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

+// 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"

misc/cgo/test/testdata/issue29563/weak.go

+// 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())
+}

misc/cgo/test/testdata/issue29563/weak1.c

+// 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;
+}

misc/cgo/test/testdata/issue29563/weak2.c

+// 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;
+}

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: