runtime: support symbolic backtrace of C code in a cgo crash

The new function runtime.SetCgoTraceback may be used to register stack
traceback and symbolizer functions, written in C, to do a stack
traceback from cgo code.

There is a sample implementation of runtime.SetCgoSymbolizer at
github.com/ianlancetaylor/cgosymbolizer.  Just importing that package is
sufficient to get symbolic C backtraces.

Currently only supported on linux/amd64.

Change-Id: If96ee2eb41c6c7379d407b9561b87557bfe47341
Reviewed-on: https://go-review.googlesource.com/17761
Reviewed-by: Austin Clements <austin@google.com>

15 files changed, 489 insertions, 5 deletions

diff --git a/src/runtime/cgo.go b/src/runtime/cgo.go
index 8ba31cd481..35d7a07e15 100644
--- a/src/runtime/cgo.go
+++ b/src/runtime/cgo.go
@@ -16,6 +16,7 @@ import "unsafe"
 //go:linkname _cgo_thread_start _cgo_thread_start
 //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
 
 var (
 	_cgo_init                     unsafe.Pointer
@@ -24,6 +25,7 @@ var (
 	_cgo_thread_start             unsafe.Pointer
 	_cgo_sys_thread_create        unsafe.Pointer
 	_cgo_notify_runtime_init_done unsafe.Pointer
+	_cgo_callers                  unsafe.Pointer
 )
 
 // iscgo is set to true by the runtime/cgo package
diff --git a/src/runtime/cgo/callbacks.go b/src/runtime/cgo/callbacks.go
index 47bd2b0edc..4f31d1c82c 100644
--- a/src/runtime/cgo/callbacks.go
+++ b/src/runtime/cgo/callbacks.go
@@ -92,5 +92,13 @@ 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
 
+// Calls the traceback function passed to SetCgoTraceback.
+
+//go:cgo_import_static x_cgo_callers
+//go:linkname x_cgo_callers x_cgo_callers
+//go:linkname _cgo_callers _cgo_callers
+var x_cgo_callers byte
+var _cgo_callers = &x_cgo_callers
+
 //go:cgo_export_static _cgo_topofstack
 //go:cgo_export_dynamic _cgo_topofstack
diff --git a/src/runtime/cgo/gcc_traceback.c b/src/runtime/cgo/gcc_traceback.c
new file mode 100644
index 0000000000..4fdfbe4d9f
--- /dev/null
+++ b/src/runtime/cgo/gcc_traceback.c
@@ -0,0 +1,29 @@
+// 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 linux
+
+#include <stdint.h>
+
+struct cgoTracebackArg {
+	uintptr_t  Context;
+	uintptr_t* Buf;
+	uintptr_t  Max;
+};
+
+// Call the user's traceback function and then call sigtramp.
+// The runtime signal handler will jump to this code.
+// We do it this way so that the user's traceback function will be called
+// by a C function with proper unwind info.
+void
+x_cgo_callers(uintptr_t sig, void *info, void *context, void (*cgoTraceback)(struct cgoTracebackArg*), uintptr_t* cgoCallers, void (*sigtramp)(uintptr_t, void*, void*)) {
+	struct cgoTracebackArg arg;
+
+	arg.Context = 0;
+	arg.Buf = cgoCallers;
+	arg.Max = 32; // must match len(runtime.cgoCallers)
+	(*cgoTraceback)(&arg);
+	sigtramp(sig, info, context);
+}
diff --git a/src/runtime/cgocall.go b/src/runtime/cgocall.go
index 7a683d7524..7cffa83617 100644
--- a/src/runtime/cgocall.go
+++ b/src/runtime/cgocall.go
@@ -84,6 +84,10 @@ import (
 	"unsafe"
 )
 
+// Addresses collected in a cgo backtrace when crashing.
+// Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c.
+type cgoCallers [32]uintptr
+
 // Call from Go to C.
 //go:nosplit
 func cgocall(fn, arg unsafe.Pointer) int32 {
@@ -109,6 +113,14 @@ func cgocall(fn, arg unsafe.Pointer) int32 {
 	mp.ncgo++
 	defer endcgo(mp)
 
