MOVD	(8*1)(RSP), R1
 	MOVD	(8*2)(RSP), R2
-	MOVD	(8*3)(RSP), R19
-	MOVD	(8*4)(RSP), R20
-	MOVD	(8*5)(RSP), R21
-	MOVD	(8*6)(RSP), R22
-	MOVD	(8*7)(RSP), R23
-	MOVD	(8*8)(RSP), R24
-	MOVD	(8*9)(RSP), R25
-	MOVD	(8*10)(RSP), R26
-	MOVD	(8*11)(RSP), R27
-	MOVD	(8*12)(RSP), g
-	MOVD	(8*13)(RSP), R29
-	MOVD	(8*14)(RSP), R30
-	FMOVD	(8*15)(RSP), F8
-	FMOVD	(8*16)(RSP), F9
-	FMOVD	(8*17)(RSP), F10
-	FMOVD	(8*18)(RSP), F11
-	FMOVD	(8*19)(RSP), F12
-	FMOVD	(8*20)(RSP), F13
-	FMOVD	(8*21)(RSP), F14
-	FMOVD	(8*22)(RSP), F15
+	MOVD	(8*3)(RSP), R3
+	MOVD	(8*4)(RSP), R19
+	MOVD	(8*5)(RSP), R20
+	MOVD	(8*6)(RSP), R21
+	MOVD	(8*7)(RSP), R22
+	MOVD	(8*8)(RSP), R23
+	MOVD	(8*9)(RSP), R24
+	MOVD	(8*10)(RSP), R25
+	MOVD	(8*11)(RSP), R26
+	MOVD	(8*12)(RSP), R27
+	MOVD	(8*13)(RSP), g
+	MOVD	(8*14)(RSP), R29
+	MOVD	(8*15)(RSP), R30
+	FMOVD	(8*16)(RSP), F8
+	FMOVD	(8*17)(RSP), F9
+	FMOVD	(8*18)(RSP), F10
+	FMOVD	(8*19)(RSP), F11
+	FMOVD	(8*20)(RSP), F12
+	FMOVD	(8*21)(RSP), F13
+	FMOVD	(8*22)(RSP), F14
+	FMOVD	(8*23)(RSP), F15
 	ADD	$(8*24), RSP
 	RET

src/runtime/cgo/asm_ppc64x.s

@@ -7,11 +7,9 @@
 #include "textflag.h"
 #include "asm_ppc64x.h"
 
-/*
- * void crosscall2(void (*fn)(void*, int32), void*, int32)
- * Save registers and call fn with two arguments.
- * crosscall2 obeys the C ABI; fn obeys the Go ABI.
- */
+// Called by C code generated by cmd/cgo.
+// func crosscall2(fn func(a unsafe.Pointer, n int32, ctxt uintptr), a unsafe.Pointer, n int32, ctxt uintptr)
+// Saves C callee-saved registers and calls fn with three arguments.
 TEXT crosscall2(SB),NOSPLIT|NOFRAME,$0
 	// TODO(austin): ABI v1 (fn is probably a function descriptor)
 
@@ -22,7 +20,7 @@ TEXT crosscall2(SB),NOSPLIT|NOFRAME,$0
 
 	BL	saveregs2<>(SB)
 
-	MOVDU	R1, (-288-2*8-FIXED_FRAME)(R1)
+	MOVDU	R1, (-288-3*8-FIXED_FRAME)(R1)
 
 	// Initialize Go ABI environment
 	BL	runtime·reginit(SB)
@@ -32,6 +30,7 @@ TEXT crosscall2(SB),NOSPLIT|NOFRAME,$0
 	MOVD	R3, CTR
 	MOVD	R4, FIXED_FRAME+0(R1)
 	MOVD	R5, FIXED_FRAME+8(R1)
+	MOVD	R6, FIXED_FRAME+16(R1)
 	BL	(CTR)
 
 	ADD	$(288+2*8+FIXED_FRAME), R1

src/runtime/cgo/asm_s390x.s

@@ -4,11 +4,9 @@
 
 #include "textflag.h"
 
