cmd/dist, cmd/link: allow passing default dynamic linker/loader

Add an environment variable to make.bash to allow setting the default
dynamic linker/loader. This fixes alpine builds to use
/lib/ld-musl-x86_64.so.1:

  $ readelf -l ../bin/go | grep 'interpreter:' | sed -e 's/^.*interpreter: \(.*\)[]]/\1/'
  /lib/ld-musl-x86_64.so.1

Also re-enable the internal linker tests that were previously disabled
for alpine (CL 41759, CL 41678).

Fixes golang#18243
Updates golang#19938

This resurrects CL 50070 which was authored by Jessie Frazelle.

Change-Id: I132b5282045a3d60c8568e3b002a7f075eac2d93

Author: tklauser

src/cmd/dist/build.go

@@ -48,6 +48,7 @@ var (
 	defaultcflags    string
 	defaultldflags   string
 	defaultpkgconfig string
+	defaultldso      string
 
 	rebuildall   bool
 	defaultclang bool
@@ -207,6 +208,8 @@ func xinit() {
 	}
 	defaultpkgconfig = b
 
+	defaultldso = os.Getenv("GO_LDSO")
+
 	// For tools being invoked but also for os.ExpandEnv.
 	os.Setenv("GO386", go386)
 	os.Setenv("GOARCH", goarch)

src/cmd/dist/buildruntime.go

@@ -76,6 +76,7 @@ func mkzbootstrap(file string) {
 	fmt.Fprintf(&buf, "const defaultGOOS = runtime.GOOS\n")
 	fmt.Fprintf(&buf, "const defaultGOARCH = runtime.GOARCH\n")
 	fmt.Fprintf(&buf, "const defaultGO_EXTLINK_ENABLED = `%s`\n", goextlinkenabled)
+	fmt.Fprintf(&buf, "const defaultGO_LDSO = `%s`\n", defaultldso)
 	fmt.Fprintf(&buf, "const version = `%s`\n", findgoversion())
 	fmt.Fprintf(&buf, "const stackGuardMultiplier = %d\n", stackGuardMultiplier())
 	fmt.Fprintf(&buf, "const goexperiment = `%s`\n", os.Getenv("GOEXPERIMENT"))

src/cmd/dist/test.go

@@ -568,10 +568,7 @@ func (t *tester) registerTests() {
 	}
 
 	// Test internal linking of PIE binaries where it is supported.
-	if goos == "linux" && goarch == "amd64" && !isAlpineLinux() {
-		// Issue 18243: We don't have a way to set the default
-		// dynamic linker used in internal linking mode. So
-		// this test is skipped on Alpine.
+	if goos == "linux" && goarch == "amd64" {
 		t.tests = append(t.tests, distTest{
 			name:    "pie_internal",
 			heading: "internal linking of -buildmode=pie",
@@ -925,10 +922,6 @@ func (t *tester) internalLink() bool {
 	if goarch == "arm64" || goarch == "mips64" || goarch == "mips64le" || goarch == "mips" || goarch == "mipsle" {
 		return false
 	}
-	if isAlpineLinux() {
-		// Issue 18243.
-		return false
-	}
 	return true
 }
 

src/cmd/internal/objabi/util.go

@@ -28,6 +28,7 @@ var (
 	GOARM    = goarm()
 	GOMIPS   = gomips()
 	GOMIPS64 = gomips64()
+	GO_LDSO  = envOr("GO_LDSO", defaultGO_LDSO)
 	Version  = version
 )
 

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

@@ -1840,6 +1840,11 @@ func Asmbelf(ctxt *Link, symo int64) {
 		sh.type_ = SHT_PROGBITS
 		sh.flags = SHF_ALLOC
 		sh.addralign = 1
+
+		if interpreter == "" && objabi.GO_LDSO != "" {
+			interpreter = objabi.GO_LDSO
+		}
+
 		if interpreter == "" {
 			switch ctxt.HeadType {
 			case objabi.Hlinux:

src/make.bash

@@ -34,6 +34,9 @@
 # controls the default behavior of the linker's -linkmode option.  The
 # default value depends on the system.
 #
+# GO_LDSO: Sets the default dynamic linker/loader (ld.so) to be used
+# by the internal linker.
+#
 # CC: Command line to run to compile C code for GOHOSTARCH.
 # Default is "gcc". Also supported: "clang".
 #
@@ -126,6 +129,15 @@ if [ "$(uname -s)" = "GNU/kFreeBSD" ]; then
 	export CGO_ENABLED=0
 fi
 
+# On Alpine Linux, use the musl dynamic linker/loader
+if [ -f "/etc/alpine-release" ]; then
+	if type readelf >/dev/null 2>&1; then
+		echo "int main() { return 0; }" | ${CC:-gcc} -o ./test-alpine-ldso -x c -
+		export GO_LDSO=$(readelf -l ./test-alpine-ldso | grep 'interpreter:' | sed -e 's/^.*interpreter: \(.*\)[]]/\1/')
+		rm -f ./test-alpine-ldso
+	fi
+fi
+
 # Clean old generated file that will cause problems in the build.
 rm -f ./runtime/runtime_defs.go