runtime: detect stack split after fork

This check would allowed to easily prevent issue 7511.
Update #7511

LGTM=rsc
R=rsc, aram
CC=golang-codereviews
https://golang.org/cl/75260043

src/pkg/runtime/proc.c

@@ -1704,14 +1704,28 @@ syscall·runtime_BeforeFork(void)
 	m->locks++;
 	if(m->profilehz != 0)
 		runtime·resetcpuprofiler(0);
+
+	// This function is called before fork in syscall package.
+	// Code between fork and exec must not allocate memory nor even try to grow stack.
+	// Here we spoil g->stackguard to reliably detect any attempts to grow stack.
+	// runtime_AfterFork will undo this in parent process, but not in child.
+	m->forkstackguard = g->stackguard;
+	g->stackguard0 = StackPreempt-1;
+	g->stackguard = StackPreempt-1;
 }
 
 // Called from syscall package after fork in parent.
+#pragma textflag NOSPLIT
 void
 syscall·runtime_AfterFork(void)
 {
 	int32 hz;
 
+	// See the comment in runtime_BeforeFork.
+	g->stackguard0 = m->forkstackguard;
+	g->stackguard = m->forkstackguard;
+	m->forkstackguard = 0;
+
 	hz = runtime·sched.profilehz;
 	if(hz != 0)
 		runtime·resetcpuprofiler(hz);

src/pkg/runtime/runtime.h

@@ -367,6 +367,7 @@ struct	M
 	bool	needextram;
 	bool	(*waitunlockf)(G*, void*);
 	void*	waitlock;
+	uintptr	forkstackguard;
 #ifdef GOOS_windows
 	void*	thread;		// thread handle
 	// these are here because they are too large to be on the stack

src/pkg/runtime/stack.c

@@ -583,6 +583,8 @@ runtime·newstack(void)
 	Gobuf label;
 	bool newstackcall;
 
+	if(m->forkstackguard)
+		runtime·throw("split stack after fork");
 	if(m->morebuf.g != m->curg) {
 		runtime·printf("runtime: newstack called from g=%p\n"
 			"\tm=%p m->curg=%p m->g0=%p m->gsignal=%p\n",