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modinv: Avoid that signed overflow may occur when tests fail #1219

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modinv: Avoid that signed overflow may occur when tests fail #1219

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12 changes: 8 additions & 4 deletions src/modinv32_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -415,10 +415,14 @@ 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(u) <= (int32_t)1 << 30); /* |u| <= 2^30 */
VERIFY_CHECK(labs(v) <= (int32_t)1 << 30); /* |v| <= 2^30 */
VERIFY_CHECK(labs(q) <= (int32_t)1 << 30); /* |q| <= 2^30 */
VERIFY_CHECK(labs(r) <= (int32_t)1 << 30); /* |r| <= 2^30 */
/* Assuming labs() returns a non-negative value,
* the previous checks imply that the additions |u|+|v| and |q|+|r| in the following checks do not overflow. */
VERIFY_CHECK((labs(u) + labs(v)) <= (int32_t)1 << 30); /* |u|+|v| <= 2^30 */
VERIFY_CHECK((labs(q) + labs(r)) <= (int32_t)1 << 30); /* |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;
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14 changes: 10 additions & 4 deletions src/modinv64_impl.h
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Expand Up @@ -30,9 +30,11 @@ typedef struct {
/* Helper function to compute the absolute value of an int64_t.
* (we don't use abs/labs/llabs as it depends on the int sizes). */
static int64_t secp256k1_modinv64_abs(int64_t v) {
int64_t r;
VERIFY_CHECK(v > INT64_MIN);
if (v < 0) return -v;
return v;
r = (v < 0) ? -v : v;
VERIFY_CHECK(r >= 0);
return r;
}

static const secp256k1_modinv64_signed62 SECP256K1_SIGNED62_ONE = {{1}};
Expand Down Expand Up @@ -419,8 +421,12 @@ 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_t)1 << 62); /* |u| <= 2^62 */
VERIFY_CHECK(secp256k1_modinv64_abs(v) <= (int64_t)1 << 62); /* |v| <= 2^62 */
VERIFY_CHECK(secp256k1_modinv64_abs(q) <= (int64_t)1 << 62); /* |q| <= 2^62 */
VERIFY_CHECK(secp256k1_modinv64_abs(r) <= (int64_t)1 << 62); /* |r| <= 2^62 */
/* Assuming secp256k1_modinv64_abs() returns a non-negative value (which is checked within that function),
* the previous checks imply that the additions |u|+|v| and |q|+|r| in the following checks do not 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
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