cmd/compile: resurrect clobberdead mode

This CL resurrects the clobberdead debugging mode (CL 23924).
When -clobberdead flag is set (TODO: make it GOEXPERIMENT?), the
compiler inserts code that clobbers all dead stack slots that
contains pointers.

Mark windows syscall functions cgo_unsafe_args, as the code
actually does that, by taking the address of one argument and
passing it to cgocall.

Change-Id: Ie09a015f4bd14ae6053cc707866e30ae509b9d6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/301791
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/base/flag.go

@@ -90,6 +90,7 @@ type CmdFlags struct {
 	BuildID            string       "help:\"record `id` as the build id in the export metadata\""
 	CPUProfile         string       "help:\"write cpu profile to `file`\""
 	Complete           bool         "help:\"compiling complete package (no C or assembly)\""
+	ClobberDead        bool         "help:\"clobber dead stack slots (for debugging)\""
 	Dwarf              bool         "help:\"generate DWARF symbols\""
 	DwarfBASEntries    *bool        "help:\"use base address selection entries in DWARF\""                        // &Ctxt.UseBASEntries, set below
 	DwarfLocationLists *bool        "help:\"add location lists to DWARF in optimized mode\""                      // &Ctxt.Flag_locationlists, set below

src/cmd/compile/internal/liveness/plive.go

@@ -16,7 +16,9 @@ package liveness
 
 import (
 	"crypto/md5"
+	"crypto/sha1"
 	"fmt"
+	"os"
 	"sort"
 	"strings"
 
@@ -30,6 +32,7 @@ import (
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
+	"cmd/internal/src"
 )
 
 // OpVarDef is an annotation for the liveness analysis, marking a place
@@ -123,9 +126,9 @@ type liveness struct {
 	unsafePoints bitvec.BitVec
 
 	// An array with a bit vector for each safe point in the
-	// current Block during Liveness.epilogue. Indexed in Value
+	// current Block during liveness.epilogue. Indexed in Value
 	// order for that block. Additionally, for the entry block
-	// livevars[0] is the entry bitmap. Liveness.compact moves
+	// livevars[0] is the entry bitmap. liveness.compact moves
 	// these to stackMaps.
 	livevars []bitvec.BitVec
 
@@ -136,6 +139,8 @@ type liveness struct {
 	stackMaps   []bitvec.BitVec
 
 	cache progeffectscache
+
+	doClobber bool // Whether to clobber dead stack slots in this function.
 }
 
 // Map maps from *ssa.Value to LivenessIndex.
@@ -387,6 +392,9 @@ func newliveness(fn *ir.Func, f *ssa.Func, vars []*ir.Name, idx map[*ir.Name]int
 	lv.livenessMap.reset()
 
 	lv.markUnsafePoints()
+
+	lv.enableClobber()
+
 	return lv
 }
 
@@ -820,6 +828,10 @@ func (lv *liveness) epilogue() {
 			live.Or(*live, liveout)
 		}
 
+		if lv.doClobber {
+			lv.clobber(b)
+		}
+
 		// The liveness maps for this block are now complete. Compact them.
 		lv.compact(b)
 	}
@@ -873,7 +885,7 @@ func (lv *liveness) compact(b *ssa.Block) {
 	}
 	for _, v := range b.Values {
 		hasStackMap := lv.hasStackMap(v)
-		isUnsafePoint := lv.allUnsafe || lv.unsafePoints.Get(int32(v.ID))
+		isUnsafePoint := lv.allUnsafe || v.Op != ssa.OpClobber && lv.unsafePoints.Get(int32(v.ID))
 		idx := objw.LivenessIndex{StackMapIndex: objw.StackMapDontCare, IsUnsafePoint: isUnsafePoint}
 		if hasStackMap {
 			idx.StackMapIndex = lv.stackMapSet.add(lv.livevars[pos])
@@ -888,6 +900,153 @@ func (lv *liveness) compact(b *ssa.Block) {
 	lv.livevars = lv.livevars[:0]
 }
 
