Soudness fix: pointer predicates can fail in assume contexts.

Make failed predicates yield invalid pointers (instead of nondet).
Mark peformance regressing test as FUTURE.

regression/contracts-dfcc/quantifiers-loops-fresh-bound-vars-smt/test.desc

@@ -1,4 +1,4 @@
-CORE dfcc-only smt-backend broken-cprover-smt-backend
+FUTURE dfcc-only smt-backend broken-cprover-smt-backend
 main.c
 --dfcc main --apply-loop-contracts --enforce-contract foo --malloc-may-fail --malloc-fail-null _ --z3 --slice-formula --no-standard-checks
 ^EXIT=0$
@@ -7,6 +7,12 @@ main.c
 --
 ^warning: ignoring
 --
+
+Marked as FUTURE because:
+- z3 >v4.12 and up can solve the problem with `--dfcc-simple-invalid-pointer-model`,
+ but the CI uses older versions;
+- bitwuzla >v0.6 can solve the problem but we don't run bitwuzla in CI.
+
 Tests support for quantifiers in loop contracts with the SMT backend.
 When quantified loop invariants are used, they are inserted three times
 in the transformed program (base case assertion, step case assumption,

src/ansi-c/library/cprover_contracts.c

@@ -1165,7 +1165,7 @@ void __CPROVER_contracts_make_invalid_pointer(void **ptr)
 
 /// \brief Implementation of the `is_fresh` front-end predicate.
 ///
-/// The behaviour depends on the boolean flags carried by \p set
+/// The behaviour depends on the boolean flags carried by \p write_set
 /// which reflect the invocation context: checking vs. replacing a contract,
 /// in a requires or an ensures clause context.
 /// \param elem First argument of the `is_fresh` predicate
@@ -1232,6 +1232,13 @@ __CPROVER_HIDE:;
       __CPROVER_assume(size <= __CPROVER_max_malloc_size);
     }
 
+    // SOUNDNESS: allow predicate to fail
+    if(__VERIFIER_nondet___CPROVER_bool())
+    {
+      __CPROVER_contracts_make_invalid_pointer(elem);
+      return 0;
+    }
+
     void *ptr = __CPROVER_allocate(size, 0);
     *elem = ptr;
 
@@ -1245,7 +1252,6 @@ __CPROVER_HIDE:;
     // __CPROVER_bool record_may_leak = __VERIFIER_nondet___CPROVER_bool();
     // __CPROVER_memory_leak = record_may_leak ? ptr : __CPROVER_memory_leak;
 
-    // record fresh object in the object set
 #ifdef __CPROVER_DFCC_DEBUG_LIB
     // manually inlined below
     __CPROVER_contracts_obj_set_add(write_set->linked_is_fresh, ptr);
@@ -1286,6 +1292,13 @@ __CPROVER_HIDE:;
       __CPROVER_assume(size <= __CPROVER_max_malloc_size);
     }
 
+    // SOUNDNESS: allow predicate to fail
+    if(__VERIFIER_nondet___CPROVER_bool())
+    {
+      __CPROVER_contracts_make_invalid_pointer(elem);
+      return 0;
+    }
+
     void *ptr = __CPROVER_allocate(size, 0);
     *elem = ptr;
 
@@ -1300,7 +1313,6 @@ __CPROVER_HIDE:;
     __CPROVER_bool record_may_leak = __VERIFIER_nondet___CPROVER_bool();
     __CPROVER_memory_leak = record_may_leak ? ptr : __CPROVER_memory_leak;
 
-    // record fresh object in the caller's write set
 #ifdef __CPROVER_DFCC_DEBUG_LIB
     __CPROVER_contracts_obj_set_add(write_set->linked_allocated, ptr);
 #else
@@ -1338,7 +1350,7 @@ __CPROVER_HIDE:;
     if(seen->elems[object_id] != 0)
       return 0;
 #endif
-      // record fresh object in the object set
+
 #ifdef __CPROVER_DFCC_DEBUG_LIB
     // manually inlined below
     __CPROVER_contracts_obj_set_add(seen, ptr);
@@ -1360,6 +1372,23 @@ __CPROVER_HIDE:;
   }
 }
 
+/// \brief Implementation of the `pointer_in_range_dfcc` front-end predicate.
+///
+/// The behaviour depends on the boolean flags carried by \p write_set
+/// which reflect the invocation context: checking vs. replacing a contract,
+/// in a requires or an ensures clause context.
+/// \param lb Lower bound pointer
+/// \param ptr Target pointer of the predicate
+/// \param ub Upper bound pointer
+/// \param write_set Write set in which seen/allocated objects are recorded;
+///
+/// \details The behaviour is as follows:
+/// - When \p set->assume_requires_ctx or \p set->assume_ensures_ctx is `true`,
+/// the predicate checks that \p lb and \p ub are valid, into the same object,
+/// ordered, and checks that \p ptr is between \p lb and \p ub.
+/// - When \p set->assert_requires_ctx or \p set->assert_ensures_ctx is `true`,
+/// the predicate checks that \p lb and \p ub are valid, into the same object,
+/// ordered, and assigns \p ptr to some nondet offset between \p lb and \p ub.
 __CPROVER_bool __CPROVER_contracts_pointer_in_range_dfcc(
   void *lb,
   void **ptr,
@@ -1386,7 +1415,11 @@ __CPROVER_HIDE:;
   if(write_set->assume_requires_ctx | write_set->assume_ensures_ctx)
   {
     if(__VERIFIER_nondet___CPROVER_bool())
+    {
+      // SOUNDNESS: allow predicate to fail
+      __CPROVER_contracts_make_invalid_pointer(ptr);
       return 0;
+    }
 
     // add nondet offset
     __CPROVER_size_t offset = __VERIFIER_nondet_size();