cmd/link: fix bug with -newobj and "ld -r" ELF host objects

When the ELF host object loader encounters a static/hidden symbol, it
creates a sym.Symbol for it but does not enter it into the sym.Symbols
lookup table. Under -newobj mode, this was not happening correctly; we
were adding the sym via loader.LookupOrCreate, which resulted in
collisions when it encountered symbols with the same name + version +
section (this can happen for "ld -r" objects).

Fixes #35779.

Change-Id: I36d40fc1efc03fc1cd8ae6b76cb6a0d2a957389c
Reviewed-on: https://go-review.googlesource.com/c/go/+/208479
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/loadelf/ldelf.go

@@ -454,9 +454,12 @@ func parseArmAttributes(e binary.ByteOrder, data []byte) (found bool, ehdrFlags
 
 func Load(l *loader.Loader, arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length int64, pn string, flags uint32) ([]*sym.Symbol, uint32, error) {
 	newSym := func(name string, version int) *sym.Symbol {
+		return l.Create(name, syms)
+	}
+	lookup := func(name string, version int) *sym.Symbol {
 		return l.LookupOrCreate(name, version, syms)
 	}
-	return load(arch, syms.IncVersion(), newSym, newSym, f, pkg, length, pn, flags)
+	return load(arch, syms.IncVersion(), newSym, lookup, f, pkg, length, pn, flags)
 }
 
 func LoadOld(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length int64, pn string, flags uint32) ([]*sym.Symbol, uint32, error) {
@@ -1101,6 +1104,9 @@ func readelfsym(newSym, lookup lookupFunc, arch *sys.Arch, elfobj *ElfObj, i int
 				// local names and hidden global names are unique
 				// and should only be referenced by their index, not name, so we
 				// don't bother to add them into the hash table
+				// FIXME: pass empty string here for name? This would
+				// reduce mem use, but also (possibly) make it harder
+				// to debug problems.
 				s = newSym(elfsym.name, localSymVersion)
 
 				s.Attr |= sym.AttrVisibilityHidden

src/cmd/link/internal/loader/loader.go

@@ -130,6 +130,9 @@ const (
 	FlagStrictDups = 1 << iota
 )
 
+// anonVersion is used to tag symbols created by loader.Create.
+const anonVersion = -1
+
 func NewLoader(flags uint32) *Loader {
 	nbuiltin := goobj2.NBuiltin()
 	return &Loader{
@@ -842,13 +845,15 @@ func (l *Loader) ExtractSymbols(syms *sym.Symbols) {
 		l.Syms[oldI] = nil
 	}
 
-	// For now, add all symbols to ctxt.Syms.
+	// Add symbols to the ctxt.Syms lookup table. This explicitly
+	// skips things created via loader.Create (marked with
+	// anonVersion), since if we tried to add these we'd wind up with
+	// collisions.
 	for _, s := range l.Syms {
-		if s != nil && s.Name != "" {
+		if s != nil && s.Name != "" && s.Version != anonVersion {
 			syms.Add(s)
 		}
 	}
-
 }
 
 // addNewSym adds a new sym.Symbol to the i-th index in the list of symbols.
@@ -980,6 +985,28 @@ func (l *Loader) LookupOrCreate(name string, version int, syms *sym.Symbols) *sy
 	return s
 }
 
+// Create creates a symbol with the specified name, but does not
+// insert it into any lookup table (hence it is possible to create a
+// symbol name with name X when there is already an existing symbol
+// named X entered into the loader). This method is intended for
+// static/hidden symbols discovered while loading host objects.
+func (l *Loader) Create(name string, syms *sym.Symbols) *sym.Symbol {
+	i := l.max + 1
+	l.max++
+	if l.extStart == 0 {
+		l.extStart = i
+	}
+
+	// Note the use of anonVersion -- this is to mark the symbol so that
+	// it can be skipped when ExtractSymbols is adding ext syms to the
+	// sym.Symbols hash.
+	l.extSyms = append(l.extSyms, nameVer{name, anonVersion})
+	l.growSyms(int(i))
+	s := syms.Newsym(name, -1)
+	l.Syms[i] = s
+	return s
+}
+
 func loadObjFull(l *Loader, r *oReader) {
 	lib := r.unit.Lib
 	istart := l.startIndex(r)