From b282efa0221f6298f1e407ba947c5a7bc8f58871 Mon Sep 17 00:00:00 2001
From: Matthew Dempsky <mdempsky@google.com>
Date: Mon, 21 Oct 2019 12:44:42 -0700
Subject: [PATCH] cmd/compile: recognize reflect.{Slice,String}Header for
 -d=checkptr

Avoids false positive pointer arithmetic panic.

Fixes #35027.

Change-Id: Idd008caaab25fcf739327ac50a021b835ef13def
Reviewed-on: https://go-review.googlesource.com/c/go/+/202560
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
---
 src/cmd/compile/internal/gc/walk.go |  4 ++++
 test/fixedbugs/issue35027.go        | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 test/fixedbugs/issue35027.go

diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index 4f5fa38a334..8f6da254715 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -3941,6 +3941,10 @@ func walkCheckPtrArithmetic(n *Node, init *Nodes) *Node {
 		return n
 	}
 
+	if n.Left.Op == ODOTPTR && isReflectHeaderDataField(n.Left) {
+		return n
+	}
+
 	// Find original unsafe.Pointer operands involved in this
 	// arithmetic expression.
 	//
diff --git a/test/fixedbugs/issue35027.go b/test/fixedbugs/issue35027.go
new file mode 100644
index 00000000000..d4b0be52c10
--- /dev/null
+++ b/test/fixedbugs/issue35027.go
@@ -0,0 +1,23 @@
+// run -gcflags=-d=checkptr
+
+// Copyright 2019 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
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+var s []int
+
+func main() {
+	s = []int{42}
+	h := (*reflect.SliceHeader)(unsafe.Pointer(&s))
+	x := *(*int)(unsafe.Pointer(h.Data))
+	if x != 42 {
+		panic(x)
+	}
+}