cmd/link: add info.plist support via plist linker option

This change adds a flag to the linker to allow it to embed info.plist
data into the resulting binaries. This can, for example, be used to
add code signing properties to binaries.

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

@@ -2360,6 +2360,30 @@ func appendString(data []byte, s string) []byte {
 	return data
 }
 
+// addPlist will add an info.Plist file, as segment __TEXT and section __info_plist.
+// This is used when using the internal linker. Search in lib.go for __info_plist to find
+// the external linker handling of this. Adding a section with an info.plist file allows
+// Go executables to be code-signed for macOS.
+func (ctxt *Link) addPlist(fn string) {
+	if fn == "" {
+		return
+	}
+	b, err := os.ReadFile(fn)
+	if err != nil {
+		Exitf("reading %s: %s", fn, err)
+	}
+
+	ldr := ctxt.loader
+	sect := Segtext.Sections[len(Segtext.Sections)-1]
+	builder := ldr.MakeSymbolBuilder(".info.plist") // __info_list
+	builder.Addstring(string(b) + "\x00")           // asciiz
+	builder.SetType(sym.SELFRXSECT)                 // Produces standalone section, without fixups
+	builder.SetAlign(16)                            // Sections don't seem to strictly require this
+	builder.SetReachable(true)                      // Don't let the linker drop it
+	builder.SetSect(sect)
+	ldr.SetSymSect(builder.Sym(), sect)
+}
+
 // assign addresses to text
 func (ctxt *Link) textaddress() {
 	addsection(ctxt.loader, ctxt.Arch, &Segtext, ".text", 05)

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

@@ -1811,6 +1811,15 @@ func (ctxt *Link) hostlink() {
 		argv = append(argv, p)
 		checkStatic(p)
 	}
+
+	// this will add an info.Plist file, as segment __TEXT and section __info_plist.
+	// This is used when using the external linker. Search in data.go for __info_plist to find
+	// the internal linker handling of this. Adding a section with an info.plist file allows
+	// Go executables to be code-signed for macOS.
+	if ctxt.LinkMode == LinkExternal && ctxt.IsDarwin() && *flagPlist != "" {
+		argv = append(argv, "-extldflags", fmt.Sprintf(`-sectcreate __TEXT __info_plist %q`, *flagPlist))
+	}
+
 	if ctxt.HeadType == objabi.Hwindows {
 		// Determine which linker we're using. Add in the extldflags in
 		// case used has specified "-fuse-ld=...".

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

@@ -82,6 +82,8 @@ var (
 
 	flagCaptureHostObjs = flag.String("capturehostobjs", "", "capture host object files loaded during internal linking to specified dir")
 
+	flagPlist = flag.String("plist", "", "filename of info.plist to include")
+
 	flagA             = flag.Bool("a", false, "no-op (deprecated)")
 	FlagC             = flag.Bool("c", false, "dump call graph")
 	FlagD             = flag.Bool("d", false, "disable dynamic executable")
@@ -357,6 +359,8 @@ func Main(arch *sys.Arch, theArch Arch) {
 	dwarfcompress(ctxt)
 	bench.Start("layout")
 	filesize := ctxt.layout(order)
+	bench.Start("plist")
+	ctxt.addPlist(*flagPlist)
 
 	// Write out the output file.
 	// It is split into two parts (Asmb and Asmb2). The first