cmd: linker changes for shared library initialization

Suggested by iant@, this change:
  - looks for a symbol _rt0_<GOARCH>_<GOOS>_lib,
  - if the symbol is present, adds a new entry into the .init_array ELF
    section that points to the symbol.

The end-effect is that the symbol _rt0_<GOARCH>_<GOOS>_lib will be
invoked as soon as the (ELF) shared library is loaded, which will in turn
initialize the runtime. (To be implemented.)

Change-Id: I99911a180215a6df18f8a18483d12b9b497b48f4
Reviewed-on: https://go-review.googlesource.com/7692
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/ld/data.go

@@ -936,6 +936,15 @@ func Addstring(s *LSym, str string) int64 {
 	return int64(r)
 }
 
+func addinitarrdata(s *LSym) {
+	p := s.Name + ".ptr"
+	sp := Linklookup(Ctxt, p, 0)
+	sp.Type = SINITARR
+	sp.Size = 0
+	sp.Dupok = 1
+	Addaddr(Ctxt, sp, s)
+}
+
 func dosymtype() {
 	for s := Ctxt.Allsym; s != nil; s = s.Allsym {
 		if len(s.P) > 0 {
@@ -946,6 +955,11 @@ func dosymtype() {
 				s.Type = SNOPTRDATA
 			}
 		}
+		// Create a new entry in the .init_array section that points to the
+		// library initializer function.
+		if Flag_shared != 0 && s.Name == INITENTRY {
+			addinitarrdata(s)
+		}
 	}
 }
 

src/cmd/internal/ld/lib.go

@@ -1362,6 +1362,7 @@ func genasmsym(put func(*LSym, string, int, int64, int64, int, *LSym)) {
 			SRODATA,
 			SSYMTAB,
 			SPCLNTAB,
+			SINITARR,
 			SDATA,
 			SNOPTRDATA,
 			SELFROSECT,

src/runtime/rt0_linux_amd64.s

@@ -10,6 +10,13 @@ TEXT _rt0_amd64_linux(SB),NOSPLIT,$-8
 	MOVQ	$main(SB), AX
 	JMP	AX
 
+// When linking with -shared, this symbol is called when the shared library
+// is loaded.
+TEXT _rt0_amd64_linux_lib(SB),NOSPLIT,$0
+	// TODO(spetrovic): Do something useful, like calling $main.  (Note that
+	// this has to be done in a separate thread, as main is expected to block.)
+	RET
+
 TEXT main(SB),NOSPLIT,$-8
 	MOVQ	$runtime·rt0_go(SB), AX
 	JMP	AX