Be stricter with side effects in VERIFY

Adds a rule to CONTRIBUTING.md and makes the code adhere to it.
real-or-random · Jun 27, 2024 · ee7083f · ee7083f
1 parent 4af241b
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diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -49,6 +49,10 @@ In addition, libsecp256k1 tries to maintain the following coding conventions:
 * Operations involving secret data should be tested for being constant time with respect to the secrets (see [src/ctime_tests.c](src/ctime_tests.c)).
 * Local variables containing secret data should be cleared explicitly to try to delete secrets from memory.
 * Use `secp256k1_memcmp_var` instead of `memcmp` (see [#823](https://github.com/bitcoin-core/secp256k1/issues/823)).
+* Use `VERIFY_CHECK(cond)` for test-only assertions.
+  * These are active only when the `VERIFY` macro is defined. The build system defines this macro when building test targets such as the `tests` and `exhaustive_tests` executables, but not when building the actual library.
+  * If you need additional code lines to prepare the call to `VERIFY_CHECK`, then wrap them and the call into `#ifdef VERIFY ... #endif`. Start a new block (see style conventions below) inside the `#ifdef VERIFY` if appropriate, and suffix `VERIFY`-only variables with `_ver`.
+  * In any `VERIFY` code, avoid side effects to variables used in non-`VERIFY` code. Note that `VERIFY_CHECK(cond)` is a no-op if `VERIFY` is not defined, so avoid also side effects in `cond`.
 
 #### Style conventions
 

diff --git a/src/ecmult_const_impl.h b/src/ecmult_const_impl.h
@@ -212,10 +212,13 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
     secp256k1_scalar_add(&v2, &v2, &S_OFFSET);
 
 #ifdef VERIFY
-    /* Verify that v1 and v2 are in range [0, 2^129-1]. */
-    for (i = 129; i < 256; ++i) {
-        VERIFY_CHECK(secp256k1_scalar_get_bits_limb32(&v1, i, 1) == 0);
-        VERIFY_CHECK(secp256k1_scalar_get_bits_limb32(&v2, i, 1) == 0);
+    {
+        int i_ver;
+        /* Verify that v1 and v2 are in range [0, 2^129-1]. */
+        for (i_ver = 129; i_ver < 256; ++i_ver) {
+            VERIFY_CHECK(secp256k1_scalar_get_bits_limb32(&v1, i_ver, 1) == 0);
+            VERIFY_CHECK(secp256k1_scalar_get_bits_limb32(&v2, i_ver, 1) == 0);
+        }
     }
 #endif
 

diff --git a/src/ecmult_impl.h b/src/ecmult_impl.h
@@ -202,15 +202,14 @@ static int secp256k1_ecmult_wnaf(int *wnaf, int len, const secp256k1_scalar *a,
 
         bit += now;
     }
+    VERIFY_CHECK(carry == 0);
+
 #ifdef VERIFY
     {
-        int verify_bit = bit;
-
-        VERIFY_CHECK(carry == 0);
-
-        while (verify_bit < 256) {
-            VERIFY_CHECK(secp256k1_scalar_get_bits_limb32(&s, verify_bit, 1) == 0);
-            verify_bit++;
+        int bit_ver = bit;
+        while (bit_ver < 256) {
+            VERIFY_CHECK(secp256k1_scalar_get_bits_limb32(&s, bit_ver, 1) == 0);
+            bit_ver++;
         }
     }
 #endif

diff --git a/src/field_impl.h b/src/field_impl.h
@@ -132,10 +132,13 @@ static int secp256k1_fe_sqrt(secp256k1_fe * SECP256K1_RESTRICT r, const secp256k
     ret = secp256k1_fe_equal(&t1, a);
 
