prog: generate missing syscall args when decoding

After a change in syscall description the number of syscall arguments
might change and some of the programs in corpus get invalidated.

This change makes syzkaller to generate missing arguments when decoding a
program as an attempt to fix and keep more programs from corpus.

prog/encoding.go

@@ -171,6 +171,11 @@ func Deserialize(data []byte) (prog *Prog, err error) {
 		if !p.EOF() {
 			return nil, fmt.Errorf("tailing data (line #%v)", p.l)
 		}
+		if len(c.Args) < len(meta.Args) {
+			for i := len(c.Args); i < len(meta.Args); i++ {
+				c.Args = append(c.Args, defaultArg(meta.Args[i]))
+			}
+		}
 		if len(c.Args) != len(meta.Args) {
 			return nil, fmt.Errorf("wrong call arg count: %v, want %v", len(c.Args), len(meta.Args))
 		}

prog/prog.go

@@ -266,6 +266,37 @@ func returnArg(t sys.Type) Arg {
 	return &ReturnArg{ArgCommon: ArgCommon{typ: t}}
 }
 
+func defaultArg(t sys.Type) Arg {
+	switch typ := t.(type) {
+	case *sys.IntType, *sys.ConstType, *sys.FlagsType, *sys.LenType, *sys.ProcType, *sys.CsumType:
+		return constArg(t, t.Default())
+	case *sys.ResourceType:
+		return resultArg(t, nil, typ.Desc.Type.Default())
+	case *sys.BufferType:
+		return dataArg(t, nil)
+	case *sys.ArrayType:
+		return groupArg(t, nil)
+	case *sys.StructType:
+		var inner []Arg
+		for _, field := range typ.Fields {
+			inner = append(inner, defaultArg(field))
+		}
+		return groupArg(t, nil)
+	case *sys.UnionType:
+		return unionArg(t, defaultArg(typ.Options[0]), typ.Options[0])
+	case *sys.VmaType:
+		return pointerArg(t, 0, 0, 0, nil)
+	case *sys.PtrType:
+		var res Arg
+		if !t.Optional() {
+			res = defaultArg(typ.Type)
+		}
+		return pointerArg(t, 0, 0, 1, res)
+	default:
+		panic("unknown arg type")
+	}
+}
+
 func (p *Prog) insertBefore(c *Call, calls []*Call) {
 	idx := 0
 	for ; idx < len(p.Calls); idx++ {

prog/prog_test.go

@@ -35,6 +35,15 @@ func TestGeneration(t *testing.T) {
 	}
 }
 
+func TestDefault(t *testing.T) {
+	initTest(t)
+	for _, meta := range sys.CallMap {
+		for _, t := range meta.Args {
+			defaultArg(t)
+		}
+	}
+}
+
 func TestSerialize(t *testing.T) {
 	rs, iters := initTest(t)
 	for i := 0; i < iters; i++ {