cmd/go, cmd/link: -buildmode=pie for linux/amd64

Depends on external linking right now. I have no immediate use for
this, but wanted to check how hard it is to support as android/amd64
is coming and it will require PIE.

Change-Id: I65c6b19159f40db4c79cf312cd0368c2b2527bfd
Reviewed-on: https://go-review.googlesource.com/16072Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testshared/shared_test.go

@@ -359,6 +359,36 @@ func TestCgoExecutable(t *testing.T) {
 	run(t, "cgo executable", "./bin/execgo")
 }
 
+func checkPIE(t *testing.T, name string) {
+	f, err := elf.Open(name)
+	if err != nil {
+		t.Fatal("elf.Open failed: ", err)
+	}
+	defer f.Close()
+	if f.Type != elf.ET_DYN {
+		t.Errorf("%s has type %v, want ET_DYN", name, f.Type)
+	}
+	if hasDynTag(f, elf.DT_TEXTREL) {
+		t.Errorf("%s has DT_TEXTREL set", name)
+	}
+}
+
+func TestTrivialPIE(t *testing.T) {
+	name := "trivial_pie"
+	goCmd(t, "build", "-buildmode=pie", "-o="+name, "trivial")
+	defer os.Remove(name)
+	run(t, name, "./"+name)
+	checkPIE(t, name)
+}
+
+func TestCgoPIE(t *testing.T) {
+	name := "cgo_pie"
+	goCmd(t, "build", "-buildmode=pie", "-o="+name, "execgo")
+	defer os.Remove(name)
+	run(t, name, "./"+name)
+	checkPIE(t, name)
+}
+
 // Build a GOPATH package into a shared library that links against the goroot runtime
 // and an executable that links against both.
 func TestGopathShlib(t *testing.T) {

src/cmd/go/alldocs.go

@@ -819,6 +819,11 @@ are:
 		Build the listed main packages and everything they import into
 		executables. Packages not named main are ignored.
 
+	-buildmode=pie
+		Build the listed main packages and everything they import into
+		position independent executables (PIE). Packages not named
+		main are ignored.
+
 
 File types
 

src/cmd/go/build.go

@@ -374,7 +374,7 @@ func buildModeInit() {
 			fatalf("-buildmode=pie not supported by gccgo")
 		} else {
 			switch platform {
-			case "android/arm":
+			case "android/arm", "linux/amd64":
 				codegenArg = "-shared"
 			default:
 				fatalf("-buildmode=pie not supported on %s\n", platform)

src/cmd/go/help.go

@@ -576,5 +576,10 @@ are:
 	-buildmode=exe
 		Build the listed main packages and everything they import into
 		executables. Packages not named main are ignored.
+
+	-buildmode=pie
+		Build the listed main packages and everything they import into
+		position independent executables (PIE). Packages not named
+		main are ignored.
 `,
 }

src/cmd/go/pkg.go

@@ -825,10 +825,10 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		importPaths = append(importPaths, "syscall")
 	}
 
-	// Currently build mode c-shared, or -linkshared, forces
+	// Currently build modes c-shared, pie, and -linkshared force
 	// external linking mode, and external linking mode forces an
 	// import of runtime/cgo.
-	if p.Name == "main" && !p.Goroot && (buildBuildmode == "c-shared" || buildLinkshared) {
+	if p.Name == "main" && !p.Goroot && (buildBuildmode == "c-shared" || buildBuildmode == "pie" || buildLinkshared) {
 		importPaths = append(importPaths, "runtime/cgo")
 	}
 

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

@@ -303,7 +303,7 @@ func (mode *BuildMode) Set(s string) error {
 		*mode = BuildmodeExe
 	case "pie":
 		switch goos {
-		case "android":
+		case "android", "linux":
 		default:
 			return badmode()
 		}
@@ -516,6 +516,12 @@ func loadlib() {
 			Linkmode = LinkExternal
 		}
 
+		// Force external linking for PIE executables, as
+		// internal linking does not support TLS_IE.
+		if Buildmode == BuildmodePIE {
+			Linkmode = LinkExternal
+		}
+
 		// cgo on Darwin must use external linking
 		// we can always use external linking, but then there will be circular
 		// dependency problems when compiling natively (external linking requires