 #ifdef VERIFY
-    if (!ret) {
-        secp256k1_fe_negate(&t1, &t1, 1);
-        secp256k1_fe_normalize_var(&t1);
-        VERIFY_CHECK(secp256k1_fe_equal(&t1, a));
+    {
+        secp256k1_fe t1_ver;
+        if (!ret) {
+            secp256k1_fe_negate(&t1_ver, &t1, 1);
+            secp256k1_fe_normalize_var(&t1_ver);
+            VERIFY_CHECK(secp256k1_fe_equal(&t1_ver, a));
+        }
     }
 #endif
     return ret;

diff --git a/src/group_impl.h b/src/group_impl.h
@@ -198,8 +198,11 @@ static void secp256k1_ge_set_all_gej_var(secp256k1_ge *r, const secp256k1_gej *a
     size_t i;
     size_t last_i = SIZE_MAX;
 #ifdef VERIFY
-    for (i = 0; i < len; i++) {
-        SECP256K1_GEJ_VERIFY(&a[i]);
+    {
+        size_t i_ver;
+        for (i_ver = 0; i_ver < len; i_ver++) {
+            SECP256K1_GEJ_VERIFY(&a[i_ver]);
+        }
     }
 #endif
 
@@ -240,8 +243,11 @@ static void secp256k1_ge_set_all_gej_var(secp256k1_ge *r, const secp256k1_gej *a
     }
 
 #ifdef VERIFY
-    for (i = 0; i < len; i++) {
-        SECP256K1_GE_VERIFY(&r[i]);
+    {
+        size_t i_ver;
+        for (i_ver = 0; i_ver < len; i_ver++) {
+            SECP256K1_GE_VERIFY(&r[i_ver]);
+        }
     }
 #endif
 }
@@ -250,9 +256,12 @@ static void secp256k1_ge_table_set_globalz(size_t len, secp256k1_ge *a, const se
     size_t i;
     secp256k1_fe zs;
 #ifdef VERIFY
-    for (i = 0; i < len; i++) {
-        SECP256K1_GE_VERIFY(&a[i]);
-        SECP256K1_FE_VERIFY(&zr[i]);
+    {
+        size_t i_ver;
+        for (i_ver = 0; i_ver < len; i_ver++) {
+            SECP256K1_GE_VERIFY(&a[i_ver]);
+            SECP256K1_FE_VERIFY(&zr[i_ver]);
+        }
     }
 #endif
 
@@ -273,8 +282,11 @@ static void secp256k1_ge_table_set_globalz(size_t len, secp256k1_ge *a, const se
     }
 
 #ifdef VERIFY
-    for (i = 0; i < len; i++) {
-        SECP256K1_GE_VERIFY(&a[i]);
+    {
+        size_t i_ver;
+        for (i_ver = 0; i_ver < len; i_ver++) {
+            SECP256K1_GE_VERIFY(&a[i_ver]);
+        }
     }
 #endif
 }

diff --git a/src/modinv32_impl.h b/src/modinv32_impl.h
@@ -67,14 +67,16 @@ static void secp256k1_modinv32_normalize_30(secp256k1_modinv32_signed30 *r, int3
     volatile int32_t cond_add, cond_negate;
 
 #ifdef VERIFY
-    /* Verify that all limbs are in range (-2^30,2^30). */
-    int i;
-    for (i = 0; i < 9; ++i) {
-        VERIFY_CHECK(r->v[i] >= -M30);
-        VERIFY_CHECK(r->v[i] <= M30);
+    {
+        /* Verify that all limbs are in range (-2^30,2^30). */
+        int i_ver;
+        for (i_ver = 0; i_ver < 9; ++i_ver) {
+            VERIFY_CHECK(r->v[i_ver] >= -M30);
+            VERIFY_CHECK(r->v[i_ver] <= M30);
+        }
+        VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, -2) > 0); /* r > -2*modulus */
+        VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, 1) < 0); /* r < modulus */
     }
-    VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, -2) > 0); /* r > -2*modulus */
-    VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, 1) < 0); /* r < modulus */
 #endif
 
     /* In a first step, add the modulus if the input is negative, and then negate if requested.
@@ -586,7 +588,7 @@ static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256
     secp256k1_modinv32_signed30 f = modinfo->modulus;
     secp256k1_modinv32_signed30 g = *x;
 #ifdef VERIFY
-    int i = 0;
+    int i_ver = 0;
 #endif
     int j, len = 9;
     int32_t eta = -1; /* eta = -delta; delta is initially 1 (faster for the variable-time code) */
@@ -631,7 +633,7 @@ static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256
             --len;
         }
 
