Update overflow check

One does not simply check for integer overflow.
bitcoin-core · Mar 6, 2023 · a293013 · a293013
1 parent 5757318
commit a293013
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Showing 2 changed files with 4 additions and 4 deletions.
diff --git a/src/modinv32_impl.h b/src/modinv32_impl.h
@@ -415,8 +415,8 @@ 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) <= (INT32_MAX - labs(v))); /* |u|+|v| doesn't overflow */
+    VERIFY_CHECK(labs(q) <= (INT32_MAX - labs(r))); /* |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 */
 #endif

diff --git a/src/modinv64_impl.h b/src/modinv64_impl.h
@@ -419,8 +419,8 @@ 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) <= (INT64_MAX - secp256k1_modinv64_abs(v))); /* |u|+|v| doesn't overflow */
+    VERIFY_CHECK(secp256k1_modinv64_abs(q) <= (INT64_MAX - secp256k1_modinv64_abs(r))); /* |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 */
 #endif