runtime: make typehash match compiler generated hashes exactly

If typehash (used by reflect) does not match the built-in map's hash,
then problems occur. If a map is built using reflect, and then
assigned to a variable of map type, the hash function can change. That
causes very bad things.

This issue is rare. MapOf consults a cache of all types that occur in
the binary before making a new one. To make a true new map type (with
a hash function derived from typehash) that map type must not occur in
the binary anywhere. But to cause the bug, we need a variable of that
type in order to assign to it. The only way to make that work is to
use a named map type for the variable, so it is distinct from the
unnamed version that MapOf looks for.

Fixes #37716

Change-Id: I3537bfceca8cbfa1af84202f432f3c06953fe0ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/222357
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/test/fixedbugs/issue37716.go b/test/fixedbugs/issue37716.go
new file mode 100644
index 0000000000..42d66dff1b
--- /dev/null
+++ b/test/fixedbugs/issue37716.go
@@ -0,0 +1,32 @@
+// run
+
+// Copyright 2020 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"
+
+// complicated enough to require a compile-generated hash function
+type K struct {
+	a, b int32 // these get merged by the compiler into a single field, something typehash doesn't do
+	c    float64
+}
+
+func main() {
+	k := K{a: 1, b: 2, c: 3}
+
+	// Make a reflect map.
+	m := reflect.MakeMap(reflect.MapOf(reflect.TypeOf(K{}), reflect.TypeOf(true)))
+	m.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(true))
+
+	// The binary must not contain the type map[K]bool anywhere, or reflect.MapOf
+	// will use that type instead of making a new one. So use an equivalent named type.
+	type M map[K]bool
+	var x M
+	reflect.ValueOf(&x).Elem().Set(m)
+	if !x[k] {
+		panic("key not found")
+	}
+}