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, 14 insertions, 0 deletions

diff --git a/prog/target.go b/prog/target.go
index 80a2665ae..631b756e8 100644
--- a/prog/target.go
+++ b/prog/target.go
@@ -44,6 +44,9 @@ type Target struct {
 	// Used as fallback when string type does not have own dictionary.
 	StringDictionary []string
 
+	// Additional special invalid pointer values besides NULL to use.
+	SpecialPointers []uint64
+
 	// Filled by prog package:
 	init        sync.Once
 	initArch    func(target *Target)
@@ -55,6 +58,8 @@ type Target struct {
 	any           anyTypes
 }
 
+const maxSpecialPointers = 16
+
 var targets = make(map[string]*Target)
 
 func RegisterTarget(target *Target, initArch func(target *Target)) {
@@ -101,6 +106,15 @@ func (target *Target) lazyInit() {
 	target.initTarget()
 	target.initArch(target)
 	target.ConstMap = nil // currently used only by initArch
+	// Give these 2 known addresses fixed positions and prepend target-specific ones at the end.
+	target.SpecialPointers = append([]uint64{
+		0x0000000000000000, // NULL pointer (keep this first because code uses special index=0 as NULL)
+		0xffffffffffffffff, // unmapped kernel address (keep second because serialized value will match actual pointer value)
+		0x9999999999999999, // non-canonical address
+	}, target.SpecialPointers...)
+	if len(target.SpecialPointers) > maxSpecialPointers {
+		panic("too many special pointers")
+	}
 }
 
 func (target *Target) initTarget() {