+	// Allocate memory to hold a cgo traceback if the cgo call crashes.
+	if mp.cgoCallers == nil {
+		mp.cgoCallers = new(cgoCallers)
+	}
+
+	// Reset traceback.
+	mp.cgoCallers[0] = 0
+
 	/*
 	 * Announce we are entering a system call
 	 * so that the scheduler knows to create another
diff --git a/src/runtime/crash_cgo_test.go b/src/runtime/crash_cgo_test.go
index 2f7591a8d3..6547996b43 100644
--- a/src/runtime/crash_cgo_test.go
+++ b/src/runtime/crash_cgo_test.go
@@ -209,3 +209,15 @@ func TestCgoCCodeSIGPROF(t *testing.T) {
 		t.Errorf("expected %q got %v", want, got)
 	}
 }
+
+func TestCgoCrashTraceback(t *testing.T) {
+	if runtime.GOOS != "linux" || runtime.GOARCH != "amd64" {
+		t.Skipf("not yet supported on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+	got := runTestProg(t, "testprogcgo", "CrashTraceback")
+	for i := 1; i <= 3; i++ {
+		if !strings.Contains(got, fmt.Sprintf("cgo symbolizer:%d", i)) {
+			t.Errorf("missing cgo symbolizer:%d", i)
+		}
+	}
+}
diff --git a/src/runtime/os1_linux.go b/src/runtime/os1_linux.go
index 1c1ead8790..726dd649fe 100644
--- a/src/runtime/os1_linux.go
+++ b/src/runtime/os1_linux.go
@@ -305,6 +305,7 @@ func memlimit() uintptr {
 
 func sigreturn()
 func sigtramp()
+func cgoSigtramp()
 
 //go:nosplit
 //go:nowritebarrierrec
@@ -323,7 +324,11 @@ func setsig(i int32, fn uintptr, restart bool) {
 		sa.sa_restorer = funcPC(sigreturn)
 	}
 	if fn == funcPC(sighandler) {
-		fn = funcPC(sigtramp)
+		if iscgo {
+			fn = funcPC(cgoSigtramp)
+		} else {
+			fn = funcPC(sigtramp)
+		}
 	}
 	sa.sa_handler = fn
 	rt_sigaction(uintptr(i), &sa, nil, unsafe.Sizeof(sa.sa_mask))
@@ -354,7 +359,7 @@ func getsig(i int32) uintptr {
 	if rt_sigaction(uintptr(i), nil, &sa, unsafe.Sizeof(sa.sa_mask)) != 0 {
 		throw("rt_sigaction read failure")
 	}
-	if sa.sa_handler == funcPC(sigtramp) {
+	if sa.sa_handler == funcPC(sigtramp) || sa.sa_handler == funcPC(cgoSigtramp) {
 		return funcPC(sighandler)
 	}
 	return sa.sa_handler
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 457927c804..e0137f7e97 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -373,8 +373,10 @@ type m struct {
 	newSigstack   bool // minit on C thread called sigaltstack
 	printlock     int8
 	fastrand      uint32
-	ncgocall      uint64 // number of cgo calls in total
-	ncgo          int32  // number of cgo calls currently in progress
+	ncgocall      uint64      // number of cgo calls in total
+	ncgo          int32       // number of cgo calls currently in progress
+	cgoCallersUse uint32      // if non-zero, cgoCallers in use temporarily
+	cgoCallers    *cgoCallers // cgo traceback if crashing in cgo call
 	park          note
 	alllink       *m // on allm
 	schedlink     muintptr
diff --git a/src/runtime/sys_linux_386.s b/src/runtime/sys_linux_386.s
index 3bf5eb0df4..4fe07e0837 100644
--- a/src/runtime/sys_linux_386.s
+++ b/src/runtime/sys_linux_386.s
@@ -232,6 +232,9 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$12
 	CALL	runtime·sigtrampgo(SB)
 	RET
 
