runtime: make entersyscall/exitsyscall safe for stack splits

It is fundamentally unsafe to grow the stack once someone
has made a call to syscall.Syscall. That function takes 6 uintptr
arguments, but depending on the call some are pointers.
In fact, some might be pointers to stack values, and we don't know which.
That makes it impossible to copy the stack somewhere else.
Since we want to delete all the stack splitting code, relying only
on stack copying, make sure that Syscall never needs to split the stack.

The only thing Syscall does is:
        call entersyscall
        make the system call
        call exitsyscall

As long as we make sure that entersyscall and exitsyscall
can live in the nosplit region, they won't ask for more stack.

Do this by making entersyscall and exitsyscall set up the
stack guard so that any call to a function with a split check
will cause a crash. Then move non-essential slow-path
work onto the m stack using onM and mark the rest of the
work nosplit. The linker will verify that the chain of nosplits
fits in the total nosplit budget.

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, khr, r
https://golang.org/cl/140950043

src/pkg/runtime/proc.c

@@ -502,16 +502,24 @@ runtime·castogscanstatus(G *gp, uint32 oldval, uint32 newval)
 	return false; // not reached
 }
 
+static void badcasgstatus(void);
+static void helpcasgstatus(void);
+
 // If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
 // and casfromgscanstatus instead.
 // casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that 
 // put it in the Gscan state is finished.
+#pragma textflag NOSPLIT
 void
 runtime·casgstatus(G *gp, uint32 oldval, uint32 newval)
 {
-	if(isscanstatus(oldval) || isscanstatus(newval) || oldval == newval) {
-		runtime·printf("casgstatus: oldval=%d, newval=%d\n", oldval, newval);
-		runtime·throw("casgstatus: bad incoming values");
+	void (*fn)(void);
+
+	if((oldval&Gscan) || (newval&Gscan) || oldval == newval) {
+		g->m->scalararg[0] = oldval;
+		g->m->scalararg[1] = newval;
+		fn = badcasgstatus;
+		runtime·onM(&fn);
 	}
 
 	// loop if gp->atomicstatus is in a scan state giving
@@ -520,11 +528,37 @@ runtime·casgstatus(G *gp, uint32 oldval, uint32 newval)
 		// Help GC if needed. 
 		if(gp->preemptscan && !gp->gcworkdone && (oldval == Grunning || oldval == Gsyscall)) {
 			gp->preemptscan = false;
-			runtime·gcphasework(gp);
+			g->m->ptrarg[0] = gp;
+			fn = helpcasgstatus;
+			runtime·onM(&fn);
 		}
 	}	
 }
 
+static void
+badcasgstatus(void)
+{
+	uint32 oldval, newval;
+	
+	oldval = g->m->scalararg[0];
+	newval = g->m->scalararg[1];
+	g->m->scalararg[0] = 0;
+	g->m->scalararg[1] = 0;
+
+	runtime·printf("casgstatus: oldval=%d, newval=%d\n", oldval, newval);
+	runtime·throw("casgstatus: bad incoming values");
+}
+
+static void
+helpcasgstatus(void)
+{
+	G *gp;
+	
+	gp = g->m->ptrarg[0];
+	g->m->ptrarg[0] = 0;
+	runtime·gcphasework(gp);
+}
+
 // stopg ensures that gp is stopped at a GC safe point where its stack can be scanned
 // or in the context of a moving collector the pointers can be flipped from pointing 
 // to old object to pointing to new objects. 
@@ -1770,6 +1804,10 @@ save(void *pc, uintptr sp)
 	g->sched.g = g;
 }
 
+static void entersyscall_bad(void);
+static void entersyscall_sysmon(void);
+static void entersyscall_gcwait(void);
+
 // The goroutine g is about to enter a system call.
 // Record that it's not using the cpu anymore.
 // This is called only from the go syscall library and cgocall,
@@ -1778,14 +1816,31 @@ save(void *pc, uintptr sp)
 // Entersyscall cannot split the stack: the runtime·gosave must
 // make g->sched refer to the caller's stack segment, because
 // entersyscall is going to return immediately after.
+//
+// Nothing entersyscall calls can split the stack either.
+// We cannot safely move the stack during an active call to syscall,
+// because we do not know which of the uintptr arguments are
+// really pointers (back into the stack).
+// In practice, this means that we make the fast path run through
+// entersyscall doing no-split things, and the slow path has to use onM
+// to run bigger things on the m stack.
 #pragma textflag NOSPLIT
 void
 ·entersyscall(int32 dummy)
 {
+	void (*fn)(void);
+
 	// Disable preemption because during this function g is in Gsyscall status,
 	// but can have inconsistent g->sched, do not let GC observe it.
 	g->m->locks++;
 	
