I have dug deeper into understanding what happens with both versions (method 1
& 2).
If the divisor has a high bit set, then pre[2] is not initialized inside
mpn_mod_1_1p_cps(), but it is also not used inside mpn_mod_1_1p() as there are
no leading zeroes.
In the other scenario, if the high bit is set, pre[2] is set to its appropriate
value, and it is used inside mpn_mod_1_1p().
Some commenting could be placed inside /mpn/generic/mod_1.c around lines 248
and 261, or pre[4] can be initialized to all 0's on both 248 & 261: mp_limb_t
pre[4] = { 0 };
-Brett Kuntz
From: "Vincent Lefevre" <vinc...@vinc17.net>
To: "marco bodrato" <marco.bodr...@tutanota.com>
Cc: "Brett Kuntz" <ku...@shaw.ca>, gmp-bugs@gmplib.org
Sent: Thursday, August 31, 2023 11:06:51 AM
Subject: Re: Uninitialized memory bug found in /mpn/generic/mod_1_1.c
On 2023-08-31 17:03:12 +0200, marco.bodr...@tutanota.com wrote:
> Ciao,
>
> 31 ago 2023, 16:30 da ku...@shaw.ca:
>
> > 1) Edit line 248 mpn/generic/mp_limb_t pre[4]; into:
> >
> ~/src/gmp$ hg diff mpn/generic/
> diff -r 3ac5afa36be5 mpn/generic/mod_1.c
> --- a/mpn/generic/mod_1.c Wed Nov 02 13:48:37 2022 +0100
> +++ b/mpn/generic/mod_1.c Thu Aug 31 16:46:35 2023 +0200
> @@ -245,7 +245,7 @@
> }
> else
> {
> - mp_limb_t pre[4];
> + mp_limb_t pre[4] = {-1, -1, -1, -1};
> mpn_mod_1_1p_cps (pre, b);
> return mpn_mod_1_1p (ap, n, b, pre);
> }
>
I don't think that this is sufficient for the test.
The code Brett mentioned is for MOD_1_1P_METHOD = 2.
So, in mpn/generic/mod_1_1.c, I also changed
# define MOD_1_1P_METHOD 1 /* need to make sure this is 2 for asm testing */
to
# define MOD_1_1P_METHOD 2 /* need to make sure this is 2 for asm testing */
and at the beginning of
mpn_mod_1_1p (mp_srcptr ap, mp_size_t n, mp_limb_t b, const mp_limb_t bmodb[4])
I added
ASSERT (bmodb[2] != -1);
(if bmodb[2] is not set, this would fail).
Then I configured with
./configure --disable-assembly --enable-assert
to enable the code and the assert, then "make" and "make check".
But even with that, I don't get any failure.
--
Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
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