+TEXT runtime·cgoSigtramp(SB),NOSPLIT,$0
+	JMP	runtime·sigtramp(SB)
+
 TEXT runtime·sigreturn(SB),NOSPLIT,$0
 	MOVL	$173, AX	// rt_sigreturn
 	// Sigreturn expects same SP as signal handler,
diff --git a/src/runtime/sys_linux_amd64.s b/src/runtime/sys_linux_amd64.s
index 7cab649238..031e412673 100644
--- a/src/runtime/sys_linux_amd64.s
+++ b/src/runtime/sys_linux_amd64.s
@@ -234,8 +234,65 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$24
 	CALL AX
 	RET
 
+// Used instead of sigtramp in programs that use cgo.
+// Arguments from kernel are in DI, SI, DX.
+TEXT runtime·cgoSigtramp(SB),NOSPLIT,$0
+	// If no traceback function, do usual sigtramp.
+	MOVQ	runtime·cgoTraceback(SB), AX
+	TESTQ	AX, AX
+	JZ	sigtramp
+
+	// If no traceback support function, which means that
+	// runtime/cgo was not linked in, do usual sigtramp.
+	MOVQ	_cgo_callers(SB), AX
+	TESTQ	AX, AX
+	JZ	sigtramp
+
+	// Figure out if we are currently in a cgo call.
+	// If not, just do usual sigtramp.
+	get_tls(CX)
+	MOVQ	g(CX),AX
+	TESTQ	AX, AX
+	JZ	sigtramp        // g == nil
+	MOVQ	g_m(AX), AX
+	TESTQ	AX, AX
+	JZ	sigtramp        // g.m == nil
+	MOVL	m_ncgo(AX), CX
+	TESTL	CX, CX
+	JZ	sigtramp        // g.m.ncgo == 0
+	MOVQ	m_curg(AX), CX
+	TESTQ	CX, CX
+	JZ	sigtramp        // g.m.curg == nil
+	MOVQ	g_syscallsp(CX), CX
+	TESTQ	CX, CX
+	JZ	sigtramp        // g.m.curg.syscallsp == 0
+	MOVQ	m_cgoCallers(AX), R8
+	TESTQ	R8, R8
+	JZ	sigtramp        // g.m.cgoCallers == nil
+	MOVL	m_cgoCallersUse(AX), CX
+	TESTL	CX, CX
+	JNZ	sigtramp	// g.m.cgoCallersUse != 0
+
+	// Jump to a function in runtime/cgo.
+	// That function, written in C, will call the user's traceback
+	// function with proper unwind info, and will then call back here.
+	// The first three arguments are already in registers.
+	// Set the last three arguments now.
+	MOVQ	runtime·cgoTraceback(SB), CX
+	MOVQ	$runtime·sigtramp(SB), R9
+	MOVQ	_cgo_callers(SB), AX
+	JMP	AX
+
+sigtramp:
+	JMP	runtime·sigtramp(SB)
+
+// For cgo unwinding to work, this function must look precisely like
+// the one in glibc.  The glibc source code is:
+// https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86_64/sigaction.c
+// The code that cares about the precise instructions used is:
+// https://gcc.gnu.org/viewcvs/gcc/trunk/libgcc/config/i386/linux-unwind.h?revision=219188&view=markup
 TEXT runtime·sigreturn(SB),NOSPLIT,$0
-	MOVL	$15, AX	// rt_sigreturn
+	MOVQ	$15, AX	// rt_sigreturn
 	SYSCALL
 	INT $3	// not reached
 
diff --git a/src/runtime/sys_linux_arm.s b/src/runtime/sys_linux_arm.s
index 50a551320a..5e5fcf0e6f 100644
--- a/src/runtime/sys_linux_arm.s
+++ b/src/runtime/sys_linux_arm.s
@@ -361,6 +361,10 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$12
 	BL	(R11)
 	RET
 