+	// Entersyscall must not call any function that might split/grow the stack.
+	// (See details in comment above.)
+	// Catch calls that might, by replacing the stack guard with something that
+	// will trip any stack check and leaving a flag to tell newstack to die.
+	g->stackguard0 = StackPreempt;
+	g->throwsplit = 1;
+
 	// Leave SP around for GC and traceback.
 	save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
 	g->syscallsp = g->sched.sp;
@@ -1794,18 +1849,13 @@ void
 	g->syscallguard = g->stackguard;
 	runtime·casgstatus(g, Grunning, Gsyscall);
 	if(g->syscallsp < g->syscallguard-StackGuard || g->syscallstack < g->syscallsp) {
-		// runtime·printf("entersyscall inconsistent %p [%p,%p]\n",
-		//	g->syscallsp, g->syscallguard-StackGuard, g->syscallstack);
-		runtime·throw("entersyscall");
+		fn = entersyscall_bad;
+		runtime·onM(&fn);
 	}
 
 	if(runtime·atomicload(&runtime·sched.sysmonwait)) {  // TODO: fast atomic
-		runtime·lock(&runtime·sched.lock);
-		if(runtime·atomicload(&runtime·sched.sysmonwait)) {
-			runtime·atomicstore(&runtime·sched.sysmonwait, 0);
-			runtime·notewakeup(&runtime·sched.sysmonnote);
-		}
-		runtime·unlock(&runtime·sched.lock);
+		fn = entersyscall_sysmon;
+		runtime·onM(&fn);
 		save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
 	}
 
@@ -1813,12 +1863,8 @@ void
 	g->m->p->m = nil;
 	runtime·atomicstore(&g->m->p->status, Psyscall);
 	if(runtime·sched.gcwaiting) {
-		runtime·lock(&runtime·sched.lock);
-		if (runtime·sched.stopwait > 0 && runtime·cas(&g->m->p->status, Psyscall, Pgcstop)) {
-			if(--runtime·sched.stopwait == 0)
-				runtime·notewakeup(&runtime·sched.stopnote);
-		}
-		runtime·unlock(&runtime·sched.lock);
+		fn = entersyscall_gcwait;
+		runtime·onM(&fn);
 		save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
 	}
 
@@ -1829,14 +1875,51 @@ void
 	g->m->locks--;
 }
 
+static void
+entersyscall_bad(void)
+{
+	G *gp;
+	
+	gp = g->m->curg;
+	runtime·printf("entersyscall inconsistent %p [%p,%p]\n",
+		gp->syscallsp, gp->syscallguard-StackGuard, gp->syscallstack);
+	runtime·throw("entersyscall");
+}
+
+static void
+entersyscall_sysmon(void)
+{
+	runtime·lock(&runtime·sched.lock);
+	if(runtime·atomicload(&runtime·sched.sysmonwait)) {
+		runtime·atomicstore(&runtime·sched.sysmonwait, 0);
+		runtime·notewakeup(&runtime·sched.sysmonnote);
+	}
+	runtime·unlock(&runtime·sched.lock);
+}
+
+static void
+entersyscall_gcwait(void)
+{
+	runtime·lock(&runtime·sched.lock);
+	if (runtime·sched.stopwait > 0 && runtime·cas(&g->m->p->status, Psyscall, Pgcstop)) {
+		if(--runtime·sched.stopwait == 0)
+			runtime·notewakeup(&runtime·sched.stopnote);
+	}
+	runtime·unlock(&runtime·sched.lock);
+}
+
+static void entersyscallblock_handoff(void);
+
 // The same as runtime·entersyscall(), but with a hint that the syscall is blocking.
 #pragma textflag NOSPLIT
 void
 ·entersyscallblock(int32 dummy)
 {
-	P *p;
+	void (*fn)(void);
 
 	g->m->locks++;  // see comment in entersyscall
+	g->throwsplit = 1;
+	g->stackguard0 = StackPreempt;  // see comment in entersyscall
 
 	// Leave SP around for GC and traceback.
 	save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
@@ -1846,43 +1929,22 @@ void
 	g->syscallguard = g->stackguard;
 	runtime·casgstatus(g, Grunning, Gsyscall);
 	if(g->syscallsp < g->syscallguard-StackGuard || g->syscallstack < g->syscallsp) {
-		// runtime·printf("entersyscall inconsistent %p [%p,%p]\n",
-		//	g->syscallsp, g->syscallguard-StackGuard, g->syscallstack);
-		runtime·throw("entersyscallblock");
+		fn = entersyscall_bad;
+		runtime·onM(&fn);
 	}
 	
-	p = releasep();
-	handoffp(p);
+	fn = entersyscallblock_handoff;
+	runtime·onM(&fn);
 
 	// Resave for traceback during blocked call.
 	save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
 
