cmd/link: avoid writing to read-only memory in addstrdata

When the linker's -X flag is used, it will overwrite the symbol's
content (sym.P) in addstrdata. The symbol's content may be in
read-only memory, in which case overwriting it will fault. Do
copy-on-write to fix this.

Change-Id: I34d583f44c30d187042757e19a14c1ef7d3e613c
Reviewed-on: https://go-review.googlesource.com/c/go/+/173937
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/ld/data.go

@@ -939,6 +939,10 @@ func addstrdata(ctxt *Link, name, value string) {
 
 	s.Size = 0
 	s.P = s.P[:0]
+	if s.Attr.ReadOnly() {
+		s.P = make([]byte, 0, ctxt.Arch.PtrSize*2)
+		s.Attr.Set(sym.AttrReadOnly, false)
+	}
 	s.R = s.R[:0]
 	reachable := s.Attr.Reachable()
 	s.AddAddr(ctxt.Arch, sp)

src/cmd/link/link_test.go

@@ -229,3 +229,31 @@ func TestBuildFortvOS(t *testing.T) {
 		t.Fatalf("%v: %v:\n%s", link.Args, err, out)
 	}
 }
+
+var testXFlagSrc = `
+package main
+var X = "hello"
+var Z = [99999]int{99998:12345} // make it large enough to be mmaped
+func main() { println(X) }
+`
+
+func TestXFlag(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	tmpdir, err := ioutil.TempDir("", "TestXFlag")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	src := filepath.Join(tmpdir, "main.go")
+	err = ioutil.WriteFile(src, []byte(testXFlagSrc), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-ldflags=-X=main.X=meow", "-o", filepath.Join(tmpdir, "main"), src)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Errorf("%v: %v:\n%s", cmd.Args, err, out)
+	}
+}