+func (lv *liveness) enableClobber() {
+	// The clobberdead experiment inserts code to clobber pointer slots in all
+	// the dead variables (locals and args) at every synchronous safepoint.
+	if !base.Flag.ClobberDead {
+		return
+	}
+	if lv.fn.Pragma&ir.CgoUnsafeArgs != 0 {
+		// C or assembly code uses the exact frame layout. Don't clobber.
+		return
+	}
+	if len(lv.vars) > 10000 || len(lv.f.Blocks) > 10000 {
+		// Be careful to avoid doing too much work.
+		// Bail if >10000 variables or >10000 blocks.
+		// Otherwise, giant functions make this experiment generate too much code.
+		return
+	}
+	if lv.f.Name == "forkAndExecInChild" || lv.f.Name == "wbBufFlush" {
+		// forkAndExecInChild calls vfork on some platforms.
+		// The code we add here clobbers parts of the stack in the child.
+		// When the parent resumes, it is using the same stack frame. But the
+		// child has clobbered stack variables that the parent needs. Boom!
+		// In particular, the sys argument gets clobbered.
+		//
+		// runtime.wbBufFlush must not modify its arguments. See the comments
+		// in runtime/mwbbuf.go:wbBufFlush.
+		return
+	}
+	if h := os.Getenv("GOCLOBBERDEADHASH"); h != "" {
+		// Clobber only functions where the hash of the function name matches a pattern.
+		// Useful for binary searching for a miscompiled function.
+		hstr := ""
+		for _, b := range sha1.Sum([]byte(lv.f.Name)) {
+			hstr += fmt.Sprintf("%08b", b)
+		}
+		if !strings.HasSuffix(hstr, h) {
+			return
+		}
+		fmt.Printf("\t\t\tCLOBBERDEAD %s\n", lv.f.Name)
+	}
+	lv.doClobber = true
+}
+
+// Inserts code to clobber pointer slots in all the dead variables (locals and args)
+// at every synchronous safepoint in b.
+func (lv *liveness) clobber(b *ssa.Block) {
+	// Copy block's values to a temporary.
+	oldSched := append([]*ssa.Value{}, b.Values...)
+	b.Values = b.Values[:0]
+	idx := 0
+
+	// Clobber pointer slots in all dead variables at entry.
+	if b == lv.f.Entry {
+		for len(oldSched) > 0 && len(oldSched[0].Args) == 0 {
+			// Skip argless ops. We need to skip at least
+			// the lowered ClosurePtr op, because it
+			// really wants to be first. This will also
+			// skip ops like InitMem and SP, which are ok.
+			b.Values = append(b.Values, oldSched[0])
+			oldSched = oldSched[1:]
+		}
+		clobber(lv, b, lv.livevars[0])
+		idx++
+	}
+
+	// Copy values into schedule, adding clobbering around safepoints.
+	for _, v := range oldSched {
+		if !lv.hasStackMap(v) {
+			b.Values = append(b.Values, v)
+			continue
+		}
+		clobber(lv, b, lv.livevars[idx])
+		b.Values = append(b.Values, v)
+		idx++
+	}
+}
+
+// clobber generates code to clobber pointer slots in all dead variables
+// (those not marked in live). Clobbering instructions are added to the end
+// of b.Values.
+func clobber(lv *liveness, b *ssa.Block, live bitvec.BitVec) {
+	for i, n := range lv.vars {
+		if !live.Get(int32(i)) && !n.Addrtaken() {
+			// Don't clobber stack objects (address-taken). They are
+			// tracked dynamically.
+			clobberVar(b, n)
+		}
+	}
+}
+
+// clobberVar generates code to trash the pointers in v.
+// Clobbering instructions are added to the end of b.Values.
+func clobberVar(b *ssa.Block, v *ir.Name) {
+	clobberWalk(b, v, 0, v.Type())
+}
+
+// b = block to which we append instructions
+// v = variable
+// offset = offset of (sub-portion of) variable to clobber (in bytes)
+// t = type of sub-portion of v.
+func clobberWalk(b *ssa.Block, v *ir.Name, offset int64, t *types.Type) {
+	if !t.HasPointers() {
+		return
+	}
+	switch t.Kind() {
+	case types.TPTR,
+		types.TUNSAFEPTR,
+		types.TFUNC,
+		types.TCHAN,
+		types.TMAP:
+		clobberPtr(b, v, offset)
+
+	case types.TSTRING:
+		// struct { byte *str; int len; }
+		clobberPtr(b, v, offset)
+
+	case types.TINTER:
+		// struct { Itab *tab; void *data; }
+		// or, when isnilinter(t)==true:
+		// struct { Type *type; void *data; }
+		clobberPtr(b, v, offset)
+		clobberPtr(b, v, offset+int64(types.PtrSize))
+
+	case types.TSLICE:
+		// struct { byte *array; int len; int cap; }
+		clobberPtr(b, v, offset)
+
+	case types.TARRAY:
+		for i := int64(0); i < t.NumElem(); i++ {
+			clobberWalk(b, v, offset+i*t.Elem().Size(), t.Elem())
+		}
+
+	case types.TSTRUCT:
+		for _, t1 := range t.Fields().Slice() {
+			clobberWalk(b, v, offset+t1.Offset, t1.Type)
+		}
+
+	default:
+		base.Fatalf("clobberWalk: unexpected type, %v", t)
+	}
+}
+
+// clobberPtr generates a clobber of the pointer at offset offset in v.
+// The clobber instruction is added at the end of b.
+func clobberPtr(b *ssa.Block, v *ir.Name, offset int64) {
+	b.NewValue0IA(src.NoXPos, ssa.OpClobber, types.TypeVoid, offset, v)
+}
+
 func (lv *liveness) showlive(v *ssa.Value, live bitvec.BitVec) {
 	if base.Flag.Live == 0 || ir.FuncName(lv.fn) == "init" || strings.HasPrefix(ir.FuncName(lv.fn), ".") {
 		return