-/*
- * void crosscall2(void (*fn)(void*, int32), void*, int32)
- * Save registers and call fn with two arguments.
- * crosscall2 obeys the C ABI; fn obeys the Go ABI.
- */
+// Called by C code generated by cmd/cgo.
+// func crosscall2(fn func(a unsafe.Pointer, n int32, ctxt uintptr), a unsafe.Pointer, n int32, ctxt uintptr)
+// Saves C callee-saved registers and calls fn with three arguments.
 TEXT crosscall2(SB),NOSPLIT|NOFRAME,$0
 	// Start with standard C stack frame layout and linkage
 
@@ -24,14 +22,15 @@ TEXT crosscall2(SB),NOSPLIT|NOFRAME,$0
 	XOR	R0, R0
 	BL	runtime·load_g(SB)
 
-	// Allocate 24 bytes on the stack
-	SUB	$24, R15
+	// Allocate 32 bytes on the stack
+	SUB	$32, R15
 
 	MOVD	R3, 8(R15)  // arg1
 	MOVW	R4, 16(R15) // arg2
-	BL	(R2)        // fn(arg1, arg2)
+	MOVD	R5, 24(R15) // arg3
+	BL	(R2)        // fn(arg1, arg2, arg3)
 
-	ADD	$24, R15
+	ADD	$32, R15
 
 	// Restore R6-R15, F0, F2, F4 and F6
 	LMG	48(R15), R6, R15

src/runtime/cgo/callbacks.go

@@ -11,7 +11,7 @@ import "unsafe"
 
 // cgocallback is defined in runtime
 //go:linkname _runtime_cgocallback runtime.cgocallback
-func _runtime_cgocallback(unsafe.Pointer, unsafe.Pointer, uintptr)
+func _runtime_cgocallback(unsafe.Pointer, unsafe.Pointer, uintptr, uintptr)
 
 // The declaration of crosscall2 is:
 //   void crosscall2(void (*fn)(void *, int), void *, int);
@@ -19,6 +19,10 @@ func _runtime_cgocallback(unsafe.Pointer, unsafe.Pointer, uintptr)
 // We need to export the symbol crosscall2 in order to support
 // callbacks from shared libraries. This applies regardless of
 // linking mode.
+//
+// Compatibility note: crosscall2 actually takes four arguments, but
+// it works to call it with three arguments when calling _cgo_panic.
+// That is supported for backward compatibility.
 //go:cgo_export_static crosscall2
 //go:cgo_export_dynamic crosscall2
 
@@ -39,7 +43,7 @@ var _runtime_cgo_panic_internal byte
 //go:nosplit
 //go:norace
 func _cgo_panic(a unsafe.Pointer, n int32) {
-	_runtime_cgocallback(unsafe.Pointer(&_runtime_cgo_panic_internal), a, uintptr(n))
+	_runtime_cgocallback(unsafe.Pointer(&_runtime_cgo_panic_internal), a, uintptr(n), 0)
 }
 
 //go:cgo_import_static x_cgo_init
@@ -92,5 +96,13 @@ var _cgo_sys_thread_create = &x_cgo_sys_thread_create
 var x_cgo_notify_runtime_init_done byte
 var _cgo_notify_runtime_init_done = &x_cgo_notify_runtime_init_done
 
+// Sets the traceback context function. See runtime.SetCgoTraceback.
+
+//go:cgo_import_static x_cgo_set_context_function
+//go:linkname x_cgo_set_context_function x_cgo_set_context_function
+//go:linkname _cgo_set_context_function _cgo_set_context_function
+var x_cgo_set_context_function byte
+var _cgo_set_context_function = &x_cgo_set_context_function
+
 //go:cgo_export_static _cgo_topofstack
 //go:cgo_export_dynamic _cgo_topofstack

