Hoist default C backend into separate functions(mld_poly_chknorm_c)

Signed-off-by: willieyz <willie.zhao@chelpis.com>

Author: willieyz

mldsa/src/poly.c

@@ -656,6 +656,48 @@ void mld_polyt0_unpack(mld_poly *r, const uint8_t a[MLDSA_POLYT0_PACKEDBYTES])
                    (1 << (MLDSA_D - 1)) + 1);
 }
 
+static uint32_t mld_poly_chknorm_c(const mld_poly *a, int32_t B)
+__contract__(
+  requires(memory_no_alias(a, sizeof(mld_poly)))
+  requires(0 <= B && B <= MLDSA_Q - REDUCE32_RANGE_MAX)
+  requires(array_bound(a->coeffs, 0, MLDSA_N, -REDUCE32_RANGE_MAX, REDUCE32_RANGE_MAX))
+  ensures(return_value == 0 || return_value == 0xFFFFFFFF)
+  ensures((return_value == 0) == array_abs_bound(a->coeffs, 0, MLDSA_N, B))
+)
+{
+  unsigned int i;
+  uint32_t t = 0;
+  mld_assert_bound(a->coeffs, MLDSA_N, -REDUCE32_RANGE_MAX, REDUCE32_RANGE_MAX);
+  for (i = 0; i < MLDSA_N; ++i)
+  __loop__(
+    invariant(i <= MLDSA_N)
+    invariant(t == 0 || t == 0xFFFFFFFF)
+    invariant((t == 0) == array_abs_bound(a->coeffs, 0, i, B))
+  )
+  {
+    /*
+     * Since we know that -REDUCE32_RANGE_MAX <= a < REDUCE32_RANGE_MAX,
+     * and B <= MLDSA_Q - REDUCE32_RANGE_MAX, to check if
+     * -B < (a mod± MLDSA_Q) < B, it suffices to check if -B < a < B.
+     *
+     * We prove this to be true using the following CBMC assertions.
+     * a ==> b expressed as !a || b to also allow run-time assertion.
+     */
+    mld_assert(a->coeffs[i] < B || a->coeffs[i] - MLDSA_Q <= -B);
+    mld_assert(a->coeffs[i] > -B || a->coeffs[i] + MLDSA_Q >= B);
+
+    /* Reference: Leaks which coefficient violates the bound via a conditional.
+     * We are more conservative to reduce the number of declassifications in
+     * constant-time testing.
+     */
+
+    /* if (abs(a[i]) >= B) */
+    t |= mld_ct_cmask_neg_i32(B - 1 - mld_ct_abs_i32(a->coeffs[i]));
+  }
+
+  return t;
+}
+
 /* Reference: explicitly checks the bound B to be <= (MLDSA_Q - 1) / 8).
  * This is unnecessary as it's always a compile-time constant.
  * We instead model it as a precondition.
@@ -669,8 +711,6 @@ void mld_polyt0_unpack(mld_poly *r, const uint8_t a[MLDSA_POLYT0_PACKEDBYTES])
 MLD_INTERNAL_API
 uint32_t mld_poly_chknorm(const mld_poly *a, int32_t B)
 {
-  unsigned int i;
-  uint32_t t = 0;
   mld_assert_bound(a->coeffs, MLDSA_N, -REDUCE32_RANGE_MAX, REDUCE32_RANGE_MAX);
 #if defined(MLD_USE_NATIVE_POLY_CHKNORM)
   {
@@ -700,34 +740,7 @@ uint32_t mld_poly_chknorm(const mld_poly *a, int32_t B)
     }
   }
 #endif /* MLD_USE_NATIVE_POLY_CHKNORM */
-  for (i = 0; i < MLDSA_N; ++i)
-  __loop__(
-    invariant(i <= MLDSA_N)
-    invariant(t == 0 || t == 0xFFFFFFFF)
-    invariant((t == 0) == array_abs_bound(a->coeffs, 0, i, B))
-  )
-  {
-    /*
-     * Since we know that -REDUCE32_RANGE_MAX <= a < REDUCE32_RANGE_MAX,
-     * and B <= MLDSA_Q - REDUCE32_RANGE_MAX, to check if
-     * -B < (a mod± MLDSA_Q) < B, it suffices to check if -B < a < B.
-     *
-     * We prove this to be true using the following CBMC assertions.
-     * a ==> b expressed as !a || b to also allow run-time assertion.
-     */
-    mld_assert(a->coeffs[i] < B || a->coeffs[i] - MLDSA_Q <= -B);
-    mld_assert(a->coeffs[i] > -B || a->coeffs[i] + MLDSA_Q >= B);
-
-    /* Reference: Leaks which coefficient violates the bound via a conditional.
-     * We are more conservative to reduce the number of declassifications in
-     * constant-time testing.
-     */
-
-    /* if (abs(a[i]) >= B) */
-    t |= mld_ct_cmask_neg_i32(B - 1 - mld_ct_abs_i32(a->coeffs[i]));
-  }
-
-  return t;
+  return mld_poly_chknorm_c(a, B);
 }
 
 #else  /* !MLD_CONFIG_MULTILEVEL_NO_SHARED */

proofs/cbmc/poly_chknorm/Makefile

@@ -20,7 +20,7 @@ PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
 PROJECT_SOURCES += $(SRCDIR)/mldsa/src/poly.c
 
 CHECK_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)poly_chknorm
-USE_FUNCTION_CONTRACTS=mld_ct_abs_i32 mld_ct_cmask_neg_i32
+USE_FUNCTION_CONTRACTS=mld_poly_chknorm_c
 APPLY_LOOP_CONTRACTS=on
 USE_DYNAMIC_FRAMES=1
 

proofs/cbmc/poly_chknorm_c/Makefile

@@ -0,0 +1,55 @@
+# Copyright (c) The mldsa-native project authors
+# SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE =poly_chknorm_c_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 = poly_chknorm_c
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mldsa/src/poly.c
+
+CHECK_FUNCTION_CONTRACTS=mld_poly_chknorm_c
+USE_FUNCTION_CONTRACTS=mld_ct_abs_i32 mld_ct_cmask_neg_i32
+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 = mld_poly_chknorm_c
+
+# 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/src/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/poly_chknorm_c/poly_chknorm_c_harness.c

@@ -0,0 +1,15 @@
+// Copyright (c) The mldsa-native project authors
+// SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT
+
+#include "poly_kl.h"
+
+// Prototype for the function under test
+uint32_t mld_poly_chknorm_c(mld_poly *a, int32_t B);
+
+void harness(void)
+{
+  mld_poly *a;
+  uint32_t r;
+  int32_t B;
+  r = mld_poly_chknorm_c(a, B);
+}