cmd/compile: avoid past-the-end pointer when zeroing

When we optimize append(s, make([]T, n)...), we have to be careful
not to pass &s[0] + len(s)*sizeof(T) as the argument to memclr, as that
pointer might be past-the-end. This can only happen if n is zero, so
just special-case n==0 in the generated code.

Fixes golang#67255

Change-Id: Ic680711bb8c38440eba5e759363ef65f5945658b
Reviewed-on: https://go-review.googlesource.com/c/go/+/584116
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -623,21 +623,23 @@ func isAppendOfMake(n ir.Node) bool {
 //	    panicmakeslicelen()
 //	  }
 //	  s := l1
-//	  n := len(s) + l2
-//	  // Compare n and s as uint so growslice can panic on overflow of len(s) + l2.
-//	  // cap is a positive int and n can become negative when len(s) + l2
-//	  // overflows int. Interpreting n when negative as uint makes it larger
-//	  // than cap(s). growslice will check the int n arg and panic if n is
-//	  // negative. This prevents the overflow from being undetected.
-//	  if uint(n) <= uint(cap(s)) {
-//	    s = s[:n]
-//	  } else {
-//	    s = growslice(T, s.ptr, n, s.cap, l2, T)
+//	  if l2 != 0 {
+//	    n := len(s) + l2
+//	    // Compare n and s as uint so growslice can panic on overflow of len(s) + l2.
+//	    // cap is a positive int and n can become negative when len(s) + l2
+//	    // overflows int. Interpreting n when negative as uint makes it larger
+//	    // than cap(s). growslice will check the int n arg and panic if n is
+//	    // negative. This prevents the overflow from being undetected.
+//	    if uint(n) <= uint(cap(s)) {
+//	      s = s[:n]
+//	    } else {
+//	      s = growslice(T, s.ptr, n, s.cap, l2, T)
+//	    }
+//	    // clear the new portion of the underlying array.
+//	    hp := &s[len(s)-l2]
+//	    hn := l2 * sizeof(T)
+//	    memclr(hp, hn)
 //	  }
-//	  // clear the new portion of the underlying array.
-//	  hp := &s[len(s)-l2]
-//	  hn := l2 * sizeof(T)
-//	  memclr(hp, hn)
 //	}
 //	s
 //
@@ -671,11 +673,18 @@ func extendSlice(n *ir.CallExpr, init *ir.Nodes) ir.Node {
 	s := typecheck.TempAt(base.Pos, ir.CurFunc, l1.Type())
 	nodes = append(nodes, ir.NewAssignStmt(base.Pos, s, l1))
 
+	// if l2 != 0 {
+	// Avoid work if we're not appending anything. But more importantly,
+	// avoid allowing hp to be a past-the-end pointer when clearing. See issue 67255.
+	nifnz := ir.NewIfStmt(base.Pos, ir.NewBinaryExpr(base.Pos, ir.ONE, l2, ir.NewInt(base.Pos, 0)), nil, nil)
+	nifnz.Likely = true
+	nodes = append(nodes, nifnz)
+
 	elemtype := s.Type().Elem()
 
 	// n := s.len + l2
 	nn := typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TINT])
-	nodes = append(nodes, ir.NewAssignStmt(base.Pos, nn, ir.NewBinaryExpr(base.Pos, ir.OADD, ir.NewUnaryExpr(base.Pos, ir.OLEN, s), l2)))
+	nifnz.Body = append(nifnz.Body, ir.NewAssignStmt(base.Pos, nn, ir.NewBinaryExpr(base.Pos, ir.OADD, ir.NewUnaryExpr(base.Pos, ir.OLEN, s), l2)))
 
 	// if uint(n) <= uint(s.cap)
 	nuint := typecheck.Conv(nn, types.Types[types.TUINT])
@@ -697,7 +706,7 @@ func extendSlice(n *ir.CallExpr, init *ir.Nodes) ir.Node {
 			l2)),
 	}
 
-	nodes = append(nodes, nif)
+	nifnz.Body = append(nifnz.Body, nif)
 
 	// hp := &s[s.len - l2]
 	// TODO: &s[s.len] - hn?
@@ -723,7 +732,7 @@ func extendSlice(n *ir.CallExpr, init *ir.Nodes) ir.Node {
 		// if growslice isn't called do we need to do the zeroing ourselves.
 		nif.Body = append(nif.Body, clr...)
 	} else {
-		nodes = append(nodes, clr...)
+		nifnz.Body = append(nifnz.Body, clr...)
 	}
 
 	typecheck.Stmts(nodes)

test/fixedbugs/issue67255.go

@@ -0,0 +1,33 @@
+// run
+
+// Copyright 2024 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
+
+var zero int
+
+var sink any
+
+func main() {
+	var objs [][]*byte
+	for i := 10; i < 200; i++ {
+		// The objects we're allocating here are pointer-ful. Some will
+		// max out their size class, which are the ones we want.
+		// We also allocate from small to large, so that the object which
+		// maxes out its size class is the last one allocated in that class.
+		// This allocation pattern leaves the next object in the class
+		// unallocated, which we need to reproduce the bug.
+		objs = append(objs, make([]*byte, i))
+	}
+	sink = objs // force heap allocation
+
+	// Bug will happen as soon as the write barrier turns on.
+	for range 10000 {
+		sink = make([]*byte, 1024)
+		for _, s := range objs {
+			s = append(s, make([]*byte, zero)...)
+		}
+	}
+}