-        VERIFY_CHECK(++i < 25); /* We should never need more than 25*30 = 750 divsteps */
+        VERIFY_CHECK(++i_ver < 25); /* We should never need more than 25*30 = 750 divsteps */
         VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, -1) > 0); /* f > -modulus */
         VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, 1) <= 0); /* f <= modulus */
         VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, len, &modinfo->modulus, -1) > 0); /* g > -modulus */

diff --git a/src/modinv64_impl.h b/src/modinv64_impl.h
@@ -91,14 +91,16 @@ static void secp256k1_modinv64_normalize_62(secp256k1_modinv64_signed62 *r, int6
     volatile int64_t cond_add, cond_negate;
 
 #ifdef VERIFY
-    /* Verify that all limbs are in range (-2^62,2^62). */
-    int i;
-    for (i = 0; i < 5; ++i) {
-        VERIFY_CHECK(r->v[i] >= -M62);
-        VERIFY_CHECK(r->v[i] <= M62);
+    {
+        /* Verify that all limbs are in range (-2^62,2^62). */
+        int i_ver;
+        for (i_ver = 0; i_ver < 5; ++i_ver) {
+            VERIFY_CHECK(r->v[i_ver] >= -M62);
+            VERIFY_CHECK(r->v[i_ver] <= M62);
+        }
+        VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, -2) > 0); /* r > -2*modulus */
+        VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, 1) < 0); /* r < modulus */
     }
-    VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, -2) > 0); /* r > -2*modulus */
-    VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, 1) < 0); /* r < modulus */
 #endif
 
     /* In a first step, add the modulus if the input is negative, and then negate if requested.
@@ -642,7 +644,7 @@ static void secp256k1_modinv64_var(secp256k1_modinv64_signed62 *x, const secp256
     secp256k1_modinv64_signed62 f = modinfo->modulus;
     secp256k1_modinv64_signed62 g = *x;
 #ifdef VERIFY
-    int i = 0;
+    int i_ver = 0;
 #endif
     int j, len = 5;
     int64_t eta = -1; /* eta = -delta; delta is initially 1 */
@@ -686,7 +688,7 @@ static void secp256k1_modinv64_var(secp256k1_modinv64_signed62 *x, const secp256
             --len;
         }
 
-        VERIFY_CHECK(++i < 12); /* We should never need more than 12*62 = 744 divsteps */
+        VERIFY_CHECK(++i_ver < 12); /* We should never need more than 12*62 = 744 divsteps */
         VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, -1) > 0); /* f > -modulus */
         VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, 1) <= 0); /* f <= modulus */
         VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, len, &modinfo->modulus, -1) > 0); /* g > -modulus */

diff --git a/src/modules/ellswift/main_impl.h b/src/modules/ellswift/main_impl.h
@@ -187,7 +187,7 @@ static int secp256k1_ellswift_xswiftec_inv_var(secp256k1_fe *t, const secp256k1_
      * - If (c & 5) = 5: return -w*(c4*u + v).
      */
     secp256k1_fe x = *x_in, u = *u_in, g, v, s, m, r, q;
-    int ret;
+    int ret_ver;
 
     secp256k1_fe_normalize_weak(&x);
     secp256k1_fe_normalize_weak(&u);
@@ -266,11 +266,11 @@ static int secp256k1_ellswift_xswiftec_inv_var(secp256k1_fe *t, const secp256k1_
         secp256k1_fe_mul(&q, &q, &s);                   /* q = s*(4*(u^3+7)+3*u^2*s) */
         secp256k1_fe_negate(&q, &q, 1);                 /* q = -s*(4*(u^3+7)+3*u^2*s) */
         if (!secp256k1_fe_is_square_var(&q)) return 0;
-        ret = secp256k1_fe_sqrt(&r, &q);                /* r = sqrt(-s*(4*(u^3+7)+3*u^2*s)) */
+        ret_ver = secp256k1_fe_sqrt(&r, &q);            /* r = sqrt(-s*(4*(u^3+7)+3*u^2*s)) */
 #ifdef VERIFY
-        VERIFY_CHECK(ret);
+        VERIFY_CHECK(ret_ver);
 #else
-        (void)ret;
+        (void)ret_ver;
 #endif
 
