cmd/go: pass -no-pie along with -Wl,-r to gcc when supported on linux

Go fails to build on a system which has PIE enabled by default like this:

/usr/bin/ld: -r and -pie may not be used together
collect2: error: ld returned 1 exit status

The only system I know that has this property right now is Ubuntu Xenial
running on s390x, which is hardly the most accessible system, but it's planned
to enable this on amd64 soon too. The fix is to pass -no-pie along with -Wl,-r
to the compiler, but unfortunately that flag is very new as well. So this does
a test compile of a trivial file to see if the flag is supported.

Change-Id: I1345571142b7c3a96212e43297d19e84ec4a3d41
Reviewed-on: https://go-review.googlesource.com/18359Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/build.go

@@ -2711,6 +2711,10 @@ func (tools gccgoToolchain) ld(b *builder, root *action, out string, allactions
 		// libffi.
 		ldflags = append(ldflags, "-Wl,-r", "-nostdlib", "-Wl,--whole-archive", "-lgolibbegin", "-Wl,--no-whole-archive")
 
+		if b.gccSupportsNoPie() {
+			ldflags = append(ldflags, "-no-pie")
+		}
+
 		// We are creating an object file, so we don't want a build ID.
 		ldflags = b.disableBuildID(ldflags)
 
@@ -2902,6 +2906,36 @@ func (b *builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 	return a
 }
 
+// On systems with PIE (position independent executables) enabled by default,
+// -no-pie must be passed when doing a partial link with -Wl,-r. But -no-pie is
+// not supported by all compilers.
+func (b *builder) gccSupportsNoPie() bool {
+	if goos != "linux" {
+		// On some BSD platforms, error messages from the
+		// compiler make it to the console despite cmd.Std*
+		// all being nil. As -no-pie is only required on linux
+		// systems so far, we only test there.
+		return false
+	}
+	src := filepath.Join(b.work, "trivial.c")
+	if err := ioutil.WriteFile(src, []byte{}, 0666); err != nil {
+		return false
+	}
+	cmdArgs := b.gccCmd(b.work)
+	cmdArgs = append(cmdArgs, "-no-pie", "-c", "trivial.c")
+	if buildN || buildX {
+		b.showcmd(b.work, "%s", joinUnambiguously(cmdArgs))
+		if buildN {
+			return false
+		}
+	}
+	cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
+	cmd.Dir = b.work
+	cmd.Env = envForDir(cmd.Dir, os.Environ())
+	err := cmd.Run()
+	return err == nil
+}
+
 // gccArchArgs returns arguments to pass to gcc based on the architecture.
 func (b *builder) gccArchArgs() []string {
 	switch goarch {
@@ -3158,6 +3192,10 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, pcCFLAGS, pcLDFLAGS, cgofi
 	}
 	ldflags := stringList(bareLDFLAGS, "-Wl,-r", "-nostdlib", staticLibs)
 
+	if b.gccSupportsNoPie() {
+		ldflags = append(ldflags, "-no-pie")
+	}
+
 	// We are creating an object file, so we don't want a build ID.
 	ldflags = b.disableBuildID(ldflags)