From c90f6dd4966087d85e1dcafc02b64ecb0c7f4e7e Mon Sep 17 00:00:00 2001
From: Joel Sing <joel@sing.id.au>
Date: Thu, 28 Mar 2019 00:53:19 +1100
Subject: [PATCH] cmd/link: permit duplicate weak symbols

Permit weak symbols to be duplicates - most external linkers allow
this and there are various situations where they can occur (including
retpoline and retguard).

Fixes #29563

Change-Id: I355493c847fbc8f670a85a643db65a4cf8f9883d
Reviewed-on: https://go-review.googlesource.com/c/go/+/169658
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
---
 misc/cgo/test/testdata/issue29563.go      | 12 ++++++++++++
 misc/cgo/test/testdata/issue29563/weak.go | 13 +++++++++++++
 misc/cgo/test/testdata/issue29563/weak1.c | 11 +++++++++++
 misc/cgo/test/testdata/issue29563/weak2.c | 11 +++++++++++
 src/cmd/link/internal/loadelf/ldelf.go    |  5 +++++
 5 files changed, 52 insertions(+)
 create mode 100644 misc/cgo/test/testdata/issue29563.go
 create mode 100644 misc/cgo/test/testdata/issue29563/weak.go
 create mode 100644 misc/cgo/test/testdata/issue29563/weak1.c
 create mode 100644 misc/cgo/test/testdata/issue29563/weak2.c

diff --git a/misc/cgo/test/testdata/issue29563.go b/misc/cgo/test/testdata/issue29563.go
new file mode 100644
index 00000000000..84def3ca444
--- /dev/null
+++ b/misc/cgo/test/testdata/issue29563.go
@@ -0,0 +1,12 @@
+// 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.
+
+// +build !windows
+
+// Issue 29563: internal linker fails on duplicate weak symbols.
+// No runtime test; just make sure it compiles.
+
+package cgotest
+
+import _ "cgotest/issue29563"
diff --git a/misc/cgo/test/testdata/issue29563/weak.go b/misc/cgo/test/testdata/issue29563/weak.go
new file mode 100644
index 00000000000..21cf635ccae
--- /dev/null
+++ b/misc/cgo/test/testdata/issue29563/weak.go
@@ -0,0 +1,13 @@
+// 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 issue29563
+
+//int foo1();
+//int foo2();
+import "C"
+
+func Bar() int {
+	return int(C.foo1()) + int(C.foo2())
+}
diff --git a/misc/cgo/test/testdata/issue29563/weak1.c b/misc/cgo/test/testdata/issue29563/weak1.c
new file mode 100644
index 00000000000..86a22734adc
--- /dev/null
+++ b/misc/cgo/test/testdata/issue29563/weak1.c
@@ -0,0 +1,11 @@
+// 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.
+
+extern int weaksym __attribute__((__weak__));
+int weaksym = 42;
+
+int foo1()
+{
+	return weaksym;
+}
diff --git a/misc/cgo/test/testdata/issue29563/weak2.c b/misc/cgo/test/testdata/issue29563/weak2.c
new file mode 100644
index 00000000000..e01eae8b58f
--- /dev/null
+++ b/misc/cgo/test/testdata/issue29563/weak2.c
@@ -0,0 +1,11 @@
+// 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.
+
+extern int weaksym __attribute__((__weak__));
+int weaksym = 42;
+
+int foo2()
+{
+	return weaksym;
+}
diff --git a/src/cmd/link/internal/loadelf/ldelf.go b/src/cmd/link/internal/loadelf/ldelf.go
index d85d91948a7..916b7cf9f2d 100644
--- a/src/cmd/link/internal/loadelf/ldelf.go
+++ b/src/cmd/link/internal/loadelf/ldelf.go
@@ -1088,6 +1088,11 @@ func readelfsym(arch *sys.Arch, syms *sym.Symbols, elfobj *ElfObj, i int, elfsym
 				if elfsym.other == 2 {
 					s.Attr |= sym.AttrVisibilityHidden
 				}
+
+				// Allow weak symbols to be duplicated when already defined.
+				if s.Outer != nil {
+					s.Attr |= sym.AttrDuplicateOK
+				}
 			}
 
 		default: