Add null checks for pointer derefs to avoid map_value_or_null verifier errors

Author: r41k0u

pythonbpf/expr_pass.py

@@ -139,38 +139,84 @@ def _get_base_type_and_depth(ir_type):
     return cur_type, depth
 
 
-def _deref_to_depth(builder, val, target_depth):
+def _deref_to_depth(func, builder, val, target_depth):
     """Dereference a pointer to a certain depth."""
 
     cur_val = val
-    for _ in range(target_depth):
+    cur_type = val.type
+
+    for depth in range(target_depth):
         if not isinstance(val.type, ir.PointerType):
             logger.error("Cannot dereference further, non-pointer type")
             return None
-        cur_val = builder.load(cur_val)
+
+        # dereference with null check
+        pointee_type = cur_type.pointee
+        null_check_block = builder.block
+        not_null_block = func.append_basic_block(name=f"deref_not_null_{depth}")
+        merge_block = func.append_basic_block(name=f"deref_merge_{depth}")
+
+        null_ptr = ir.Constant(cur_type, None)
+        is_not_null = builder.icmp_signed("!=", cur_val, null_ptr)
+        logger.debug(f"Inserted null check for pointer at depth {depth}")
+
+        builder.cbranch(is_not_null, not_null_block, merge_block)
+
+        builder.position_at_end(not_null_block)
+        dereferenced_val = builder.load(cur_val)
+        logger.debug(f"Dereferenced to depth {depth - 1}, type: {pointee_type}")
+        builder.branch(merge_block)
+
+        builder.position_at_end(merge_block)
+        phi = builder.phi(pointee_type, name=f"deref_result_{depth}")
+
+        zero_value = (
+            ir.Constant(pointee_type, 0)
+            if isinstance(pointee_type, ir.IntType)
+            else ir.Constant(pointee_type, None)
+        )
+        phi.add_incoming(zero_value, null_check_block)
+
+        phi.add_incoming(dereferenced_val, not_null_block)
+
+        # Continue with phi result
+        cur_val = phi
+        cur_type = pointee_type
     return cur_val
 
 
-def _normalize_types(builder, lhs, rhs):
+def _normalize_types(func, builder, lhs, rhs):
     """Normalize types for comparison."""
 
+    logger.info(f"Normalizing types: {lhs.type} vs {rhs.type}")
     if isinstance(lhs.type, ir.IntType) and isinstance(rhs.type, ir.IntType):
         if lhs.type.width < rhs.type.width:
             lhs = builder.sext(lhs, rhs.type)
         else:
             rhs = builder.sext(rhs, lhs.type)
         return lhs, rhs
-
-    logger.error(f"Type mismatch: {lhs.type} vs {rhs.type}")
-    return None, None
+    elif not isinstance(lhs.type, ir.PointerType) and not isinstance(
+        rhs.type, ir.PointerType
+    ):
+        logger.error(f"Type mismatch: {lhs.type} vs {rhs.type}")
+        return None, None
+    else:
+        lhs_base, lhs_depth = _get_base_type_and_depth(lhs.type)
+        rhs_base, rhs_depth = _get_base_type_and_depth(rhs.type)
+        if lhs_base == rhs_base:
+            if lhs_depth < rhs_depth:
+                rhs = _deref_to_depth(func, builder, rhs, rhs_depth - lhs_depth)
+            elif rhs_depth < lhs_depth:
+                lhs = _deref_to_depth(func, builder, lhs, lhs_depth - rhs_depth)
+            return _normalize_types(func, builder, lhs, rhs)
 
 
-def _handle_comparator(builder, op, lhs, rhs):
+def _handle_comparator(func, builder, op, lhs, rhs):
     """Handle comparison operations."""
 
     # NOTE: For now assume same types
     if lhs.type != rhs.type:
-        lhs, rhs = _normalize_types(builder, lhs, rhs)
+        lhs, rhs = _normalize_types(func, builder, lhs, rhs)
 
     if lhs is None or rhs is None:
         return None
@@ -227,7 +273,7 @@ def _handle_compare(
 
     lhs, _ = lhs
     rhs, _ = rhs
-    return _handle_comparator(builder, cond.ops[0], lhs, rhs)
+    return _handle_comparator(func, builder, cond.ops[0], lhs, rhs)
 
 
 def convert_to_bool(builder, val):