[dev.link] cmd/link: fix payload pointer liveness

Currently, the symbol updater uses a pointer pointing to the
loader's payloads array. If the payloads slice grows (and moves),
the pointer may become stale and no longer point to the symbol's
actual payload. Specifically, consider

	sb, sym := l.MakeSymbolUpdater(...)
	// add a bunch of external symbols, which grows payload slice
	sb.SetType(t)
	l.SymType(sym) // may not return t

sb.SetType on line 3 may not have the desired effect, as
sb.extSymPayload may no longer point to the right payload. As a
result, the type we get on line 4 may be not the one we set.

Fix this by making the payload's address permanent. Once it is
allocated it will never move.

Change-Id: Iab190ea5aceb5c37f91d09ad4ffd458e881b03f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/217063
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

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

@@ -158,7 +158,8 @@ type Loader struct {
 	extStaticSyms map[nameVer]Sym   // externally defined static symbols, keyed by name
 	overwrite     map[Sym]Sym       // overwrite[i]=j if symbol j overwrites symbol i
 
-	payloads []extSymPayload // contents of linker-materialized external syms
+	payloadBatch []extSymPayload
+	payloads     []*extSymPayload // contents of linker-materialized external syms
 	values       []int64          // symbol values, indexed by global sym index
 
 	itablink map[Sym]struct{} // itablink[j] defined if j is go.itablink.*
@@ -349,6 +350,7 @@ func (l *Loader) newExtSym(name string, ver int) Sym {
 	}
 	l.growSyms(int(i))
 	pi := i - l.extStart
+	l.payloads[pi] = l.allocPayload()
 	l.payloads[pi].name = name
 	l.payloads[pi].ver = ver
 	return i
@@ -405,7 +407,18 @@ func (l *Loader) getPayload(i Sym) *extSymPayload {
 		return nil
 	}
 	pi := i - l.extStart
-	return &l.payloads[pi]
+	return l.payloads[pi]
+}
+
+// allocPayload allocates a new payload.
+func (l *Loader) allocPayload() *extSymPayload {
+	batch := l.payloadBatch
+	if len(batch) == 0 {
+		batch = make([]extSymPayload, 1000)
+	}
+	p := &batch[0]
+	l.payloadBatch = batch[1:]
+	return p
 }
 
 func (ms *extSymPayload) Grow(siz int64) {
@@ -431,7 +444,7 @@ func (l *Loader) growSyms(i int) {
 		return
 	}
 	l.Syms = append(l.Syms, make([]*sym.Symbol, i+1-n)...)
-	l.payloads = append(l.payloads, make([]extSymPayload, i+1-n)...)
+	l.payloads = append(l.payloads, make([]*extSymPayload, i+1-n)...)
 	l.growValues(int(i) + 1)
 	l.growAttrBitmaps(int(i) + 1)
 }
@@ -1752,7 +1765,7 @@ func (l *Loader) LoadFull(arch *sys.Arch, syms *sym.Symbols) {
 	for _, i := range toConvert {
 
 		// Copy kind/size/value etc.
-		pp := &l.payloads[i-l.extStart]
+		pp := l.payloads[i-l.extStart]
 		s := l.Syms[i]
 		s.Version = int16(pp.ver)
 		s.Type = pp.kind
@@ -2021,7 +2034,7 @@ func (l *Loader) cloneToExternal(symIdx Sym) Sym {
 
 	// Create new symbol, update version and kind.
 	ns := l.newExtSym(sname, sver)
-	pp := &l.payloads[ns-l.extStart]
+	pp := l.payloads[ns-l.extStart]
 	pp.kind = skind
 	pp.ver = sver
 	pp.size = int64(osym.Siz)

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

@@ -63,15 +63,24 @@ func TestAddMaterializedSymbol(t *testing.T) {
 	sb2, es2 := ldr.MakeSymbolUpdater(es2)
 	sb3, es3 := ldr.MakeSymbolUpdater(es3)
 
+	// Suppose we create some more symbols, which triggers a grow.
+	// Make sure the symbol builder's payload pointer is valid,
+	// even across a grow.
+	ldr.growSyms(9999)
+
 	// Check get/set symbol type
 	es3typ := sb3.Type()
 	if es3typ != sym.Sxxx {
-		t.Errorf("SymType(es3): expected %d, got %d", sym.Sxxx, es3typ)
+		t.Errorf("SymType(es3): expected %v, got %v", sym.Sxxx, es3typ)
 	}
-	sb2.SetType(sym.SRODATA)
-	es3typ = sb2.Type()
+	sb3.SetType(sym.SRODATA)
+	es3typ = sb3.Type()
 	if es3typ != sym.SRODATA {
-		t.Errorf("SymType(es3): expected %d, got %d", sym.SRODATA, es3typ)
+		t.Errorf("SymType(es3): expected %v, got %v", sym.SRODATA, es3typ)
+	}
+	es3typ = ldr.SymType(es3)
+	if es3typ != sym.SRODATA {
+		t.Errorf("SymType(es3): expected %v, got %v", sym.SRODATA, es3typ)
 	}
 
 	// New symbols should not initially be reachable.

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

@@ -28,7 +28,7 @@ func (l *Loader) MakeSymbolBuilder(name string) *SymbolBuilder {
 		panic("can't build if sym.Symbol already present")
 	}
 	sb := &SymbolBuilder{l: l, symIdx: symIdx}
-	sb.extSymPayload = &l.payloads[symIdx-l.extStart]
+	sb.extSymPayload = l.payloads[symIdx-l.extStart]
 	return sb
 }
 
@@ -53,7 +53,7 @@ func (l *Loader) MakeSymbolUpdater(symIdx Sym) (*SymbolBuilder, Sym) {
 
 	// Construct updater and return.
 	sb := &SymbolBuilder{l: l, symIdx: symIdx}
-	sb.extSymPayload = &l.payloads[symIdx-l.extStart]
+	sb.extSymPayload = l.payloads[symIdx-l.extStart]
 	return sb, symIdx
 }