-	g->stackguard0 = StackPreempt;  // see comment in entersyscall
 	g->m->locks--;
 }
 
-// The same as runtime·entersyscallblock(), but called on g0 stack.
-void
-runtime·entersyscallblock_m(void)
+static void
+entersyscallblock_handoff(void)
 {
-	G *gp;
-
-	gp = g->m->curg;
-	// sched.{g,pc,sp,lr} are already set by mcall.
-	gp->stackguard0 = StackPreempt;  // we are on g0, the goroutine must not touch its stack until exitsyscall
-	gp->sched.ret = 0;
-	gp->sched.ctxt = 0;
-	gp->syscallsp = gp->sched.sp;
-	gp->syscallpc = gp->sched.pc;
-	gp->syscallstack = gp->stackbase;
-	gp->syscallguard = gp->stackguard;
-	runtime·casgstatus(gp, Grunning, Gsyscall);
-	if(gp->syscallsp < gp->syscallguard-StackGuard || gp->syscallstack < gp->syscallsp) {
-		// runtime·printf("entersyscall inconsistent %p [%p,%p]\n",
-		//	gp->syscallsp, gp->syscallguard-StackGuard, gp->syscallstack);
-		runtime·throw("entersyscall_m");
-	}
-
 	handoffp(releasep());
 }
 
@@ -1917,6 +1979,7 @@ runtime·exitsyscall(void)
 			// otherwise restore the real stackguard, we've spoiled it in entersyscall/entersyscallblock
 			g->stackguard0 = g->stackguard;
 		}
+		g->throwsplit = 0;
 		return;
 	}
 
@@ -1935,13 +1998,16 @@ runtime·exitsyscall(void)
 	g->syscallstack = (uintptr)nil;
 	g->syscallsp = (uintptr)nil;
 	g->m->p->syscalltick++;
+	g->throwsplit = 0;
 }
 
+static void exitsyscallfast_pidle(void);
+
 #pragma textflag NOSPLIT
 static bool
 exitsyscallfast(void)
 {
-	P *p;
+	void (*fn)(void);
 
 	// Freezetheworld sets stopwait but does not retake P's.
 	if(runtime·sched.stopwait) {
@@ -1959,6 +2025,21 @@ exitsyscallfast(void)
 	// Try to get any other idle P.
 	g->m->p = nil;
 	if(runtime·sched.pidle) {
+		fn = exitsyscallfast_pidle;
+		runtime·onM(&fn);
+		if(g->m->scalararg[0]) {
+			g->m->scalararg[0] = 0;
+			return true;
+		}
+	}
+	return false;
+}
+
+static void
+exitsyscallfast_pidle(void)
+{
+	P *p;
+
 	runtime·lock(&runtime·sched.lock);
 	p = pidleget();
 	if(p && runtime·atomicload(&runtime·sched.sysmonwait)) {
@@ -1968,10 +2049,9 @@ exitsyscallfast(void)
 	runtime·unlock(&runtime·sched.lock);
 	if(p) {
 		acquirep(p);
-			return true;
-		}
-	}
-	return false;
+		g->m->scalararg[0] = 1;
+	} else
+		g->m->scalararg[0] = 0;
 }
 
 // runtime·exitsyscall slow path on g0.

src/pkg/runtime/runtime.h

@@ -293,6 +293,7 @@ struct	G
 	bool	paniconfault;	// panic (instead of crash) on unexpected fault address
 	bool    preemptscan;    // preempted g does scan for GC
 	bool    gcworkdone;     // debug: cleared at begining of gc work phase cycle, set by gcphasework, tested at end of cycle
+	bool	throwsplit; // must not split stack
 	int8	raceignore;	// ignore race detection events
 	M*	m;		// for debuggers, but offset not hard-coded
 	M*	lockedm;
@@ -867,7 +868,6 @@ int32	runtime·asmcgocall_errno(void (*fn)(void*), void*);
 void	runtime·entersyscall(void);
 void	runtime·entersyscallblock(void);
 void	runtime·exitsyscall(void);
-void	runtime·entersyscallblock_m(void);
 G*	runtime·newproc1(FuncVal*, byte*, int32, int32, void*);
 bool	runtime·sigsend(int32 sig);
 intgo	runtime·callers(intgo, uintptr*, intgo);

src/pkg/runtime/stack.c

@@ -876,6 +876,8 @@ runtime·newstack(void)
 			g->m->morebuf.g, g->m, g->m->curg, g->m->g0, g->m->gsignal);
 		runtime·throw("runtime: wrong goroutine in newstack");
 	}
+	if(g->throwsplit)
+		runtime·throw("runtime: stack split at bad time");
 
 	// The goroutine must be executing in order to call newstack, so the possible states are
 	// Grunning and Gsyscall (and, due to GC, also Gscanrunning and Gscansyscall).