Update overflow check

One does not simply check for integer overlow.

Author: roconnor-blockstream

src/modinv32_impl.h

@@ -415,10 +415,10 @@ static void secp256k1_modinv32_update_de_30(secp256k1_modinv32_signed30 *d, secp
     VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, 9, &modinfo->modulus, 1) < 0);  /* d <    modulus */
     VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, 9, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
     VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, 9, &modinfo->modulus, 1) < 0);  /* e <    modulus */
-    VERIFY_CHECK((labs(u) + labs(v)) >= 0); /* |u|+|v| doesn't overflow */
-    VERIFY_CHECK((labs(q) + labs(r)) >= 0); /* |q|+|r| doesn't overflow */
-    VERIFY_CHECK((labs(u) + labs(v)) <= M30 + 1); /* |u|+|v| <= 2^30 */
-    VERIFY_CHECK((labs(q) + labs(r)) <= M30 + 1); /* |q|+|r| <= 2^30 */
+    VERIFY_CHECK(labs(v) <= M30 + 1);             /*     |v| <= 2^30 */
+    VERIFY_CHECK(labs(u) <= (M30 + 1 - labs(v))); /* |u|+|v| <= 2^30 */
+    VERIFY_CHECK(labs(r) <= M30 + 1);             /*     |r| <= 2^30 */
+    VERIFY_CHECK(labs(q) <= (M30 + 1 - labs(r))); /* |q|+|r| <= 2^30 */
 #endif
     /* [md,me] start as zero; plus [u,q] if d is negative; plus [v,r] if e is negative. */
     sd = d->v[8] >> 31;

src/modinv64_impl.h

@@ -419,10 +419,10 @@ static void secp256k1_modinv64_update_de_62(secp256k1_modinv64_signed62 *d, secp
     VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(d, 5, &modinfo->modulus, 1) < 0);  /* d <    modulus */
     VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(e, 5, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
     VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(e, 5, &modinfo->modulus, 1) < 0);  /* e <    modulus */
-    VERIFY_CHECK((secp256k1_modinv64_abs(u) + secp256k1_modinv64_abs(v)) >= 0); /* |u|+|v| doesn't overflow */
-    VERIFY_CHECK((secp256k1_modinv64_abs(q) + secp256k1_modinv64_abs(r)) >= 0); /* |q|+|r| doesn't overflow */
-    VERIFY_CHECK((secp256k1_modinv64_abs(u) + secp256k1_modinv64_abs(v)) <= (int64_t)1 << 62); /* |u|+|v| <= 2^62 */
-    VERIFY_CHECK((secp256k1_modinv64_abs(q) + secp256k1_modinv64_abs(r)) <= (int64_t)1 << 62); /* |q|+|r| <= 2^62 */
+    VERIFY_CHECK(secp256k1_modinv64_abs(v) <= (int64_t)1 << 62);                                 /*     |v| <= 2^62 */
+    VERIFY_CHECK(secp256k1_modinv64_abs(u) <= (((int64_t)1 << 62) - secp256k1_modinv64_abs(v))); /* |u|+|v| <= 2^62 */
+    VERIFY_CHECK(secp256k1_modinv64_abs(r) <= (int64_t)1 << 62);                                 /*     |r| <= 2^62 */
+    VERIFY_CHECK(secp256k1_modinv64_abs(q) <= (((int64_t)1 << 62) - secp256k1_modinv64_abs(r))); /* |q|+|r| <= 2^62 */
 #endif
     /* [md,me] start as zero; plus [u,q] if d is negative; plus [v,r] if e is negative. */
     sd = d4 >> 63;