src/runtime/cgo/gcc_context.c

+// Copyright 2016 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.
+
+// +build cgo
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris windows
+
+#include "libcgo.h"
+
+// The context function, used when tracing back C calls into Go.
+void (*x_cgo_context_function)(struct context_arg*);
+
+// Sets the context function to call to record the traceback context
+// when calling a Go function from C code. Called from runtime.SetCgoTraceback.
+void x_cgo_set_context_function(void (*context)(struct context_arg*)) {
+	x_cgo_context_function = context;
+}
+
+// Releases the cgo traceback context.
+void _cgo_release_context(uintptr_t ctxt) {
+	if (ctxt != 0 && x_cgo_context_function != nil) {
+		struct context_arg arg;
+
+		arg.Context = ctxt;
+		(*x_cgo_context_function)(&arg);
+	}
+}

src/runtime/cgo/gcc_libinit.c

@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h> // strerror
+#include "libcgo.h"
 
 static pthread_cond_t runtime_init_cond = PTHREAD_COND_INITIALIZER;
 static pthread_mutex_t runtime_init_mu = PTHREAD_MUTEX_INITIALIZER;
@@ -24,13 +25,21 @@ x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
 	}
 }
 
-void
+uintptr_t
 _cgo_wait_runtime_init_done() {
 	pthread_mutex_lock(&runtime_init_mu);
 	while (runtime_init_done == 0) {
 		pthread_cond_wait(&runtime_init_cond, &runtime_init_mu);
 	}
 	pthread_mutex_unlock(&runtime_init_mu);
+	if (x_cgo_context_function != nil) {
+		struct context_arg arg;
+
+		arg.Context = 0;
+		(*x_cgo_context_function)(&arg);
+		return arg.Context;
+	}
+	return 0;
 }
 
 void

src/runtime/cgo/gcc_libinit_openbsd.c

@@ -4,6 +4,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include "libcgo.h"
 
 void
 x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
@@ -11,12 +12,20 @@ x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
 	abort();
 }
 
-void
+uintptr_t
 _cgo_wait_runtime_init_done() {
 	// TODO(spetrovic): implement this method.
+	if (x_cgo_context_function != nil) {
+		struct context_arg arg;
+
+		arg.Context = 0;
+		(*x_cgo_context_function)(&arg);
+		return arg.Context;
+	}
+	return 0;
 }
 
 void
 x_cgo_notify_runtime_init_done(void* dummy) {
 	// TODO(spetrovic): implement this method.
-}
\ No newline at end of file
+}

src/runtime/cgo/gcc_libinit_windows.c

@@ -10,6 +10,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#include "libcgo.h"
+
 static volatile long runtime_init_once_gate = 0;
 static volatile long runtime_init_once_done = 0;
 
@@ -66,12 +68,20 @@ _cgo_is_runtime_initialized() {
 	 return status;
 }
 
-void
+uintptr_t
 _cgo_wait_runtime_init_done() {
 	 _cgo_maybe_run_preinit();
 	while (!_cgo_is_runtime_initialized()) {
 			WaitForSingleObject(runtime_init_wait, INFINITE);
 	}
+	if (x_cgo_context_function != nil) {
+		struct context_arg arg;
+
+		arg.Context = 0;
+		(*x_cgo_context_function)(&arg);
+		return arg.Context;
+	}
+	return 0;
 }
 
 void

src/runtime/cgo/libcgo.h

@@ -57,8 +57,10 @@ void _cgo_sys_thread_start(ThreadStart *ts);
 
 /*
  * Waits for the Go runtime to be initialized (OS dependent).
+ * If runtime.SetCgoTraceback is used to set a context function,
+ * calls the context function and returns the context value.
  */
-void _cgo_wait_runtime_init_done();
+uintptr_t _cgo_wait_runtime_init_done();
 
 /*
  * Call fn in the 6c world.
@@ -84,3 +86,11 @@ void darwin_arm_init_thread_exception_port(void);
  * Starts a mach message server processing EXC_BAD_ACCESS.
  */
 void darwin_arm_init_mach_exception_handler(void);
+
+/*
+ * The cgo context function. See runtime.SetCgoTraceback.
+ */
+struct context_arg {
+	uintptr_t Context;
+};
+extern void (*x_cgo_context_function)(struct context_arg*);

src/runtime/cgocall.go

@@ -166,7 +166,7 @@ func cfree(p unsafe.Pointer) {
 
 // Call from C back to Go.
 //go:nosplit
-func cgocallbackg() {
+func cgocallbackg(ctxt uintptr) {
 	gp := getg()
 	if gp != gp.m.curg {
 		println("runtime: bad g in cgocallback")
@@ -184,20 +184,43 @@ func cgocallbackg() {
 	savedsp := unsafe.Pointer(gp.syscallsp)
 	savedpc := gp.syscallpc
 	exitsyscall(0) // coming out of cgo call
-	cgocallbackg1()
+
+	cgocallbackg1(ctxt)
+
 	// going back to cgo call
 	reentersyscall(savedpc, uintptr(savedsp))
 
 	gp.m.syscall = syscall
 }
 
-func cgocallbackg1() {
+func cgocallbackg1(ctxt uintptr) {
 	gp := getg()
 	if gp.m.needextram {
 		gp.m.needextram = false
 		systemstack(newextram)
 	}
 
+	if ctxt != 0 {
+		s := append(gp.cgoCtxt, ctxt)
+
+		// Now we need to set gp.cgoCtxt = s, but we could get
+		// a SIGPROF signal while manipulating the slice, and
+		// the SIGPROF handler could pick up gp.cgoCtxt while
+		// tracing up the stack.  We need to ensure that the
+		// handler always sees a valid slice, so set the
+		// values in an order such that it always does.
+		p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
+		atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
+		p.cap = cap(s)
+		p.len = len(s)
+
+		defer func(gp *g) {
+			// Decrease the length of the slice by one, safely.
+			p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
+			p.len--
+		}(gp)
+	}
+
 	if gp.m.ncgo == 0 {
 		// The C call to Go came from a thread not currently running
 		// any Go. In the case of -buildmode=c-archive or c-shared,
@@ -236,13 +259,13 @@ func cgocallbackg1() {
 		// SP and the stack frame and between the stack frame and the arguments.
 		cb = (*args)(unsafe.Pointer(sp + 5*sys.PtrSize))
 	case "amd64":
-		// On amd64, stack frame is one word, plus caller PC.
+		// On amd64, stack frame is two words, plus caller PC.
 		if framepointer_enabled {
 			// In this case, there's also saved BP.
-			cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize))
+			cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
 			break
 		}
-		cb = (*args)(unsafe.Pointer(sp + 2*sys.PtrSize))
+		cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize))
 	case "386":
 		// On 386, stack frame is three words, plus caller PC.
 		cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))

src/runtime/crash_cgo_test.go

@@ -221,3 +221,14 @@ func TestCgoCrashTraceback(t *testing.T) {
 		}
 	}
 }
+
+func TestCgoTracebackContext(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("test does not work on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+	got := runTestProg(t, "testprogcgo", "TracebackContext")
+	want := "OK\n"
+	if got != want {
+		t.Errorf("expected %q got %v", want, got)
+	}
+}

src/runtime/runtime2.go

@@ -352,7 +352,8 @@ type g struct {
 	gopc           uintptr // pc of go statement that created this goroutine
 	startpc        uintptr // pc of goroutine function
 	racectx        uintptr
-	waiting        *sudog // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
+	waiting        *sudog    // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
+	cgoCtxt        []uintptr // cgo traceback context
 
 	// Per-G GC state
 

src/runtime/symtab.go

@@ -18,6 +18,10 @@ type Frames struct {
 	// ci.callers[0] is the address of the faulting instruction
 	// instead of the return address of the call.
 	wasPanic bool
+
+	// Frames to return for subsequent calls to the Next method.
+	// Used for non-Go frames.
+	frames *[]Frame
 }
 
 // Frame is the information returned by Frames for each call frame.
@@ -46,12 +50,23 @@ type Frame struct {
 // prepares to return function/file/line information.
 // Do not change the slice until you are done with the Frames.
 func CallersFrames(callers []uintptr) *Frames {
-	return &Frames{callers, false}
+	return &Frames{callers: callers}
 }
 
 // Next returns frame information for the next caller.
 // If more is false, there are no more callers (the Frame value is valid).
 func (ci *Frames) Next() (frame Frame, more bool) {
+	if ci.frames != nil {
+		// We have saved up frames to return.
+		f := (*ci.frames)[0]
+		if len(*ci.frames) == 1 {
+			ci.frames = nil
+		} else {
+			*ci.frames = (*ci.frames)[1:]
+		}
+		return f, ci.frames != nil || len(ci.callers) > 0
+	}
+
 	if len(ci.callers) == 0 {
 		ci.wasPanic = false
 		return Frame{}, false
@@ -62,6 +77,9 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 	f := FuncForPC(pc)
 	if f == nil {
 		ci.wasPanic = false
+		if cgoSymbolizer != nil {
+			return ci.cgoNext(pc, more)
+		}
 		return Frame{}, more
 	}
 
@@ -87,6 +105,54 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 	return frame, more
 }
 
+// cgoNext returns frame information for pc, known to be a non-Go function,
+// using the cgoSymbolizer hook.
+func (ci *Frames) cgoNext(pc uintptr, more bool) (Frame, bool) {
+	arg := cgoSymbolizerArg{pc: pc}
+	callCgoSymbolizer(&arg)
+
+	if arg.file == nil && arg.funcName == nil {
+		// No useful information from symbolizer.
+		return Frame{}, more
+	}
+
+	var frames []Frame
+	for {
+		frames = append(frames, Frame{
+			PC:       pc,
+			Func:     nil,
+			Function: gostring(arg.funcName),
+			File:     gostring(arg.file),
+			Line:     int(arg.lineno),
+			Entry:    arg.entry,
+		})
+		if arg.more == 0 {
+			break
+		}
+		callCgoSymbolizer(&arg)
+	}
+
+	// No more frames for this PC. Tell the symbolizer we are done.
+	// We don't try to maintain a single cgoSymbolizerArg for the
+	// whole use of Frames, because there would be no good way to tell
+	// the symbolizer when we are done.
+	arg.pc = 0
+	callCgoSymbolizer(&arg)
+
+	if len(frames) == 1 {
+		// Return a single frame.
+		return frames[0], more
+	}
+
+	// Return the first frame we saw and store the rest to be
+	// returned by later calls to Next.
+	rf := frames[0]
+	frames = frames[1:]
+	ci.frames = new([]Frame)
+	*ci.frames = frames
+	return rf, true
+}
+
 // NOTE: Func does not expose the actual unexported fields, because we return *Func
 // values to users, and we want to keep them from being able to overwrite the data
 // with (say) *f = Func{}.