src/cmd/compile/internal/test/clobberdead_test.go

@@ -0,0 +1,47 @@
+// Copyright 2021 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 test
+
+import (
+	"internal/testenv"
+	"io/ioutil"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+const helloSrc = `
+package main
+import "fmt"
+func main() { fmt.Println("hello") }
+`
+
+func TestClobberDead(t *testing.T) {
+	// Test that clobberdead mode generates correct program.
+
+	if testing.Short() {
+		// This test rebuilds the runtime with a special flag, which
+		// takes a while.
+		t.Skip("skip in short mode")
+	}
+	testenv.MustHaveGoRun(t)
+	t.Parallel()
+
+	tmpdir := t.TempDir()
+	src := filepath.Join(tmpdir, "x.go")
+	err := ioutil.WriteFile(src, []byte(helloSrc), 0644)
+	if err != nil {
+		t.Fatalf("write file failed: %v", err)
+	}
+
+	cmd := exec.Command(testenv.GoToolPath(t), "run", "-gcflags=all=-clobberdead", src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("go run failed: %v\n%s", err, out)
+	}
+	if string(out) != "hello\n" {
+		t.Errorf("wrong output: got %q, want %q", out, "hello\n")
+	}
+}

src/runtime/syscall_windows.go

@@ -263,6 +263,7 @@ const _LOAD_LIBRARY_SEARCH_SYSTEM32 = 0x00000800
 // to the full path inside of system32 for use with vanilla LoadLibrary.
 //go:linkname syscall_loadsystemlibrary syscall.loadsystemlibrary
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_loadsystemlibrary(filename *uint16, absoluteFilepath *uint16) (handle, err uintptr) {
 	lockOSThread()
 	c := &getg().m.syscall
@@ -293,6 +294,7 @@ func syscall_loadsystemlibrary(filename *uint16, absoluteFilepath *uint16) (hand
 
 //go:linkname syscall_loadlibrary syscall.loadlibrary
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_loadlibrary(filename *uint16) (handle, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
@@ -310,6 +312,7 @@ func syscall_loadlibrary(filename *uint16) (handle, err uintptr) {
 
 //go:linkname syscall_getprocaddress syscall.getprocaddress
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_getprocaddress(handle uintptr, procname *byte) (outhandle, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
@@ -327,6 +330,7 @@ func syscall_getprocaddress(handle uintptr, procname *byte) (outhandle, err uint
 
 //go:linkname syscall_Syscall syscall.Syscall
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_Syscall(fn, nargs, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
@@ -340,6 +344,7 @@ func syscall_Syscall(fn, nargs, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 
 //go:linkname syscall_Syscall6 syscall.Syscall6
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_Syscall6(fn, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
@@ -353,6 +358,7 @@ func syscall_Syscall6(fn, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err ui
 
 //go:linkname syscall_Syscall9 syscall.Syscall9
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_Syscall9(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
@@ -366,6 +372,7 @@ func syscall_Syscall9(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1
 
 //go:linkname syscall_Syscall12 syscall.Syscall12
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_Syscall12(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12 uintptr) (r1, r2, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
@@ -379,6 +386,7 @@ func syscall_Syscall12(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11,
 
 //go:linkname syscall_Syscall15 syscall.Syscall15
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_Syscall15(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15 uintptr) (r1, r2, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()
@@ -392,6 +400,7 @@ func syscall_Syscall15(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11,
 
 //go:linkname syscall_Syscall18 syscall.Syscall18
 //go:nosplit
+//go:cgo_unsafe_args
 func syscall_Syscall18(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18 uintptr) (r1, r2, err uintptr) {
 	lockOSThread()
 	defer unlockOSThread()

test/codegen/clobberdead.go

@@ -0,0 +1,32 @@
+// asmcheck -gcflags=-clobberdead
+
+// +build amd64
+
+// Copyright 2021 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 codegen
+
+type T [2]*int // contain pointer, not SSA-able (so locals are not registerized)
+
+var p1, p2, p3 T
+
+func F() {
+	// 3735936685 is 0xdeaddead
+	// clobber x, y at entry. not clobber z (stack object).
+	// amd64:`MOVL\t\$3735936685, ""\.x`, `MOVL\t\$3735936685, ""\.y`, -`MOVL\t\$3735936685, ""\.z`
+	x, y, z := p1, p2, p3
+	addrTaken(&z)
+	// x is dead at the call (the value of x is loaded before the CALL), y is not
+	// amd64:`MOVL\t\$3735936685, ""\.x`, -`MOVL\t\$3735936685, ""\.y`
+	use(x)
+	// amd64:`MOVL\t\$3735936685, ""\.x`, `MOVL\t\$3735936685, ""\.y`
+	use(y)
+}
+
+//go:noinline
+func use(T) {}
+
+//go:noinline
+func addrTaken(*T) {}