runtime: add GODEBUG invalidptr setting

Fixes #8861.
Fixes #8911.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/165780043

src/runtime/extern.go

@@ -39,6 +39,12 @@ a comma-separated list of name=val pairs. Supported names are:
 	gcdead: setting gcdead=1 causes the garbage collector to clobber all stack slots
 	that it thinks are dead.
 
+	invalidptr: defaults to invalidptr=1, causing the garbage collector and stack
+	copier to crash the program if an invalid pointer value (for example, 1)
+	is found in a pointer-typed location. Setting invalidptr=0 disables this check.
+	This should only be used as a temporary workaround to diagnose buggy code.
+	The real fix is to not store integers in pointer-typed locations.
+
 	scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit
 	detailed multiline info every X milliseconds, describing state of the scheduler,
 	processors, threads and goroutines.

src/runtime/mgc0.c

@@ -330,7 +330,7 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 			if(obj == nil)
 				continue;
 			if(obj < arena_start || obj >= arena_used) {
-				if((uintptr)obj < PhysPageSize) {
+				if((uintptr)obj < PhysPageSize && runtime·invalidptr) {
 					s = nil;
 					goto badobj;
 				}
@@ -375,7 +375,7 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 					else
 						runtime·printf(" span=%p-%p-%p state=%d\n", (uintptr)s->start<<PageShift, s->limit, (uintptr)(s->start+s->npages)<<PageShift, s->state);
 					if(ptrmask != nil)
-						runtime·throw("bad pointer");
+						runtime·throw("invalid heap pointer");
 					// Add to badblock list, which will cause the garbage collection
 					// to keep repeating until it has traced the chain of pointers
 					// leading to obj all the way back to a root.

src/runtime/runtime.c

@@ -276,9 +276,13 @@ struct DbgVar
 	int32*	value;
 };
 
+// Do we report invalid pointers found during stack or heap scans?
+int32 runtime·invalidptr = 1;
+
 #pragma dataflag NOPTR /* dbgvar has no heap pointers */
 static DbgVar dbgvar[] = {
 	{"allocfreetrace", &runtime·debug.allocfreetrace},
+	{"invalidptr", &runtime·invalidptr},
 	{"efence", &runtime·debug.efence},
 	{"gctrace", &runtime·debug.gctrace},
 	{"gcdead", &runtime·debug.gcdead},

src/runtime/runtime.h

@@ -657,6 +657,8 @@ enum {
 byte*	runtime·startup_random_data;
 uint32	runtime·startup_random_data_len;
 
+int32	runtime·invalidptr;
+
 enum {
 	// hashinit wants this many random bytes
 	HashRandomBytes = 32

src/runtime/stack.c

@@ -401,12 +401,12 @@ adjustpointers(byte **scanp, BitVector *bv, AdjustInfo *adjinfo, Func *f)
 			break;
 		case BitsPointer:
 			p = scanp[i];
-			if(f != nil && (byte*)0 < p && (p < (byte*)PageSize || (uintptr)p == PoisonGC || (uintptr)p == PoisonStack)) {
+			if(f != nil && (byte*)0 < p && (p < (byte*)PageSize && runtime·invalidptr || (uintptr)p == PoisonGC || (uintptr)p == PoisonStack)) {
 				// Looks like a junk value in a pointer slot.
 				// Live analysis wrong?
 				g->m->traceback = 2;
 				runtime·printf("runtime: bad pointer in frame %s at %p: %p\n", runtime·funcname(f), &scanp[i], p);
-				runtime·throw("bad pointer!");
+				runtime·throw("invalid stack pointer");
 			}
 			if(minp <= p && p < maxp) {
 				if(StackDebug >= 3)