cmd/ld: fix bug with "runtime/cgo" in external link mode

In external link mode the linker explicitly adds the string
constant "runtime/cgo".  It adds the string constant using the
same symbol name as the compiler, but a different format.  The
compiler assumes that the string data immediately follows the
string header, but the linker puts the two in different
sections.  The result is bad string data when the compiler
sees "runtime/cgo" used as a string constant.

The compiler assumption is in datastring in [568]g/gobj.c.
The linker layout is in addstrdata in ld/data.c.  The compiler
assumption is valid for string literals.  The linker is not
creating a string literal, so its assumption is also valid.

There are a few ways to avoid this problem.  This patch fixes
it by only doing the fake import of runtime/cgo if necessary,
and by only creating the string symbol if necessary.

Fixes #7234.

LGTM=dvyukov
R=golang-codereviews, dvyukov, bradfitz
CC=golang-codereviews
https://golang.org/cl/58410043

misc/cgo/test/issue7234_test.go

+// Copyright 2014 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 cgotest
+
+import "testing"
+
+// This test actually doesn't have anything to do with cgo.  It is a
+// test of http://golang.org/issue/7234, a compiler/linker bug in
+// handling string constants when using -linkmode=external.  The test
+// is in this directory because we routinely test -linkmode=external
+// here.
+
+var v7234 = [...]string{"runtime/cgo"}
+
+func TestIssue7234(t *testing.T) {
+	if v7234[0] != "runtime/cgo" {
+		t.Errorf("bad string constant %q", v7234[0])
+	}
+}

src/cmd/ld/lib.c

@@ -164,6 +164,7 @@ loadlib(void)
 {
 	int i, w, x;
 	LSym *s, *gmsym;
+	char* cgostrsym;
 
 	if(flag_shared) {
 		s = linklookup(ctxt, "runtime.islibrary", 0);
@@ -176,7 +177,15 @@ loadlib(void)
 		loadinternal("math");
 	if(flag_race)
 		loadinternal("runtime/race");
-	if(linkmode == LinkExternal) {
+
+	for(i=0; i<ctxt->libraryp; i++) {
+		if(debug['v'] > 1)
+			Bprint(&bso, "%5.2f autolib: %s (from %s)\n", cputime(), ctxt->library[i].file, ctxt->library[i].objref);
+		iscgo |= strcmp(ctxt->library[i].pkg, "runtime/cgo") == 0;
+		objfile(ctxt->library[i].file, ctxt->library[i].pkg);
+	}
+	
+	if(linkmode == LinkExternal && !iscgo) {
 		// This indicates a user requested -linkmode=external.
 		// The startup code uses an import of runtime/cgo to decide
 		// whether to initialize the TLS.  So give it one.  This could
@@ -184,7 +193,6 @@ loadlib(void)
 		loadinternal("runtime/cgo");
 
 		// Pretend that we really imported the package.
-		// This will do no harm if we did in fact import it.
 		s = linklookup(ctxt, "go.importpath.runtime/cgo.", 0);
 		s->type = SDATA;
 		s->dupok = 1;
@@ -192,16 +200,11 @@ loadlib(void)
 
 		// Provided by the code that imports the package.
 		// Since we are simulating the import, we have to provide this string.
-		addstrdata("go.string.\"runtime/cgo\"", "runtime/cgo");
+		cgostrsym = "go.string.\"runtime/cgo\"";
+		if(linkrlookup(ctxt, cgostrsym, 0) == nil)
+			addstrdata(cgostrsym, "runtime/cgo");
 	}
 
-	for(i=0; i<ctxt->libraryp; i++) {
-		if(debug['v'] > 1)
-			Bprint(&bso, "%5.2f autolib: %s (from %s)\n", cputime(), ctxt->library[i].file, ctxt->library[i].objref);
-		iscgo |= strcmp(ctxt->library[i].pkg, "runtime/cgo") == 0;
-		objfile(ctxt->library[i].file, ctxt->library[i].pkg);
-	}
-	
 	if(linkmode == LinkAuto) {
 		if(iscgo && externalobj)
 			linkmode = LinkExternal;