src/runtime/testdata/testprogcgo/tracebackctxt.go

+// Copyright 2016 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.
+
+// The __attribute__((weak)) used below doesn't seem to work on Windows.
+
+// +build !windows
+
+package main
+
+// Test the context argument to SetCgoTraceback.
+// Use fake context, traceback, and symbolizer functions.
+
+/*
+#include <stdlib.h>
+#include <stdint.h>
+
+// Use weak declarations so that we can define functions here even
+// though we use //export in the Go code.
+extern void tcContext(void*) __attribute__((weak));
+extern void tcTraceback(void*) __attribute__((weak));
+extern void tcSymbolizer(void*) __attribute__((weak));
+
+extern void G1(void);
+extern void G2(void);
+
+static void C1() {
+	G1();
+}
+
+static void C2() {
+	G2();
+}
+
+struct cgoContextArg {
+	uintptr_t context;
+};
+
+struct cgoTracebackArg {
+	uintptr_t  context;
+	uintptr_t* buf;
+	uintptr_t  max;
+};
+
+struct cgoSymbolizerArg {
+	uintptr_t   pc;
+	const char* file;
+	uintptr_t   lineno;
+	const char* func;
+	uintptr_t   entry;
+	uintptr_t   more;
+	uintptr_t   data;
+};
+
+// Global so that there is only one, weak so that //export works.
+// Uses atomic adds and subtracts to catch the possibility of
+// erroneous calls from multiple threads; that should be impossible in
+// this test case, but we check just in case.
+int contextCount __attribute__((weak));
+
+static int getContextCount() {
+	return __sync_add_and_fetch(&contextCount, 0);
+}
+
+void tcContext(void* parg) {
+	struct cgoContextArg* arg = (struct cgoContextArg*)(parg);
+	if (arg->context == 0) {
+		arg->context = __sync_add_and_fetch(&contextCount, 1);
+	} else {
+		if (arg->context != __sync_add_and_fetch(&contextCount, 0)) {
+			abort();
+		}
+		__sync_sub_and_fetch(&contextCount, 1);
+	}
+}
+
+void tcTraceback(void* parg) {
+	int base, i;
+	struct cgoTracebackArg* arg = (struct cgoTracebackArg*)(parg);
+	if (arg->context == 0) {
+		// This shouldn't happen in this program.
+		abort();
+	}
+	// Return a variable number of PC values.
+	base = arg->context << 8;
+	for (i = 0; i < arg->context; i++) {
+		if (i < arg->max) {
+			arg->buf[i] = base + i;
+		}
+	}
+}
+
+void tcSymbolizer(void *parg) {
+	struct cgoSymbolizerArg* arg = (struct cgoSymbolizerArg*)(parg);
+	if (arg->pc == 0) {
+		return;
+	}
+	// Report two lines per PC returned by traceback, to test more handling.
+	arg->more = arg->file == NULL;
+	arg->file = "tracebackctxt.go";
+	arg->func = "cFunction";
+	arg->lineno = arg->pc + (arg->more << 16);
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"runtime"
+	"unsafe"
+)
+
+func init() {
+	register("TracebackContext", TracebackContext)
+}
+
+var tracebackOK bool
+
+func TracebackContext() {
+	runtime.SetCgoTraceback(0, unsafe.Pointer(C.tcTraceback), unsafe.Pointer(C.tcContext), unsafe.Pointer(C.tcSymbolizer))
+	C.C1()
+	if got := C.getContextCount(); got != 0 {
+		fmt.Printf("at end contextCount == %d, expected 0\n", got)
+		tracebackOK = false
+	}
+	if tracebackOK {
+		fmt.Println("OK")
+	}
+}
+
+//export G1
+func G1() {
+	C.C2()
+}
+
+//export G2
+func G2() {
+	pc := make([]uintptr, 32)
+	n := runtime.Callers(0, pc)
+	cf := runtime.CallersFrames(pc[:n])
+	var frames []runtime.Frame
+	for {
+		frame, more := cf.Next()
+		frames = append(frames, frame)
+		if !more {
+			break
+		}
+	}
+
+	want := []struct {
+		function string
+		line     int
+	}{
+		{"main.G2", 0},
+		{"cFunction", 0x10200},
+		{"cFunction", 0x200},
+		{"cFunction", 0x10201},
+		{"cFunction", 0x201},
+		{"main.G1", 0},
+		{"cFunction", 0x10100},
+		{"cFunction", 0x100},
+		{"main.TracebackContext", 0},
+	}
+
+	ok := true
+	i := 0
+wantLoop:
+	for _, w := range want {
+		for ; i < len(frames); i++ {
+			if w.function == frames[i].Function {
+				if w.line != 0 && w.line != frames[i].Line {
+					fmt.Printf("found function %s at wrong line %#x (expected %#x)\n", w.function, frames[i].Line, w.line)
+					ok = false
+				}
+				i++
+				continue wantLoop
+			}
+		}
+		fmt.Printf("did not find function %s in\n", w.function)
+		for _, f := range frames {
+			fmt.Println(f)
+		}
+		ok = false
+		break
+	}
+	tracebackOK = ok
+	if got := C.getContextCount(); got != 2 {
+		fmt.Printf("at bottom contextCount == %d, expected 2\n", got)
+		tracebackOK = false
+	}
+}

src/runtime/traceback.go

@@ -172,6 +172,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		frame.lr = lr0
 	}
 	waspanic := false
+	cgoCtxt := gp.cgoCtxt
 	printing := pcbuf == nil && callback == nil
 	_defer := gp._defer
 
