From: Paul Eggert <eggert@cs.ucla.edu>
Date: Thu, 10 Jul 2025 04:22:12 +0000 (-0700)
Subject: factor: fix mp_factor_using_pollard_rho aliasing
X-Git-Tag: v9.8~179
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=4171ca33536219248afa1b74a561f1adc5739b64;p=thirdparty%2Fcoreutils.git

factor: fix mp_factor_using_pollard_rho aliasing

* src/factor.c (mp_factor_using_pollard_rho):
Fix recently-introduced aliasing bug by computing q
before g gets updated in place.  Problem discovered
on a hypothetical platform where W_TYPE_SIZE
is neither 32 nor 64.
---

diff --git a/src/factor.c b/src/factor.c
index 134c092f84..fe3a639a14 100644
--- a/src/factor.c
+++ b/src/factor.c
@@ -939,7 +939,7 @@ mp_finish_in_single (struct mp_factors *mp_factors, mpz_t n)
   return mp_finish_up_in_single (mp_factors, n, 0);
 }
 
-/* Return some or all factors of T, where B^2 / 2 <= T.
+/* Return some or all factors of T.
    Divide T by the factors found.  */
 static struct mp_factors
 mp_factor_using_division (mpz_t t)
@@ -1534,6 +1534,10 @@ mp_factor_using_pollard_rho (struct mp_factors *factors,
   while (gn == 1 && gp[0] == 1);
 
   mpz_t g = MPZ_ROINIT_N (gp, gn);
+  mpz_t m = MPZ_ROINIT_N ((mp_limb_t *) mp, n), q;
+  mpz_init (q);
+  mpz_divexact (q, m, g);
+
   if (!mp_finish_in_single (factors, g))
     {
       if (mp_prime_p (g))
@@ -1542,10 +1546,6 @@ mp_factor_using_pollard_rho (struct mp_factors *factors,
         mp_factor_using_pollard_rho (factors, gp, gn, a + 1);
     }
 
-  mpz_t m = MPZ_ROINIT_N ((mp_limb_t *) mp, n), q;
-  mpz_init (q);
-  mpz_divexact (q, m, g);
-
   if (!mp_finish_in_single (factors, q))
     {
       if (mp_prime_p (q))
@@ -1601,7 +1601,7 @@ factor (struct factors *factors, mp_limb_t t1, mp_limb_t t0)
   return factor_up (factors, t1, t0, 0);
 }
 
-/* Return the prime factors of T, where B^2 / 2 <= T.  */
+/* Return the prime factors of T.  */
 static struct mp_factors
 mp_factor (mpz_t t)
 {