runtime/debug: add SetPanicOnFault

SetPanicOnFault allows recovery from unexpected memory faults.
This can be useful if you are using a memory-mapped file
or probing the address space of the current program.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/66590044

14 files changed, 52 insertions, 15 deletions

diff --git a/doc/go1.3.txt b/doc/go1.3.txt
index 31dfd38a1..613d26197 100644
--- a/doc/go1.3.txt
+++ b/doc/go1.3.txt
@@ -8,3 +8,4 @@ testing: add b.RunParallel function (CL 57270043)
 misc/benchcmp has been replaced by go tool benchcmp (CL 47980043)
 cmd/go, go/build: support .m files (CL 60590044)
 unicode: upgrade from Unicode 6.2.0 to 6.3.0 (CL 65400044)
+runtime/debug: add SetPanicOnFault (CL 66590044)
diff --git a/src/pkg/runtime/debug/garbage.go b/src/pkg/runtime/debug/garbage.go
index a724fdf8f..093e7e7b1 100644
--- a/src/pkg/runtime/debug/garbage.go
+++ b/src/pkg/runtime/debug/garbage.go
@@ -135,3 +135,14 @@ func SetMaxStack(bytes int) int {
 func SetMaxThreads(threads int) int {
 	return setMaxThreads(threads)
 }
+
+// SetPanicOnFault controls the runtime's behavior when a program faults
+// at an unexpected (non-nil) address. Such faults are typically caused by
+// bugs such as runtime memory corruption, so the default response is to crash
+// the program. Programs working with memory-mapped files or unsafe
+// manipulation of memory may cause faults at non-nil addresses in less
+// dramatic situations; SetPanicOnFault allows such programs to request
+// that the runtime trigger only a panic, not a crash.
+// SetPanicOnFault applies only to the current goroutine.
+// It returns the previous setting.
+func SetPanicOnFault(enabled bool) bool
diff --git a/src/pkg/runtime/os_darwin.c b/src/pkg/runtime/os_darwin.c
index 5a38a77a3..f2262618d 100644
--- a/src/pkg/runtime/os_darwin.c
+++ b/src/pkg/runtime/os_darwin.c
@@ -436,7 +436,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case SIGBUS:
-		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000) {
+		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
@@ -444,7 +444,7 @@ runtime·sigpanic(void)
 		runtime·printf("unexpected fault address %p\n", g->sigcode1);
 		runtime·throw("fault");
 	case SIGSEGV:
-		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000) {
+		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/os_dragonfly.c b/src/pkg/runtime/os_dragonfly.c
index ade3adb26..f96ea8948 100644
--- a/src/pkg/runtime/os_dragonfly.c
+++ b/src/pkg/runtime/os_dragonfly.c
@@ -171,7 +171,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case SIGBUS:
-		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000) {
+		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
@@ -179,7 +179,7 @@ runtime·sigpanic(void)
 		runtime·printf("unexpected fault address %p\n", g->sigcode1);
 		runtime·throw("fault");
 	case SIGSEGV:
-		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000) {
+		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/os_freebsd.c b/src/pkg/runtime/os_freebsd.c
index 442c6d767..8b7b0f626 100644
--- a/src/pkg/runtime/os_freebsd.c
+++ b/src/pkg/runtime/os_freebsd.c
@@ -179,7 +179,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case SIGBUS:
-		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000) {
+		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
@@ -187,7 +187,7 @@ runtime·sigpanic(void)
 		runtime·printf("unexpected fault address %p\n", g->sigcode1);
 		runtime·throw("fault");
 	case SIGSEGV:
-		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000) {
+		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/os_linux.c b/src/pkg/runtime/os_linux.c
index 130e1059e..b4be9406e 100644
--- a/src/pkg/runtime/os_linux.c
+++ b/src/pkg/runtime/os_linux.c
@@ -220,7 +220,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case SIGBUS:
-		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000) {
+		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
@@ -228,7 +228,7 @@ runtime·sigpanic(void)
 		runtime·printf("unexpected fault address %p\n", g->sigcode1);
 		runtime·throw("fault");
 	case SIGSEGV:
-		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000) {
+		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/os_netbsd.c b/src/pkg/runtime/os_netbsd.c
index 745818183..f8ae30985 100644
--- a/src/pkg/runtime/os_netbsd.c
+++ b/src/pkg/runtime/os_netbsd.c