         /* If (c & 1) = 1 and r = 0, fail. */
@@ -287,8 +287,8 @@ static int secp256k1_ellswift_xswiftec_inv_var(secp256k1_fe *t, const secp256k1_
     }
 
     /* Let w = sqrt(s). */
-    ret = secp256k1_fe_sqrt(&m, &s);                    /* m = sqrt(s) = w */
-    VERIFY_CHECK(ret);
+    ret_ver = secp256k1_fe_sqrt(&m, &s);                /* m = sqrt(s) = w */
+    VERIFY_CHECK(ret_ver);
 
     /* Return logic. */
     if ((c & 5) == 0 || (c & 5) == 5) {
@@ -311,7 +311,7 @@ static void secp256k1_ellswift_prng(unsigned char* out32, const secp256k1_sha256
     secp256k1_sha256 hash = *hasher;
     unsigned char buf4[4];
 #ifdef VERIFY
-    size_t blocks = hash.bytes >> 6;
+    size_t blocks_ver = hash.bytes >> 6;
 #endif
     buf4[0] = cnt;
     buf4[1] = cnt >> 8;
@@ -321,7 +321,7 @@ static void secp256k1_ellswift_prng(unsigned char* out32, const secp256k1_sha256
     secp256k1_sha256_finalize(&hash, out32);
 
     /* Writing and finalizing together should trigger exactly one SHA256 compression. */
-    VERIFY_CHECK(((hash.bytes) >> 6) == (blocks + 1));
+    VERIFY_CHECK(((hash.bytes) >> 6) == (blocks_ver + 1));
 }
 
 /** Find an ElligatorSwift encoding (u, t) for X coordinate x, and random Y coordinate.
@@ -407,17 +407,17 @@ int secp256k1_ellswift_encode(const secp256k1_context *ctx, unsigned char *ell64
         secp256k1_fe t;
         unsigned char p64[64] = {0};
         size_t ser_size;
-        int ser_ret;
+        int ser_ret_ver;
         secp256k1_sha256 hash;
 
         /* Set up hasher state; the used RNG is H(pubkey || "\x00"*31 || rnd32 || cnt++), using
          * BIP340 tagged hash with tag "secp256k1_ellswift_encode". */
         secp256k1_ellswift_sha256_init_encode(&hash);
-        ser_ret = secp256k1_eckey_pubkey_serialize(&p, p64, &ser_size, 1);
+        ser_ret_ver = secp256k1_eckey_pubkey_serialize(&p, p64, &ser_size, 1);
 #ifdef VERIFY
-        VERIFY_CHECK(ser_ret && ser_size == 33);
+        VERIFY_CHECK(ser_ret_ver && ser_size == 33);
 #else
-        (void)ser_ret;
+        (void)ser_ret_ver;
 #endif
         secp256k1_sha256_write(&hash, p64, sizeof(p64));
         secp256k1_sha256_write(&hash, rnd32, 32);

diff --git a/src/scalar_4x64_impl.h b/src/scalar_4x64_impl.h
@@ -228,12 +228,15 @@ static void secp256k1_scalar_half(secp256k1_scalar *r, const secp256k1_scalar *a
     r->d[2] = secp256k1_u128_to_u64(&t); secp256k1_u128_rshift(&t, 64);
     r->d[3] = secp256k1_u128_to_u64(&t) + (a->d[3] >> 1) + (SECP256K1_N_H_3 & mask);
 #ifdef VERIFY
-    /* The line above only computed the bottom 64 bits of r->d[3]; redo the computation
-     * in full 128 bits to make sure the top 64 bits are indeed zero. */
-    secp256k1_u128_accum_u64(&t, a->d[3] >> 1);
-    secp256k1_u128_accum_u64(&t, SECP256K1_N_H_3 & mask);
-    secp256k1_u128_rshift(&t, 64);
-    VERIFY_CHECK(secp256k1_u128_to_u64(&t) == 0);
+    {
+        /* The line above only computed the bottom 64 bits of r->d[3]; redo the computation
+         * in full 128 bits to make sure the top 64 bits are indeed zero. */
+        secp256k1_uint128 t_ver = t;
+        secp256k1_u128_accum_u64(&t_ver, a->d[3] >> 1);
+        secp256k1_u128_accum_u64(&t_ver, SECP256K1_N_H_3 & mask);
+        secp256k1_u128_rshift(&t_ver, 64);
+        VERIFY_CHECK(secp256k1_u128_to_u64(&t_ver) == 0);
+    }
 
