cmd/link: add -msan option

The -msan option causes the linker to link against the runtime/msan
package in order to use the C/C++ memory sanitizer.

This CL passes tests but is not usable by itself.  The actual
runtime/msan package, and support for -msan in the go tool and the
compiler, and tests, are in separate CLs.

Change-Id: I02c097393b98c5b80e40ee3dbc167a8b4d23efe0
Reviewed-on: https://go-review.googlesource.com/16161
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/doc.go

@@ -72,6 +72,8 @@ Flags:
 		Write memory profile to file.
 	-memprofilerate rate
 		Set runtime.MemProfileRate to rate.
+	-msan
+		Link with C/C++ memory sanitizer support.
 	-o file
 		Write output to file (default a.out, or a.out.exe on Windows).
 	-r dir1:dir2:...

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

@@ -198,6 +198,7 @@ var (
 	elfglobalsymndx    int
 	flag_installsuffix string
 	flag_race          int
+	flag_msan          int
 	Buildmode          BuildMode
 	Linkshared         bool
 	tracksym           string
@@ -373,6 +374,9 @@ func libinit() {
 	} else if flag_race != 0 {
 		suffixsep = "_"
 		suffix = "race"
+	} else if flag_msan != 0 {
+		suffixsep = "_"
+		suffix = "msan"
 	}
 
 	Lflag(fmt.Sprintf("%s/pkg/%s_%s%s%s", goroot, goos, goarch, suffixsep, suffix))
@@ -483,6 +487,9 @@ func loadlib() {
 	if flag_race != 0 {
 		loadinternal("runtime/race")
 	}
+	if flag_msan != 0 {
+		loadinternal("runtime/msan")
+	}
 
 	var i int
 	for i = 0; i < len(Ctxt.Library); i++ {
@@ -517,6 +524,11 @@ func loadlib() {
 		if (Thearch.Thechar == '5' || Thearch.Thechar == '7') && HEADTYPE == obj.Hdarwin && iscgo {
 			Linkmode = LinkExternal
 		}
+
+		// Force external linking for msan.
+		if flag_msan != 0 {
+			Linkmode = LinkExternal
+		}
 	}
 
 	// cmd/7l doesn't support cgo internal linking
@@ -797,6 +809,7 @@ var internalpkg = []string{
 	"os/user",
 	"runtime/cgo",
 	"runtime/race",
+	"runtime/msan",
 }
 
 func ldhostobj(ld func(*obj.Biobuf, string, int64, string), f *obj.Biobuf, pkg string, length int64, pn string, file string) {

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

@@ -98,6 +98,7 @@ func Ldmain() {
 	obj.Flagstr("k", "set field tracking `symbol`", &tracksym)
 	obj.Flagfn1("linkmode", "set link `mode` (internal, external, auto)", setlinkmode)
 	flag.BoolVar(&Linkshared, "linkshared", false, "link against installed Go shared libraries")
+	obj.Flagcount("msan", "enable MSan interface", &flag_msan)
 	obj.Flagcount("n", "dump symbol table", &Debug['n'])
 	obj.Flagstr("o", "write output to `file`", &outfile)
 	flag.Var(&rpath, "r", "set the ELF dynamic linker search `path` to dir1:dir2:...")