From aa5540cd82170f82c6fe11511e12de96aa58cbc1 Mon Sep 17 00:00:00 2001
From: Than McIntosh <thanm@google.com>
Date: Tue, 8 Jun 2021 20:09:49 -0400
Subject: [PATCH] cmd/compile: make map.zero symbol content-addressable

The compiler machinery that generates "map.zero" symbols marks them as
RODATA and DUPOK, which is problematic when a given application has
multiple map zero symbols (from different packages) with varying
sizes: the dupok path in the loader assumes that if two symbols have
the same name, it is safe to pick any of the versions. In the case of
map.zero, the link needs to select the largest symbol, not an
arbitrary sym.

To fix this problem, mark map.zero symbols as content-addressable,
since the loader's content addressability processing path already
supports selection of the larger symbol in cases where there are dups.

Fixes #46653.

Change-Id: Iabd2feef01d448670ba795c7eaddc48c191ea276
Reviewed-on: https://go-review.googlesource.com/c/go/+/326211
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
---
 src/cmd/compile/internal/gc/obj.go       |  1 +
 test/fixedbugs/issue46653.dir/bad/bad.go | 64 ++++++++++++++++++++++++
 test/fixedbugs/issue46653.dir/main.go    | 27 ++++++++++
 test/fixedbugs/issue46653.go             | 10 ++++
 4 files changed, 102 insertions(+)
 create mode 100644 test/fixedbugs/issue46653.dir/bad/bad.go
 create mode 100644 test/fixedbugs/issue46653.dir/main.go
 create mode 100644 test/fixedbugs/issue46653.go

diff --git a/src/cmd/compile/internal/gc/obj.go b/src/cmd/compile/internal/gc/obj.go
index 0b10cb8a9e1..55a0ab7da79 100644
--- a/src/cmd/compile/internal/gc/obj.go
+++ b/src/cmd/compile/internal/gc/obj.go
@@ -148,6 +148,7 @@ func dumpdata() {
 	if reflectdata.ZeroSize > 0 {
 		zero := base.PkgLinksym("go.map", "zero", obj.ABI0)
 		objw.Global(zero, int32(reflectdata.ZeroSize), obj.DUPOK|obj.RODATA)
+		zero.Set(obj.AttrContentAddressable, true)
 	}
 
 	staticdata.WriteFuncSyms()
diff --git a/test/fixedbugs/issue46653.dir/bad/bad.go b/test/fixedbugs/issue46653.dir/bad/bad.go
new file mode 100644
index 00000000000..c1611b8347a
--- /dev/null
+++ b/test/fixedbugs/issue46653.dir/bad/bad.go
@@ -0,0 +1,64 @@
+// 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 a
+
+func Bad() {
+	m := make(map[int64]A)
+	a := m[0]
+	if len(a.B.C1.D2.E2.F1) != 0 ||
+		len(a.B.C1.D2.E2.F2) != 0 ||
+		len(a.B.C1.D2.E2.F3) != 0 ||
+		len(a.B.C1.D2.E2.F4) != 0 ||
+		len(a.B.C1.D2.E2.F5) != 0 ||
+		len(a.B.C1.D2.E2.F6) != 0 ||
+		len(a.B.C1.D2.E2.F7) != 0 ||
+		len(a.B.C1.D2.E2.F8) != 0 ||
+		len(a.B.C1.D2.E2.F9) != 0 ||
+		len(a.B.C1.D2.E2.F10) != 0 ||
+		len(a.B.C1.D2.E2.F11) != 0 ||
+		len(a.B.C1.D2.E2.F16) != 0 {
+		panic("bad")
+	}
+}
+
+type A struct {
+	B
+}
+
+type B struct {
+	C1 C
+	C2 C
+}
+
+type C struct {
+	D1 D
+	D2 D
+}
+
+type D struct {
+	E1 E
+	E2 E
+	E3 E
+	E4 E
+}
+
+type E struct {
+	F1  string
+	F2  string
+	F3  string
+	F4  string
+	F5  string
+	F6  string
+	F7  string
+	F8  string
+	F9  string
+	F10 string
+	F11 string
+	F12 string
+	F13 string
+	F14 string
+	F15 string
+	F16 string
+}
diff --git a/test/fixedbugs/issue46653.dir/main.go b/test/fixedbugs/issue46653.dir/main.go
new file mode 100644
index 00000000000..e2a96e54ec5
--- /dev/null
+++ b/test/fixedbugs/issue46653.dir/main.go
@@ -0,0 +1,27 @@
+// 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 main
+
+import (
+	bad "issue46653.dir/bad"
+)
+
+func main() {
+	bad.Bad()
+}
+
+func neverCalled() L {
+	m := make(map[string]L)
+	return m[""]
+}
+
+type L struct {
+	A Data
+	B Data
+}
+
+type Data struct {
+	F1 [22][]string
+}
diff --git a/test/fixedbugs/issue46653.go b/test/fixedbugs/issue46653.go
new file mode 100644
index 00000000000..e6283b1de50
--- /dev/null
+++ b/test/fixedbugs/issue46653.go
@@ -0,0 +1,10 @@
+// runindir
+
+// 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.
+
+// Test to verify compiler and linker handling of multiple
+// competing map.zero symbol definitions.
+
+package ignored