prog: add concept of "special pointers"

Currently we only generate either valid user-space pointers or NULL.
Extend NULL to a set of special pointers that we will use in programs.
All targets now contain 3 special values:
 - NULL
 - 0xfffffffffffffff (invalid kernel pointer)
 - 0x999999999999999 (non-canonical address)
Each target can add additional special pointers on top of this.

Also generate NULL/special pointers for non-opt ptr's.
This restriction was always too restrictive. We may want to generate
them with very low probability, but we do want to generate them.

Also change pointers to NULL/special during mutation
(but still not in the opposite direction).

1 files changed, 4 insertions, 6 deletions

diff --git a/prog/encoding.go b/prog/encoding.go
index 92e2c47f0..c968fa0b7 100644
--- a/prog/encoding.go
+++ b/prog/encoding.go
@@ -85,8 +85,8 @@ func (a *ConstArg) serialize(ctx *serializer) {
 }
 
 func (a *PointerArg) serialize(ctx *serializer) {
-	if a.IsNull() {
-		ctx.printf("0x0")
+	if a.IsSpecial() {
+		ctx.printf("0x%x", a.Address)
 		return
 	}
 	target := ctx.target
@@ -345,10 +345,8 @@ func (target *Target) parseArgInt(typ Type, p *parser) (Arg, error) {
 	case *ResourceType:
 		return MakeResultArg(typ, nil, v), nil
 	case *PtrType, *VmaType:
-		if typ.Optional() {
-			return MakeNullPointerArg(typ), nil
-		}
-		return typ.makeDefaultArg(), nil
+		index := -v % uint64(len(target.SpecialPointers))
+		return MakeSpecialPointerArg(typ, index), nil
 	default:
 		eatExcessive(p, true)
 		return typ.makeDefaultArg(), nil