CBMC: Add contract and proof for polyveck_sub

Resolves #127

Unrolling resulted in too poor performance. The direct proof works, but it
requires to workaround the CBMC limitation described in:
#102
diffblue/cbmc#8617

This also moves the correctness post-condition in poly_sub into a assert at
the end of the function. This vastly improves performance.

Signed-off-by: Matthias J. Kannwischer <[email protected]>

Author: mkannwischer

mldsa/poly.c

@@ -62,7 +62,10 @@ void poly_sub(poly *c, const poly *a, const poly *b)
   __loop__(
     invariant(i <= MLDSA_N)
     invariant(forall(k1, 0, i, c->coeffs[k1] == a->coeffs[k1] - b->coeffs[k1])))
-  c->coeffs[i] = a->coeffs[i] - b->coeffs[i];
+  {
+    c->coeffs[i] = a->coeffs[i] - b->coeffs[i];
+  }
+  cassert(forall(k, 0, MLDSA_N, c->coeffs[k] == a->coeffs[k] - b->coeffs[k]));
 }
 
 void poly_shiftl(poly *a)

mldsa/poly.h

@@ -90,7 +90,6 @@ __contract__(
   requires(memory_no_alias(b, sizeof(poly)))
   requires(forall(k0, 0, MLDSA_N, (int64_t) a->coeffs[k0] - b->coeffs[k0] <= INT32_MAX))
   requires(forall(k1, 0, MLDSA_N, (int64_t) a->coeffs[k1] - b->coeffs[k1] >= INT32_MIN))
-  ensures(forall(k, 0, MLDSA_N, c->coeffs[k] == a->coeffs[k] - b->coeffs[k]))
   assigns(memory_slice(c, sizeof(poly))));
 
 #define poly_shiftl MLD_NAMESPACE(poly_shiftl)

mldsa/polyvec.c

@@ -226,8 +226,16 @@ void polyveck_sub(polyveck *w, const polyveck *u, const polyveck *v)
   unsigned int i;
 
   for (i = 0; i < MLDSA_K; ++i)
+  __loop__(
+    assigns(i, object_whole(w))
+    invariant(i <= MLDSA_K))
   {
-    poly_sub(&w->vec[i], &u->vec[i], &v->vec[i]);
+    poly t;
+    poly_sub(&t, &u->vec[i], &v->vec[i]);
+    /* Full struct assignment from local variables to simplify proof */
+    /* TODO: eliminate once CBMC resolves
+     * https://github.com/diffblue/cbmc/issues/8617 */
+    w->vec[i] = t;
   }
 }
 

mldsa/polyvec.h

@@ -180,6 +180,7 @@ __contract__(
   assigns(memory_slice(w, sizeof(polyveck)))
 );
 
+#define polyveck_sub MLD_NAMESPACE(polyveck_sub)
 /*************************************************
  * Name:        polyveck_sub
  *
@@ -191,8 +192,18 @@ __contract__(
  *              - const polyveck *v: pointer to second input vector to be
  *                                   subtracted from first input vector
  **************************************************/
-#define polyveck_sub MLD_NAMESPACE(polyveck_sub)
-void polyveck_sub(polyveck *w, const polyveck *u, const polyveck *v);
+void polyveck_sub(polyveck *w, const polyveck *u, const polyveck *v)
+__contract__(
+  requires(memory_no_alias(w, sizeof(polyveck)))
+  requires(memory_no_alias(u, sizeof(polyveck)))
+  requires(memory_no_alias(v, sizeof(polyveck)))
+  requires(forall(k0, 0, MLDSA_K,
+    forall(k1, 0, MLDSA_N, (int64_t) u->vec[k0].coeffs[k1] - v->vec[k0].coeffs[k1] <= INT32_MAX)))
+  requires(forall(k2, 0, MLDSA_K,
+    forall(k3, 0, MLDSA_N, (int64_t) u->vec[k2].coeffs[k3] - v->vec[k2].coeffs[k3] >= INT32_MIN)))
+  assigns(memory_slice(w, sizeof(polyveck)))
+);
+
 #define polyveck_shiftl MLD_NAMESPACE(polyveck_shiftl)
 /*************************************************
  * Name:        polyveck_shiftl

proofs/cbmc/polyveck_sub/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = polyveck_sub_harness
+
+# This should be a unique identifier for this proof, and will appear on the
+# Litani dashboard. It can be human-readable and contain spaces if you wish.
+PROOF_UID = polyveck_sub
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET += 
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mldsa/polyvec.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)polyveck_sub
+USE_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)poly_sub
+APPLY_LOOP_CONTRACTS=on
+USE_DYNAMIC_FRAMES=1
+
+# Disable any setting of EXTERNAL_SAT_SOLVER, and choose SMT backend instead
+EXTERNAL_SAT_SOLVER=
+CBMCFLAGS=--smt2
+
+FUNCTION_NAME = polyveck_sub
+
+# If this proof is found to consume huge amounts of RAM, you can set the
+# EXPENSIVE variable. With new enough versions of the proof tools, this will
+# restrict the number of EXPENSIVE CBMC jobs running at once. See the
+# documentation in Makefile.common under the "Job Pools" heading for details.
+# EXPENSIVE = true
+
+# This function is large enough to need...
+CBMC_OBJECT_BITS = 8
+
+# If you require access to a file-local ("static") function or object to conduct
+# your proof, set the following (and do not include the original source file
+# ("mldsa/poly.c") in PROJECT_SOURCES).
+# REWRITTEN_SOURCES = $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i
+# include ../Makefile.common
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_SOURCE = $(SRCDIR)/mldsa/poly.c
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_FUNCTIONS = foo bar
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_OBJECTS = baz
+# Care is required with variables on the left-hand side: REWRITTEN_SOURCES must
+# be set before including Makefile.common, but any use of variables on the
+# left-hand side requires those variables to be defined. Hence, _SOURCE,
+# _FUNCTIONS, _OBJECTS is set after including Makefile.common.
+
+include ../Makefile.common

proofs/cbmc/polyveck_sub/polyveck_sub_harness.c

@@ -0,0 +1,10 @@
+// Copyright (c) 2025 The mldsa-native project authors
+// SPDX-License-Identifier: Apache-2.0
+
+#include "polyvec.h"
+
+void harness(void)
+{
+  polyveck *a, *b, *c;
+  polyveck_sub(a, b, c);
+}