runtime: restore r2 when restoring state from gobuf in gogo on ppc64x

When using plugins with goroutines calling cgo, we hit a case where
an intermittent SIGSEGV occurs when referencing an address that is based
on r2 (TOC address). When the failure can be generated in gdb, the
contents of r2 is wrong even though the value in the current stack's
slot for r2 is correct. So that means it somehow switched to start
running the code in this function without passing through the beginning
of the function which had the correct value of r2 and stored it there.

It was noted that in runtime.gogo when the state is restored from
gobuf, r2 is not restored from its slot on the stack. Adding the
instruction to restore r2 prevents the SIGSEGV.

This adds a testcase under testplugin which reproduces the problem if
the program is run multiple times. The team who reported this problem
has verified it fixes the issue on their larger, more complex
application.

Fixes #25756

Change-Id: I6028b6f1f8775d5c23f4ebb57ae273330a28eb8f
Reviewed-on: https://go-review.googlesource.com/117515
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testplugin/src/issue25756/main.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Run the game of life in C using Go for parallelization.
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"plugin"
+)
+
+const MAXDIM = 100
+
+var dim = flag.Int("dim", 16, "board dimensions")
+var gen = flag.Int("gen", 10, "generations")
+
+func main() {
+	flag.Parse()
+
+	var a [MAXDIM * MAXDIM]int32
+	for i := 2; i < *dim; i += 8 {
+		for j := 2; j < *dim-3; j += 8 {
+			for y := 0; y < 3; y++ {
+				a[i**dim+j+y] = 1
+			}
+		}
+	}
+
+	p, err := plugin.Open("life.so")
+	if err != nil {
+		panic(err)
+	}
+	f, err := p.Lookup("Run")
+	if err != nil {
+		panic(err)
+	}
+	f.(func(int, int, int, []int32))(*gen, *dim, *dim, a[:])
+
+	for i := 0; i < *dim; i++ {
+		for j := 0; j < *dim; j++ {
+			if a[i**dim+j] == 0 {
+				fmt.Print(" ")
+			} else {
+				fmt.Print("X")
+			}
+		}
+		fmt.Print("\n")
+	}
+}

misc/cgo/testplugin/src/issue25756/plugin/c-life.c

+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include <assert.h>
+#include "life.h"
+#include "_cgo_export.h"
+
+const int MYCONST = 0;
+
+// Do the actual manipulation of the life board in C.  This could be
+// done easily in Go, we are just using C for demonstration
+// purposes.
+void
+Step(int x, int y, int *a, int *n)
+{
+	struct GoStart_return r;
+
+	// Use Go to start 4 goroutines each of which handles 1/4 of the
+	// board.
+	r = GoStart(0, x, y, 0, x / 2, 0, y / 2, a, n);
+	assert(r.r0 == 0 && r.r1 == 100);	// test multiple returns
+	r = GoStart(1, x, y, x / 2, x, 0, y / 2, a, n);
+	assert(r.r0 == 1 && r.r1 == 101);	// test multiple returns
+	GoStart(2, x, y, 0, x / 2, y / 2, y, a, n);
+	GoStart(3, x, y, x / 2, x, y / 2, y, a, n);
+	GoWait(0);
+	GoWait(1);
+	GoWait(2);
+	GoWait(3);
+}
+
+// The actual computation.  This is called in parallel.
+void
+DoStep(int xdim, int ydim, int xstart, int xend, int ystart, int yend, int *a, int *n)
+{
+	int x, y, c, i, j;
+
+	for(x = xstart; x < xend; x++) {
+		for(y = ystart; y < yend; y++) {
+			c = 0;
+			for(i = -1; i <= 1; i++) {
+				for(j = -1; j <= 1; j++) {
+				  if(x+i >= 0 && x+i < xdim &&
+					y+j >= 0 && y+j < ydim &&
+					(i != 0 || j != 0))
+				    c += a[(x+i)*xdim + (y+j)] != 0;
+				}
+			}
+			if(c == 3 || (c == 2 && a[x*xdim + y] != 0))
+				n[x*xdim + y] = 1;
+			else
+				n[x*xdim + y] = 0;
+		}
+	}
+}

misc/cgo/testplugin/src/issue25756/plugin/life.go

+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+// #include "life.h"
+import "C"
+
+import "unsafe"
+
+func Run(gen, x, y int, a []int32) {
+	n := make([]int32, x*y)
+	for i := 0; i < gen; i++ {
+		C.Step(C.int(x), C.int(y), (*C.int)(unsafe.Pointer(&a[0])), (*C.int)(unsafe.Pointer(&n[0])))
+		copy(a, n)
+	}
+}
+
+// Keep the channels visible from Go.
+var chans [4]chan bool
+
+//export GoStart
+// Double return value is just for testing.
+func GoStart(i, xdim, ydim, xstart, xend, ystart, yend C.int, a *C.int, n *C.int) (int, int) {
+	c := make(chan bool, int(C.MYCONST))
+	go func() {
+		C.DoStep(xdim, ydim, xstart, xend, ystart, yend, a, n)
+		c <- true
+	}()
+	chans[i] = c
+	return int(i), int(i + 100)
+}
+
+//export GoWait
+func GoWait(i C.int) {
+	<-chans[i]
+	chans[i] = nil
+}

misc/cgo/testplugin/src/issue25756/plugin/life.h

+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+extern void Step(int, int, int *, int *);
+extern void DoStep(int, int, int, int, int, int, int *, int *);
+extern const int MYCONST;

misc/cgo/testplugin/test.bash

@@ -15,7 +15,7 @@ goos=$(go env GOOS)
 goarch=$(go env GOARCH)
 
 function cleanup() {
-	rm -f plugin*.so unnamed*.so iface*.so issue*
+	rm -f plugin*.so unnamed*.so iface*.so life.so issue*
 	rm -rf host pkg sub iface
 }
 trap cleanup EXIT
@@ -90,3 +90,12 @@ GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue22295 src/issue22295.pkg/m
 GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -buildmode=plugin -o issue24351.so src/issue24351/plugin.go
 GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue24351 src/issue24351/main.go
 ./issue24351
+
+# Test for issue 25756
+GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -buildmode=plugin -o life.so issue25756/plugin
+GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue25756 src/issue25756/main.go
+# Fails intermittently, but 20 runs should cause the failure
+for i in `seq 1 20`;
+do
+  ./issue25756 > /dev/null
+done

src/runtime/asm_ppc64x.s

@@ -139,6 +139,7 @@ TEXT runtime·gogo(SB), NOSPLIT, $16-8
 	MOVD	0(g), R4
 	MOVD	gobuf_sp(R5), R1
 	MOVD	gobuf_lr(R5), R31
+	MOVD	24(R1), R2	// restore R2
 	MOVD	R31, LR
 	MOVD	gobuf_ret(R5), R3
 	MOVD	gobuf_ctxt(R5), R11