@@ -239,7 +239,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case SIGBUS:
-		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000) {
+		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
@@ -247,7 +247,7 @@ runtime·sigpanic(void)
 		runtime·printf("unexpected fault address %p\n", g->sigcode1);
 		runtime·throw("fault");
 	case SIGSEGV:
-		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000) {
+		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/os_openbsd.c b/src/pkg/runtime/os_openbsd.c
index 45f5040c9..c16276aca 100644
--- a/src/pkg/runtime/os_openbsd.c
+++ b/src/pkg/runtime/os_openbsd.c
@@ -216,7 +216,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case SIGBUS:
-		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000) {
+		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
@@ -224,7 +224,7 @@ runtime·sigpanic(void)
 		runtime·printf("unexpected fault address %p\n", g->sigcode1);
 		runtime·throw("fault");
 	case SIGSEGV:
-		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000) {
+		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/os_solaris.c b/src/pkg/runtime/os_solaris.c
index 4e212f544..b8cd4d90c 100644
--- a/src/pkg/runtime/os_solaris.c
+++ b/src/pkg/runtime/os_solaris.c
@@ -211,7 +211,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case SIGBUS:
-		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000) {
+		if(g->sigcode0 == BUS_ADRERR && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
@@ -219,7 +219,7 @@ runtime·sigpanic(void)
 		runtime·printf("unexpected fault address %p\n", g->sigcode1);
 		runtime·throw("fault");
 	case SIGSEGV:
-		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000) {
+		if((g->sigcode0 == 0 || g->sigcode0 == SEGV_MAPERR || g->sigcode0 == SEGV_ACCERR) && g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/os_windows.c b/src/pkg/runtime/os_windows.c
index 8815aee9f..a84b4e283 100644
--- a/src/pkg/runtime/os_windows.c
+++ b/src/pkg/runtime/os_windows.c
@@ -348,7 +348,7 @@ runtime·sigpanic(void)
 {
 	switch(g->sig) {
 	case EXCEPTION_ACCESS_VIOLATION:
-		if(g->sigcode1 < 0x1000) {
+		if(g->sigcode1 < 0x1000 || g->paniconfault) {
 			if(g->sigpc == 0)
 				runtime·panicstring("call of nil func value");
 			runtime·panicstring("invalid memory address or nil pointer dereference");
diff --git a/src/pkg/runtime/proc.c b/src/pkg/runtime/proc.c
index 7799dc8b5..986136d7e 100644
--- a/src/pkg/runtime/proc.c
+++ b/src/pkg/runtime/proc.c
@@ -1449,6 +1449,7 @@ goexit0(G *gp)
 	gp->status = Gdead;
 	gp->m = nil;
 	gp->lockedm = nil;
+	gp->paniconfault = 0;
 	m->curg = nil;
 	m->lockedg = nil;
 	if(m->locked & ~LockExternal) {
diff --git a/src/pkg/runtime/rdebug.goc b/src/pkg/runtime/rdebug.goc
index e5b57c481..042b30ace 100644
--- a/src/pkg/runtime/rdebug.goc
+++ b/src/pkg/runtime/rdebug.goc
@@ -20,3 +20,8 @@ func setGCPercent(in int) (out int) {
 func setMaxThreads(in int) (out int) {
 	out = runtime·setmaxthreads(in);
 }
+
+func SetPanicOnFault(enabled bool) (old bool) {
+	old = g->paniconfault;
+	g->paniconfault = enabled;
+}
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 3d5b6007a..8d07294e8 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -273,6 +273,7 @@ struct	G
 	bool	issystem;	// do not output in stack dump
 	bool	isbackground;	// ignore in deadlock detector
 	bool	preempt;	// preemption signal, duplicates stackguard0 = StackPreempt
+	bool	paniconfault;	// panic (instead of crash) on unexpected fault address
 	int8	raceignore;	// ignore race detection events
 	M*	m;		// for debuggers, but offset not hard-coded
 	M*	lockedm;
diff --git a/src/pkg/runtime/runtime_test.go b/src/pkg/runtime/runtime_test.go
index c67327562..83489480d 100644
--- a/src/pkg/runtime/runtime_test.go
+++ b/src/pkg/runtime/runtime_test.go
@@ -10,9 +10,11 @@ import (
 	"os"
 	"os/exec"
 	. "runtime"
+	"runtime/debug"
 	"strconv"
 	"strings"
 	"testing"
+	"unsafe"
 )
 
 var errf error
@@ -131,3 +133,19 @@ func TestRuntimeGogoBytes(t *testing.T) {
 func TestStopCPUProfilingWithProfilerOff(t *testing.T) {
 	SetCPUProfileRate(0)
 }
+
+func TestSetPanicOnFault(t *testing.T) {
+	old := debug.SetPanicOnFault(true)
+	defer debug.SetPanicOnFault(old)
+
+	defer func() {
+		if err := recover(); err == nil {
+			t.Fatalf("did not find error in recover")
+		}
+	}()
+
+	var p *int
+	p = (*int)(unsafe.Pointer(^uintptr(0)))
+	println(*p)
+	t.Fatalf("still here - should have faulted")
+}