cmd/compile,runtime: change unsafe.Slice((*T)(nil), 0) to return []T(…

…nil)

This CL removes the unconditional OCHECKNIL check added in
walkUnsafeSlice by instead passing it as a pointer to
runtime.unsafeslice, and hiding the check behind a `len == 0` check.

While here, this CL also implements checkptr functionality for
unsafe.Slice and disallows use of unsafe.Slice with //go:notinheap
types.

Updates #46742.

Change-Id: I743a445ac124304a4d7322a7fe089c4a21b9a655
Reviewed-on: https://go-review.googlesource.com/c/go/+/331070
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/typecheck/builtin/runtime.go

@@ -183,8 +183,9 @@ func makeslice(typ *byte, len int, cap int) unsafe.Pointer
 func makeslice64(typ *byte, len int64, cap int64) unsafe.Pointer
 func makeslicecopy(typ *byte, tolen int, fromlen int, from unsafe.Pointer) unsafe.Pointer
 func growslice(typ *byte, old []any, cap int) (ary []any)
-func unsafeslice(typ *byte, len int)
-func unsafeslice64(typ *byte, len int64)
+func unsafeslice(typ *byte, ptr unsafe.Pointer, len int)
+func unsafeslice64(typ *byte, ptr unsafe.Pointer, len int64)
+func unsafeslicecheckptr(typ *byte, ptr unsafe.Pointer, len int64)
 
 func memmove(to *any, frm *any, length uintptr)
 func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)

src/cmd/compile/internal/typecheck/func.go

@@ -1018,7 +1018,14 @@ func tcUnsafeSlice(n *ir.BinaryExpr) *ir.BinaryExpr {
 	t := n.X.Type()
 	if !t.IsPtr() {
 		base.Errorf("first argument to unsafe.Slice must be pointer; have %L", t)
+	} else if t.Elem().NotInHeap() {
+		// TODO(mdempsky): This can be relaxed, but should only affect the
+		// Go runtime itself. End users should only see //go:notinheap
+		// types due to incomplete C structs in cgo, and those types don't
+		// have a meaningful size anyway.
+		base.Errorf("unsafe.Slice of incomplete (or unallocatable) type not allowed")
 	}
+
 	if !checkunsafeslice(&n.Y) {
 		n.SetType(nil)
 		return n

src/cmd/compile/internal/walk/builtin.go

@@ -654,36 +654,28 @@ func walkRecover(nn *ir.CallExpr, init *ir.Nodes) ir.Node {
 }
 
 func walkUnsafeSlice(n *ir.BinaryExpr, init *ir.Nodes) ir.Node {
+	ptr := safeExpr(n.X, init)
 	len := safeExpr(n.Y, init)
 
 	fnname := "unsafeslice64"
-	argtype := types.Types[types.TINT64]
+	lenType := types.Types[types.TINT64]
 
 	// Type checking guarantees that TIDEAL len/cap are positive and fit in an int.
 	// The case of len or cap overflow when converting TUINT or TUINTPTR to TINT
 	// will be handled by the negative range checks in unsafeslice during runtime.
-	if len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size() {
+	if ir.ShouldCheckPtr(ir.CurFunc, 1) {
+		fnname = "unsafeslicecheckptr"
+		// for simplicity, unsafeslicecheckptr always uses int64
+	} else if len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size() {
 		fnname = "unsafeslice"
-		argtype = types.Types[types.TINT]
+		lenType = types.Types[types.TINT]
 	}
 
 	t := n.Type()
 
-	// Call runtime.unsafeslice[64] to check that the length argument is
-	// non-negative and smaller than the max length allowed for the
-	// element type.
+	// Call runtime.unsafeslice{,64,checkptr} to check ptr and len.
 	fn := typecheck.LookupRuntime(fnname)
-	init.Append(mkcall1(fn, nil, init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(len, argtype)))
-
-	ptr := walkExpr(n.X, init)
-
-	c := ir.NewUnaryExpr(n.Pos(), ir.OCHECKNIL, ptr)
-	c.SetTypecheck(1)
-	init.Append(c)
-
-	// TODO(mdempsky): checkptr instrumentation. Maybe merge into length
-	// check above, along with nil check? Need to be careful about
-	// notinheap pointers though: can't pass them as unsafe.Pointer.
+	init.Append(mkcall1(fn, nil, init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(ptr, types.Types[types.TUNSAFEPTR]), typecheck.Conv(len, lenType)))
 
 	h := ir.NewSliceHeaderExpr(n.Pos(), t,
 		typecheck.Conv(ptr, types.Types[types.TUNSAFEPTR]),

src/runtime/checkptr.go

@@ -16,11 +16,30 @@ func checkptrAlignment(p unsafe.Pointer, elem *_type, n uintptr) {
 	}
 