+TEXT runtime·cgoSigtramp(SB),NOSPLIT,$0
+	MOVW  	$runtime·sigtramp(SB), R11
+	B	(R11)
+
 TEXT runtime·rtsigprocmask(SB),NOSPLIT,$0
 	MOVW	sig+0(FP), R0
 	MOVW	new+4(FP), R1
diff --git a/src/runtime/sys_linux_arm64.s b/src/runtime/sys_linux_arm64.s
index 94c101a3d4..1bee8477ed 100644
--- a/src/runtime/sys_linux_arm64.s
+++ b/src/runtime/sys_linux_arm64.s
@@ -259,6 +259,10 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$24
 	BL	(R0)
 	RET
 
+TEXT runtime·cgoSigtramp(SB),NOSPLIT,$0
+	MOVD	$runtime·sigtramp(SB), R3
+	B	(R3)
+
 TEXT runtime·mmap(SB),NOSPLIT,$-8
 	MOVD	addr+0(FP), R0
 	MOVD	n+8(FP), R1
diff --git a/src/runtime/sys_linux_mips64x.s b/src/runtime/sys_linux_mips64x.s
index 26437ddde4..f6877cb32d 100644
--- a/src/runtime/sys_linux_mips64x.s
+++ b/src/runtime/sys_linux_mips64x.s
@@ -249,6 +249,10 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$64
 	JAL	(R1)
 	RET
 
+TEXT runtime·cgoSigtramp(SB),NOSPLIT,$0
+	MOVV	$runtime·sigtramp(SB), R1
+	JMP	(R1)
+
 TEXT runtime·mmap(SB),NOSPLIT,$-8
 	MOVV	addr+0(FP), R4
 	MOVV	n+8(FP), R5
diff --git a/src/runtime/sys_linux_ppc64x.s b/src/runtime/sys_linux_ppc64x.s
index d063e025a6..56b842ac01 100644
--- a/src/runtime/sys_linux_ppc64x.s
+++ b/src/runtime/sys_linux_ppc64x.s
@@ -243,6 +243,21 @@ TEXT runtime·_sigtramp(SB),NOSPLIT,$64
 	MOVD	24(R1), R2
 	RET
 
+#ifdef GOARCH_ppc64le
+// ppc64le doesn't need function descriptors
+TEXT runtime·cgoSigtramp(SB),NOSPLIT,$0
+#else
+// function descriptor for the real sigtramp
+TEXT runtime·cgoSigtramp(SB),NOSPLIT|NOFRAME,$0
+	DWORD	$runtime·_cgoSigtramp(SB)
+	DWORD	$0
+	DWORD	$0
+TEXT runtime·_cgoSigtramp(SB),NOSPLIT,$0
+#endif
+	MOVD	$runtime·sigtramp(SB), R12
+	MOVD	R12, CTR
+	JMP	(CTR)
+
 TEXT runtime·mmap(SB),NOSPLIT|NOFRAME,$0
 	MOVD	addr+0(FP), R3
 	MOVD	n+8(FP), R4
diff --git a/src/runtime/testdata/testprogcgo/traceback.go b/src/runtime/testdata/testprogcgo/traceback.go
new file mode 100644
index 0000000000..bb3e70a44f
--- /dev/null
+++ b/src/runtime/testdata/testprogcgo/traceback.go
@@ -0,0 +1,80 @@
+// 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.
+
+package main
+
+// This program will crash.
+// We want the stack trace to include the C functions.
+// We use a fake traceback, and a symbolizer that dumps a string we recognize.
+
+/*
+#cgo CFLAGS: -g -O0
+
+#include <stdint.h>
+
+char *p;
+
+static int f3() {
+	*p = 0;
+	return 0;
+}
+
+static int f2() {
+	return f3();
+}
+
+static int f1() {
+	return f2();
+}
+
+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;
+};
+
+void cgoTraceback(void* parg) {
+	struct cgoTracebackArg* arg = (struct cgoTracebackArg*)(parg);
+	arg->buf[0] = 1;
+	arg->buf[1] = 2;
+	arg->buf[2] = 3;
+	arg->buf[3] = 0;
+}
+
+void cgoSymbolizer(void* parg) {
+	struct cgoSymbolizerArg* arg = (struct cgoSymbolizerArg*)(parg);
+	if (arg->pc != arg->data + 1) {
+		arg->file = "unexpected data";
+	} else {
+		arg->file = "cgo symbolizer";
+	}
+	arg->lineno = arg->data + 1;
+	arg->data++;
+}
+*/
+import "C"
+
+import (
+	"runtime"
+	"unsafe"
+)
+
+func init() {
+	register("CrashTraceback", CrashTraceback)
+}
+
+func CrashTraceback() {
+	runtime.SetCgoTraceback(0, unsafe.Pointer(C.cgoTraceback), nil, unsafe.Pointer(C.cgoSymbolizer))
+	C.f1()
+}
diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index 872b2ef903..16b9278641 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -5,6 +5,7 @@
 package runtime
 