     SECP256K1_SCALAR_VERIFY(r);
 #endif
@@ -970,32 +973,26 @@ static const secp256k1_modinv64_modinfo secp256k1_const_modinfo_scalar = {
 
 static void secp256k1_scalar_inverse(secp256k1_scalar *r, const secp256k1_scalar *x) {
     secp256k1_modinv64_signed62 s;
-#ifdef VERIFY
-    int zero_in = secp256k1_scalar_is_zero(x);
-#endif
     SECP256K1_SCALAR_VERIFY(x);
 
     secp256k1_scalar_to_signed62(&s, x);
     secp256k1_modinv64(&s, &secp256k1_const_modinfo_scalar);
     secp256k1_scalar_from_signed62(r, &s);
 
+    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == secp256k1_scalar_is_zero(x));
     SECP256K1_SCALAR_VERIFY(r);
-    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == zero_in);
 }
 
 static void secp256k1_scalar_inverse_var(secp256k1_scalar *r, const secp256k1_scalar *x) {
     secp256k1_modinv64_signed62 s;
-#ifdef VERIFY
-    int zero_in = secp256k1_scalar_is_zero(x);
-#endif
     SECP256K1_SCALAR_VERIFY(x);
 
     secp256k1_scalar_to_signed62(&s, x);
     secp256k1_modinv64_var(&s, &secp256k1_const_modinfo_scalar);
     secp256k1_scalar_from_signed62(r, &s);
 
+    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == secp256k1_scalar_is_zero(x));
     SECP256K1_SCALAR_VERIFY(r);
-    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == zero_in);
 }
 
 SECP256K1_INLINE static int secp256k1_scalar_is_even(const secp256k1_scalar *a) {

diff --git a/src/scalar_8x32_impl.h b/src/scalar_8x32_impl.h
@@ -790,32 +790,26 @@ static const secp256k1_modinv32_modinfo secp256k1_const_modinfo_scalar = {
 
 static void secp256k1_scalar_inverse(secp256k1_scalar *r, const secp256k1_scalar *x) {
     secp256k1_modinv32_signed30 s;
-#ifdef VERIFY
-    int zero_in = secp256k1_scalar_is_zero(x);
-#endif
     SECP256K1_SCALAR_VERIFY(x);
 
     secp256k1_scalar_to_signed30(&s, x);
     secp256k1_modinv32(&s, &secp256k1_const_modinfo_scalar);
     secp256k1_scalar_from_signed30(r, &s);
 
+    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == secp256k1_scalar_is_zero(x));
     SECP256K1_SCALAR_VERIFY(r);
-    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == zero_in);
 }
 
 static void secp256k1_scalar_inverse_var(secp256k1_scalar *r, const secp256k1_scalar *x) {
     secp256k1_modinv32_signed30 s;
-#ifdef VERIFY
-    int zero_in = secp256k1_scalar_is_zero(x);
-#endif
     SECP256K1_SCALAR_VERIFY(x);
 
     secp256k1_scalar_to_signed30(&s, x);
     secp256k1_modinv32_var(&s, &secp256k1_const_modinfo_scalar);
     secp256k1_scalar_from_signed30(r, &s);
 
+    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == secp256k1_scalar_is_zero(x));
     SECP256K1_SCALAR_VERIFY(r);
-    VERIFY_CHECK(secp256k1_scalar_is_zero(r) == zero_in);
 }
 
 SECP256K1_INLINE static int secp256k1_scalar_is_even(const secp256k1_scalar *a) {