 	// Check that (*[n]elem)(p) doesn't straddle multiple heap objects.
-	if size := n * elem.size; size > 1 && checkptrBase(p) != checkptrBase(add(p, size-1)) {
+	// TODO(mdempsky): Fix #46938 so we don't need to worry about overflow here.
+	if checkptrStraddles(p, n*elem.size) {
 		throw("checkptr: converted pointer straddles multiple allocations")
 	}
 }
 
+// checkptrStraddles reports whether the first size-bytes of memory
+// addressed by ptr is known to straddle more than one Go allocation.
+func checkptrStraddles(ptr unsafe.Pointer, size uintptr) bool {
+	if size <= 1 {
+		return false
+	}
+
+	end := add(ptr, size-1)
+	if uintptr(end) < uintptr(ptr) {
+		return true
+	}
+
+	// TODO(mdempsky): Detect when [ptr, end] contains Go allocations,
+	// but neither ptr nor end point into one themselves.
+
+	return checkptrBase(ptr) != checkptrBase(end)
+}
+
 func checkptrArithmetic(p unsafe.Pointer, originals []unsafe.Pointer) {
 	if 0 < uintptr(p) && uintptr(p) < minLegalPointer {
 		throw("checkptr: pointer arithmetic computed bad pointer value")

src/runtime/checkptr_test.go

@@ -30,6 +30,8 @@ func TestCheckPtr(t *testing.T) {
 		{"CheckPtrArithmetic2", "fatal error: checkptr: pointer arithmetic result points to invalid allocation\n"},
 		{"CheckPtrSize", "fatal error: checkptr: converted pointer straddles multiple allocations\n"},
 		{"CheckPtrSmall", "fatal error: checkptr: pointer arithmetic computed bad pointer value\n"},
+		{"CheckPtrSliceOK", ""},
+		{"CheckPtrSliceFail", "fatal error: checkptr: unsafe.Slice result straddles multiple allocations\n"},
 	}
 
 	for _, tc := range testCases {

src/runtime/slice.go

@@ -112,19 +112,37 @@ func makeslice64(et *_type, len64, cap64 int64) unsafe.Pointer {
 	return makeslice(et, len, cap)
 }
 
-func unsafeslice(et *_type, len int) {
+func unsafeslice(et *_type, ptr unsafe.Pointer, len int) {
+	if len == 0 {
+		return
+	}
+
+	if ptr == nil {
+		panic(errorString("unsafe.Slice: ptr is nil and len is not zero"))
+	}
+
 	mem, overflow := math.MulUintptr(et.size, uintptr(len))
 	if overflow || mem > maxAlloc || len < 0 {
 		panicunsafeslicelen()
 	}
 }
 
-func unsafeslice64(et *_type, len64 int64) {
+func unsafeslice64(et *_type, ptr unsafe.Pointer, len64 int64) {
 	len := int(len64)
 	if int64(len) != len64 {
 		panicunsafeslicelen()
 	}
-	unsafeslice(et, len)
+	unsafeslice(et, ptr, len)
+}
+
+func unsafeslicecheckptr(et *_type, ptr unsafe.Pointer, len64 int64) {
+	unsafeslice64(et, ptr, len64)
+
+	// Check that underlying array doesn't straddle multiple heap objects.
+	// unsafeslice64 has already checked for overflow.
+	if checkptrStraddles(ptr, uintptr(len64)*et.size) {
+		throw("checkptr: unsafe.Slice result straddles multiple allocations")
+	}
 }
 
 func panicunsafeslicelen() {

src/runtime/testdata/testprog/checkptr.go

@@ -13,6 +13,8 @@ func init() {
 	register("CheckPtrArithmetic2", CheckPtrArithmetic2)
 	register("CheckPtrSize", CheckPtrSize)
 	register("CheckPtrSmall", CheckPtrSmall)
+	register("CheckPtrSliceOK", CheckPtrSliceOK)
+	register("CheckPtrSliceFail", CheckPtrSliceFail)
 }
 
 func CheckPtrAlignmentNoPtr() {
@@ -49,3 +51,14 @@ func CheckPtrSize() {
 func CheckPtrSmall() {
 	sink2 = unsafe.Pointer(uintptr(1))
 }
+
+func CheckPtrSliceOK() {
+	p := new([4]int64)
+	sink2 = unsafe.Slice(&p[1], 3)
+}
+
+func CheckPtrSliceFail() {
+	p := new(int64)
+	sink2 = p
+	sink2 = unsafe.Slice(p, 100)
+}

test/unsafebuiltins.go

@@ -30,8 +30,11 @@ func main() {
 		assert(len(s) == len(p))
 		assert(cap(s) == len(p))
 
-		// nil pointer
-		mustPanic(func() { _ = unsafe.Slice((*int)(nil), 0) })
+		// nil pointer with zero length returns nil
+		assert(unsafe.Slice((*int)(nil), 0) == nil)
+
+		// nil pointer with positive length panics
+		mustPanic(func() { _ = unsafe.Slice((*int)(nil), 1) })
 
 		// negative length
 		var neg int = -1