 import (
+	"runtime/internal/atomic"
 	"runtime/internal/sys"
 	"unsafe"
 )
@@ -579,6 +580,22 @@ func tracebacktrap(pc, sp, lr uintptr, gp *g) {
 }
 
 func traceback1(pc, sp, lr uintptr, gp *g, flags uint) {
+	// If the goroutine is in cgo, and we have a cgo traceback, print that.
+	if iscgo && gp.m != nil && gp.m.ncgo > 0 && gp.syscallsp != 0 && gp.m.cgoCallers != nil && gp.m.cgoCallers[0] != 0 {
+		// Lock cgoCallers so that a signal handler won't
+		// change it, copy the array, reset it, unlock it.
+		// We are locked to the thread and are not running
+		// concurrently with a signal handler.
+		// We just have to stop a signal handler from interrupting
+		// in the middle of our copy.
+		atomic.Store(&gp.m.cgoCallersUse, 1)
+		cgoCallers := *gp.m.cgoCallers
+		gp.m.cgoCallers[0] = 0
+		atomic.Store(&gp.m.cgoCallersUse, 0)
+
+		printCgoTraceback(&cgoCallers)
+	}
+
 	var n int
 	if readgstatus(gp)&^_Gscan == _Gsyscall {
 		// Override registers if blocked in system call.
@@ -739,3 +756,233 @@ func isSystemGoroutine(gp *g) bool {
 		pc == timerprocPC ||
 		pc == gcBgMarkWorkerPC
 }
+
+// SetCgoTraceback records three C functions to use to gather
+// traceback information from C code and to convert that traceback
+// information into symbolic information. These are used when printing
+// stack traces for a program that uses cgo.
+//
+// The traceback and context functions may be called from a signal
+// handler, and must therefore use only async-signal safe functions.
+// The symbolizer function may be called while the program is
+// crashing, and so must be cautious about using memory.  None of the
+// functions may call back into Go.
+//
+// The context function will be called with a single argument, a
+// pointer to a struct:
+//
+// struct {
+//	Context uintptr
+// }
+//
+// In C syntax, this struct will be
+//
+// struct {
+//   uintptr_t Context;
+// };
+//
+// 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.
+//
+// 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
+// context is no longer needed; that is, when the Go code is returning
+// to its C code caller. This permits permits the context function to
+// release any associated resources.
+//
+// While it would be correct for the context function to record a
+// complete a stack trace whenever it is called, and simply copy that
+// out in the traceback function, in a typical program the context
+// function will be called many times without ever recording a
+// traceback for that context. Recording a complete stack trace in a
+// call to the context function is likely to be inefficient.
+//
+// The traceback function will be called with a single argument, a
+// pointer to a struct:
+//
+// struct {
+//	Context uintptr
+//	Buf     *uintptr
+//	Max     uintptr
+// }
+//
+// In C syntax, this struct will be
+//
+// struct {
+//   uintptr_t  Context;
+//   uintptr_t* Buf;
+//   uintptr_t  Max;
+// };
+//
+// The Context field will be zero to gather a traceback from the
+// current program execution point. In this case, the traceback
+// function will be called from C code.
+//
+// Otherwise Context will be a value previously returned by a call to
+// the context function. The traceback function should gather a stack
+// trace from that saved point in the program execution. The traceback
+// function may be called from an execution thread other than the one
+// that recorded the context, but only when the context is known to be
+// valid and unchanging. The traceback function may also be called
+// deeper in the call stack on the same thread that recorded the
+// context. The traceback function may be called multiple times with
+// the same Context value; it will usually be appropriate to cache the
+// result, if possible, the first time this is called for a specific
+// context value.
+//
+// Buf is where the traceback information should be stored. It should
+// be PC values, such that Buf[0] is the PC of the caller, Buf[1] is
