cmd/link: do not add duplicate symbols to Allsym

When building shared libraries, all symbols on Allsym are marked reachable.
What I didn't realize was that this includes the ".dup" symbols created when
"dupok" symbols are read from multiple package files. This breaks now because
deadcode makes some assumptions that fail for these ".dup" symbols, but in any
case was a bad idea -- I suspect this change makes libstd.so a bunch smaller,
but creating it was broken before this CL so I can't be sure.

This change simply stops adding these symbols to Allsym, which might make some
of the many iterations over Allsym the linker does a touch quicker, although
that's not the motivation here.

Add a test that no symbols called ".dup" makes it into the runtime shared
library.

Fixes #14841

Change-Id: I65dd6e88d150a770db2d01b75cfe5db5fd4f8d25
Reviewed-on: https://go-review.googlesource.com/20780
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testshared/shared_test.go

@@ -203,6 +203,26 @@ func TestNoTextrel(t *testing.T) {
 	}
 }
 
+// The shared library does not contain symbols called ".dup"
+func TestNoDupSymbols(t *testing.T) {
+	sopath := filepath.Join(gorootInstallDir, soname)
+	f, err := elf.Open(sopath)
+	if err != nil {
+		t.Fatal("elf.Open failed: ", err)
+	}
+	defer f.Close()
+	syms, err := f.Symbols()
+	if err != nil {
+		t.Errorf("error reading symbols %v", err)
+		return
+	}
+	for _, s := range syms {
+		if s.Name == ".dup" {
+			t.Fatalf("%s contains symbol called .dup", sopath)
+		}
+	}
+}
+
 // The install command should have created a "shlibname" file for the
 // listed packages (and runtime/cgo, and math on arm) indicating the
 // name of the shared library containing it.

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

@@ -348,7 +348,7 @@ func decodetype_methods(s *LSym) []methodsig {
 	numMethods := int(decode_inuxi(s.P[off+2*Thearch.Ptrsize:], Thearch.Intsize))
 	r := decode_reloc(s, int32(off+Thearch.Ptrsize))
 	if r.Sym != s {
-		panic(fmt.Sprintf("method slice pointer in %q leads to a different symbol", s.Name))
+		panic(fmt.Sprintf("method slice pointer in %s leads to a different symbol %s", s, r.Sym))
 	}
 	off = int(r.Add)                    // array of reflect.method values
 	sizeofMethod := 5 * Thearch.Ptrsize // sizeof reflect.method in program

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

@@ -175,8 +175,6 @@ func ldobjfile(ctxt *Link, f *obj.Biobuf, pkg string, length int64, pn string) {
 	}
 }
 
-var readsym_ndup int
-
 func readsym(ctxt *Link, f *obj.Biobuf, pkg string, pn string) {
 	if obj.Bgetc(f) != 0xfe {
 		log.Fatalf("readsym out of sync")
@@ -211,8 +209,7 @@ func readsym(ctxt *Link, f *obj.Biobuf, pkg string, pn string) {
 		}
 		if len(s.P) > 0 {
 			dup = s
-			s = linknewsym(ctxt, ".dup", readsym_ndup)
-			readsym_ndup++ // scratch
+			s = linknewsym(ctxt, ".dup", -1)
 		}
 	}
 

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

@@ -173,7 +173,11 @@ func linknewsym(ctxt *Link, symb string, v int) *LSym {
 	s.Version = int16(v)
 	ctxt.Nsymbol++
 
-	ctxt.Allsym = append(ctxt.Allsym, s)
+	if v != -1 {
+		ctxt.Allsym = append(ctxt.Allsym, s)
+	} else if v < -1 {
+		ctxt.Diag("invalid version %d in linknewsym", v)
+	}
 	return s
 }