@@ -252,6 +253,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 				sp = gp.m.curg.sched.sp
 				stkbarG = gp.m.curg
 				stkbar = stkbarG.stkbar[stkbarG.stkbarPos:]
+				cgoCtxt = gp.m.curg.cgoCtxt
 			}
 			frame.fp = sp + uintptr(funcspdelta(f, frame.pc, &cache))
 			if !usesLR {
@@ -413,6 +415,18 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		n++
 
 	skipped:
+		if f.entry == cgocallback_gofuncPC && len(cgoCtxt) > 0 {
+			ctxt := cgoCtxt[len(cgoCtxt)-1]
+			cgoCtxt = cgoCtxt[:len(cgoCtxt)-1]
+
+			// skip only applies to Go frames.
+			// callback != nil only used when we only care
+			// about Go frames.
+			if skip == 0 && callback == nil {
+				n = tracebackCgoContext(pcbuf, printing, ctxt, n, max)
+			}
+		}
+
 		waspanic = f.entry == sigpanicPC
 
 		// Do not unwind past the bottom of the stack.
@@ -546,6 +560,39 @@ func getArgInfo(frame *stkframe, f *_func, needArgMap bool) (arglen uintptr, arg
 	return
 }
 
+// tracebackCgoContext handles tracing back a cgo context value, from
+// the context argument to setCgoTraceback, for the gentraceback
+// function. It returns the new value of n.
+func tracebackCgoContext(pcbuf *uintptr, printing bool, ctxt uintptr, n, max int) int {
+	var cgoPCs [32]uintptr
+	cgoContextPCs(ctxt, cgoPCs[:])
+	var arg cgoSymbolizerArg
+	anySymbolized := false
+	for _, pc := range cgoPCs {
+		if pc == 0 || n >= max {
+			break
+		}
+		if pcbuf != nil {
+			(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = pc
+		}
+		if printing {
+			if cgoSymbolizer == nil {
+				print("non-Go function at pc=", hex(pc), "\n")
+			} else {
+				c := printOneCgoTraceback(pc, max-n, &arg)
+				n += c - 1 // +1 a few lines down
+				anySymbolized = true
+			}
+		}
+		n++
+	}
+	if anySymbolized {
+		arg.pc = 0
+		callCgoSymbolizer(&arg)
+	}
+	return n
+}
+
 func printcreatedby(gp *g) {
 	// Show what created goroutine, except main goroutine (goid 1).
 	pc := gp.gopc
@@ -782,10 +829,11 @@ func isSystemGoroutine(gp *g) bool {
 //	};
 //
 // If the Context field is 0, the context function is being called to
-// record the current traceback context. It should record whatever
-// information is needed about the current point of execution to later
-// produce a stack trace, probably the stack pointer and PC. In this
-// case the context function will be called from C code.
+// record the current traceback context. It should record in the
+// Context field whatever information is needed about the current
+// point of execution to later produce a stack trace, probably the
+// stack pointer and PC. In this case the context function will be
+// called from C code.
 //
 // If the Context field is not 0, then it is a value returned by a
 // previous call to the context function. This case is called when the
@@ -903,16 +951,18 @@ func SetCgoTraceback(version int, traceback, context, symbolizer unsafe.Pointer)
 	if version != 0 {
 		panic("unsupported version")
 	}
-	if context != nil {
-		panic("SetCgoTraceback: context function not yet implemented")
-	}
+
 	cgoTraceback = traceback
-	cgoContext = context
 	cgoSymbolizer = symbolizer
+
+	// The context function is called when a C function calls a Go
+	// function. As such it is only called by C code in runtime/cgo.
+	if _cgo_set_context_function != nil {
+		cgocall(_cgo_set_context_function, context)
+	}
 }
 
 var cgoTraceback unsafe.Pointer
-var cgoContext unsafe.Pointer
 var cgoSymbolizer unsafe.Pointer
 