+// the PC of that function's caller, and so on.  Max is the maximum
+// number of entries to store.  The function should store a zero to
+// indicate the top of the stack, or that the caller is on a different
+// stack, presumably a Go stack.
+//
+// Unlike runtime.Callers, the PC values returned should, when passed
+// to the symbolizer function, return the file/line of the call
+// instruction.  No additional subtraction is required or appropriate.
+//
+// The symbolizer function will be called with a single argument, a
+// pointer to a struct:
+//
+// struct {
+//	PC      uintptr // program counter to fetch information for
+//	File    *byte   // file name (NUL terminated)
+//	Lineno  uintptr // line number
+//	Func    *byte   // function name (NUL terminated)
+//	Entry   uintptr // function entry point
+//	More    uintptr // set non-zero if more info for this PC
+//	Data    uintptr // unused by runtime, available for function
+// }
+//
+// In C syntax, this struct will be
+//
+// struct {
+//   uintptr_t PC;
+//   char*     File;
+//   uintptr_t Lineno;
+//   char*     Func;
+//   uintptr_t Entry;
+//   uintptr_t More;
+//   uintptr_t Data;
+// };
+//
+// The PC field will be a value returned by a call to the traceback
+// function.
+//
+// The first time the function is called for a particular traceback,
+// all the fields except PC will be 0. The function should fill in the
+// other fields if possible, setting them to 0/nil if the information
+// is not available. The Data field may be used to store any useful
+// information across calls. The More field should be set to non-zero
+// if there is more information for this PC, zero otherwise. If More
+// is set non-zero, the function will be called again with the same
+// PC, and may return different information (this is intended for use
+// with inlined functions). If More is zero, the function will be
+// called with the next PC value in the traceback. When the traceback
+// is complete, the function will be called once more with PC set to
+// zero; this may be used to free any information. Each call will
+// leave the fields of the struct set to the same values they had upon
+// return, except for the PC field when the More field is zero. The
+// function must not keep a copy of the struct pointer between calls.
+//
+// When calling SetCgoTraceback, the version argument is the version
+// number of the structs that the functions expect to receive.
+// Currently this must be zero.
+//
+// The symbolizer function may be nil, in which case the results of
+// the traceback function will be displayed as numbers. If the
+// traceback function is nil, the symbolizer function will never be
+// called. The context function may be nil, in which case the
+// traceback function will only be called with the context field set
+// to zero.  If the context function is nil, then calls from Go to C
+// to Go will not show a traceback for the C portion of the call stack.
+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
+}
+
+var cgoTraceback unsafe.Pointer
+var cgoContext unsafe.Pointer
+var cgoSymbolizer unsafe.Pointer
+
+// cgoTracebackArg is the type passed to cgoTraceback.
+type cgoTracebackArg struct {
+	context uintptr
+	buf     *uintptr
+	max     uintptr
+}
+
+// cgoContextArg is the type passed to cgoContext.
+type cgoContextArg struct {
+	context uintptr
+}
+
+// cgoSymbolizerArg is the type passed to cgoSymbolizer.
+type cgoSymbolizerArg struct {
+	pc       uintptr
+	file     *byte
+	lineno   uintptr
+	funcName *byte
+	entry    uintptr
+	more     uintptr
+	data     uintptr
+}
+
+// cgoTraceback prints a traceback of callers.
+func printCgoTraceback(callers *cgoCallers) {
+	if cgoSymbolizer == nil {
+		for _, c := range callers {
+			if c == 0 {
+				break
+			}
+			print("non-Go function at pc=", hex(c), "\n")
+		}
+		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
+			}
+		}
+	}
+	arg.pc = 0
+	call(cgoSymbolizer, noescape(unsafe.Pointer(&arg)))
+}