runtime: abort stack scan in cases that we cannot unwind the stack

In signal-triggered stack scan, if the signal is delivered at
    certain bad time (e.g. in vdso, or in the middle of setcontext?),
    the unwinder may not be able to unwind the whole stack, while it
    still reports _URC_END_OF_STACK. So we cannot rely on _URC_END_OF_STACK
    to tell if it successfully scanned the stack. Instead, we check
    the last Go frame to see it actually reached the end of the stack.
    For Go-created stack, this is runtime.kickoff. For C-created
    stack, we need to record the outermost Go frame when it enters
    the Go side.
    
    Also we cannot unwind the stack if the signal is delivered in the
    middle of runtime.gogo, halfway through a goroutine switch, where
    the g and the stack don't match. Give up in this case as well.
    
    Reviewed-on: https://go-review.googlesource.com/c/159098

From-SVN: r269018

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-9605c2efd99aa9c744652a9153e208e0653b8596
+672572130ba7a7b277a4c9c8f93576fc42accf63
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/cgo_gccgo.go

@@ -80,6 +80,10 @@ func CgocallBack() {
 		gp = getg()
 		mp := gp.m
 		mp.dropextram = true
+
+		// This is a C-created stack.
+		// Record the outermost Go frame to help stack scan.
+		gp.entrysp = getcallersp()
 	}
 
 	lockOSThread()

libgo/go/runtime/proc.go

@@ -1192,6 +1192,9 @@ func kickoff() {
 		gp.param = nil
 	}
 
+	// Record the entry SP to help stack scan.
+	gp.entrysp = getsp()
+
 	fv(param)
 	goexit1()
 }

libgo/go/runtime/runtime2.go

@@ -433,6 +433,7 @@ type g struct {
 
 	entry    func(unsafe.Pointer) // goroutine function to run
 	entryfn  uintptr              // function address passed to __go_go
+	entrysp  uintptr              // the stack pointer of the outermost Go frame
 	fromgogo bool                 // whether entered from gogo function
 
 	scanningself bool // whether goroutine is scanning its own stack

libgo/go/runtime/stubs.go

@@ -231,6 +231,10 @@ func getcallerpc() uintptr
 //go:noescape
 func getcallersp() uintptr // implemented as an intrinsic on all platforms
 
+// getsp returns the stack pointer (SP) of the caller of getsp.
+//go:noinline
+func getsp() uintptr { return getcallersp() }
+
 func asmcgocall(fn, arg unsafe.Pointer) int32 {
 	throw("asmcgocall")
 	return 0

libgo/runtime/go-unwind.c

@@ -649,6 +649,19 @@ findstackmaps (struct _Unwind_Context *context, _Unwind_Ptr *ip, _Unwind_Ptr *sp
   _sleb128_t index;
   int size;
 
+#ifdef HAVE_GETIPINFO
+  ip1 = _Unwind_GetIPInfo (context, &ip_before_insn);
+#else
+  ip1 = _Unwind_GetIP (context);
+#endif
+  if (! ip_before_insn)
+    --ip1;
+
+  if (ip != NULL)
+    *ip = ip1;
+  if (sp != NULL)
+    *sp = _Unwind_GetCFA (context);
+
 #ifdef __ARM_EABI_UNWINDER__
   {
     _Unwind_Control_Block *ucbp;
@@ -672,14 +685,6 @@ findstackmaps (struct _Unwind_Context *context, _Unwind_Ptr *ip, _Unwind_Ptr *sp
   if (info.TType == NULL)
     return NOTFOUND_OK;
 
-#ifdef HAVE_GETIPINFO
-  ip1 = _Unwind_GetIPInfo (context, &ip_before_insn);
-#else
-  ip1 = _Unwind_GetIP (context);
-#endif
-  if (! ip_before_insn)
-    --ip1;
-
   size = value_size (info.ttype_encoding);
 
   action_record = NULL;
@@ -738,15 +743,16 @@ findstackmaps (struct _Unwind_Context *context, _Unwind_Ptr *ip, _Unwind_Ptr *sp
   if (stackmap1 == NULL)
     return NOTFOUND_BAD;
 
-  if (ip != NULL)
-    *ip = ip1;
-  if (sp != NULL)
-    *sp = _Unwind_GetCFA (context);
   if (stackmap != NULL)
     *stackmap = stackmap1;
   return FOUND;
 }
 
+struct scanstate {
+  void* gcw;      // the GC worker, passed into scanstackwithmap_callback
+  uintptr lastsp; // the last (outermost) SP of Go function seen in a traceback, set by the callback
+};
+
 // Callback function to scan a stack frame with stack maps.
 // It skips non-Go functions.
 static _Unwind_Reason_Code
@@ -755,7 +761,11 @@ scanstackwithmap_callback (struct _Unwind_Context *context, void *arg)
   struct _stackmap *stackmap;
   _Unwind_Ptr ip, sp;
   G* gp;
-  void *gcw = arg;
+  struct scanstate* state = (struct scanstate*) arg;
+  void *gcw;
+
+  gp = runtime_g ();
+  gcw = state->gcw;
 
   switch (findstackmaps (context, &ip, &sp, &stackmap))
     {
@@ -767,7 +777,6 @@ scanstackwithmap_callback (struct _Unwind_Context *context, void *arg)
           // No stack map found.
           // If we're scanning from the signal stack, the goroutine
           // may be not stopped at a safepoint. Allow this case.
-          gp = runtime_g ();
           if (gp != gp->m->gsignal)
             {
               // TODO: print gp, pc, sp
@@ -781,6 +790,7 @@ scanstackwithmap_callback (struct _Unwind_Context *context, void *arg)
         abort ();
     }
 
+  state->lastsp = sp;
   runtime_scanstackblockwithmap (ip, sp, (uintptr)(stackmap->len) * sizeof(uintptr), stackmap->data, gcw);
 
   return _URC_NO_REASON;
@@ -792,10 +802,30 @@ bool
 scanstackwithmap (void *gcw)
 {
   _Unwind_Reason_Code code;
+  bool ret;
+  struct scanstate state;
+  G* gp;
+  G* curg;
+
+  state.gcw = gcw;
+  state.lastsp = 0;
+  gp = runtime_g ();
+  curg = gp->m->curg;
+
   runtime_xadd (&__go_runtime_in_callers, 1);
-  code = _Unwind_Backtrace (scanstackwithmap_callback, gcw);
+  code = _Unwind_Backtrace (scanstackwithmap_callback, (void*)&state);
   runtime_xadd (&__go_runtime_in_callers, -1);
-  return code == _URC_END_OF_STACK;
+  ret = (code == _URC_END_OF_STACK);
+  if (ret && gp == gp->m->gsignal)
+    {
+      // For signal-triggered scan, the unwinder may not be able to unwind
+      // the whole stack while it still reports _URC_END_OF_STACK (e.g.
+      // signal is delivered in vdso). Check that we actually reached the
+      // the end of the stack, that is, the SP on entry.
+      if (state.lastsp != curg->entrysp)
+        ret = false;
+    }
+  return ret;
 }
 
 // Returns whether stack map is enabled.