 // cgoTracebackArg is the type passed to cgoTraceback.
@@ -922,7 +972,7 @@ type cgoTracebackArg struct {
 	max     uintptr
 }
 
-// cgoContextArg is the type passed to cgoContext.
+// cgoContextArg is the type passed to the context function.
 type cgoContextArg struct {
 	context uintptr
 }
@@ -950,39 +1000,75 @@ func printCgoTraceback(callers *cgoCallers) {
 		return
 	}
 
-	call := cgocall
-	if panicking > 0 {
-		// We do not want to call into the scheduler when panicking.
-		call = asmcgocall
-	}
-
 	var arg cgoSymbolizerArg
 	for _, c := range callers {
 		if c == 0 {
 			break
 		}
-		arg.pc = c
-		for {
-			call(cgoSymbolizer, noescape(unsafe.Pointer(&arg)))
-			if arg.funcName != nil {
-				// Note that we don't print any argument
-				// information here, not even parentheses.
-				// The symbolizer must add that if
-				// appropriate.
-				println(gostringnocopy(arg.funcName))
-			} else {
-				println("non-Go function")
-			}
-			print("\t")
-			if arg.file != nil {
-				print(gostringnocopy(arg.file), ":", arg.lineno, " ")
-			}
-			print("pc=", hex(c), "\n")
-			if arg.more == 0 {
-				break
-			}
-		}
+		printOneCgoTraceback(c, 0x7fffffff, &arg)
 	}
 	arg.pc = 0
-	call(cgoSymbolizer, noescape(unsafe.Pointer(&arg)))
+	callCgoSymbolizer(&arg)
+}
+
+// printOneCgoTraceback prints the traceback of a single cgo caller.
+// This can print more than one line because of inlining.
+// Returns the number of frames printed.
+func printOneCgoTraceback(pc uintptr, max int, arg *cgoSymbolizerArg) int {
+	c := 0
+	arg.pc = pc
+	for {
+		if c > max {
+			break
+		}
+		callCgoSymbolizer(arg)
+		if arg.funcName != nil {
+			// Note that we don't print any argument
+			// information here, not even parentheses.
+			// The symbolizer must add that if appropriate.
+			println(gostringnocopy(arg.funcName))
+		} else {
+			println("non-Go function")
+		}
+		print("\t")
+		if arg.file != nil {
+			print(gostringnocopy(arg.file), ":", arg.lineno, " ")
+		}
+		print("pc=", hex(c), "\n")
+		c++
+		if arg.more == 0 {
+			break
+		}
+	}
+	return c
+}
+
+// callCgoSymbolizer calls the cgoSymbolizer function.
+func callCgoSymbolizer(arg *cgoSymbolizerArg) {
+	call := cgocall
+	if panicking > 0 || getg().m.curg != getg() {
+		// We do not want to call into the scheduler when panicking
+		// or when on the system stack.
+		call = asmcgocall
+	}
+	call(cgoSymbolizer, noescape(unsafe.Pointer(arg)))
+}
+
+// cgoContextPCs gets the PC values from a cgo traceback.
+func cgoContextPCs(ctxt uintptr, buf []uintptr) {
+	if cgoTraceback == nil {
+		return
+	}
+	call := cgocall
+	if panicking > 0 || getg().m.curg != getg() {
+		// We do not want to call into the scheduler when panicking
+		// or when on the system stack.
+		call = asmcgocall
+	}
+	arg := cgoTracebackArg{
+		context: ctxt,
+		buf:     (*uintptr)(noescape(unsafe.Pointer(&buf[0]))),
+		max:     uintptr(len(buf)),
+	}
+	call(cgoTraceback, noescape(unsafe.Pointer(&arg)))
 }