cmd/internal/ld: allow -r to override rpath when -linkshared

Including having -r "" preventing rpath from being set at all.

Change-Id: Ib40d7bf93a6e9ef21985c4a05b5703e4fbd1cd1b
Reviewed-on: https://go-review.googlesource.com/8806
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/ld/elf.go

@@ -1818,8 +1818,8 @@ func doelf() {
 			Elfwritedynent(s, DT_RELENT, ELF32RELSIZE)
 		}
 
-		if rpath != "" {
+		if rpath.val != "" {
-			Elfwritedynent(s, DT_RUNPATH, uint64(Addstring(dynstr, rpath)))
+			Elfwritedynent(s, DT_RUNPATH, uint64(Addstring(dynstr, rpath.val)))
 		}
 
 		if Thearch.Thechar == '9' {

src/cmd/internal/ld/lib.go

@@ -108,12 +108,27 @@ type Arch struct {
 	Vput             func(uint64)
 }
 
+type Rpath struct {
+	set bool
+	val string
+}
+
+func (r *Rpath) Set(val string) error {
+	r.set = true
+	r.val = val
+	return nil
+}
+
+func (r *Rpath) String() string {
+	return r.val
+}
+
 var (
 	Thearch Arch
 	datap   *LSym
 	Debug   [128]int
 	Lcsize  int32
-	rpath   string
+	rpath   Rpath
 	Spsize  int32
 	Symsize int32
 )
@@ -935,8 +950,8 @@ func hostlink() {
 	argv = append(argv, "-o")
 	argv = append(argv, outfile)
 
-	if rpath != "" {
+	if rpath.val != "" {
-		argv = append(argv, fmt.Sprintf("-Wl,-rpath,%s", rpath))
+		argv = append(argv, fmt.Sprintf("-Wl,-rpath,%s", rpath.val))
 	}
 
 	// Force global symbols to be exported for dlopen, etc.
@@ -955,7 +970,9 @@ func hostlink() {
 		for _, shlib := range Ctxt.Shlibs {
 			dir, base := filepath.Split(shlib)
 			argv = append(argv, "-L"+dir)
+			if !rpath.set {
 				argv = append(argv, "-Wl,-rpath="+dir)
+			}
 			base = strings.TrimSuffix(base, ".so")
 			base = strings.TrimPrefix(base, "lib")
 			argv = append(argv, "-l"+base)

src/cmd/internal/ld/pobj.go

@@ -115,7 +115,7 @@ func Ldmain() {
 	flag.BoolVar(&Linkshared, "linkshared", false, "link against installed Go shared libraries")
 	obj.Flagcount("n", "dump symbol table", &Debug['n'])
 	obj.Flagstr("o", "outfile: set output file", &outfile)
-	obj.Flagstr("r", "dir1:dir2:...: set ELF dynamic linker search path", &rpath)
+	flag.Var(&rpath, "r", "dir1:dir2:...: set ELF dynamic linker search path")
 	obj.Flagcount("race", "enable race detector", &flag_race)
 	obj.Flagcount("s", "disable symbol table", &Debug['s'])
 	var flagShared int