Arithmetic bug on ARM (STM 32 F4)

[Titouan Coladon]

Hi,

I will try to describe as good as possible.

The bug itself is really simple: The "mod" operation below gives a wrong
result on stm32F4

    mpz_t t; mpz_init(t);
    mpz_t tt; mpz_init(tt);
    mpz_t ttt; mpz_init(ttt);
    mpz_set_str(tt,
"25975415027868818028037643145183730078267964363630309107863975221668725232906664910091356673487591948237515304861352649057814029879720636142810833722202669061030484458925649047021859875730105532595466846094449735851132072400310565642",
10);
    mpz_set_str(t,
"6864797660130609714981900799081393217269435300143305409394463459185543183397655394245057746333217197532963996371363321113864768612440380340372808892707005449",
10);
    mpz_mod(ttt, tt, t);
    printf("tt = "); gmp_printf("%Zd", tt); printf("\n");
    printf("t = "); gmp_printf("%Zd", t); printf("\n");
    printf("ttt = "); gmp_printf("%Zd", ttt); printf("\n");
    mpz_clear(t);
    mpz_clear(tt);
    mpz_clear(ttt);


The result is:

tt =
25975415027868818028037643145183730078267964363630309107863975221668725232906664910091356673487591948237515304861352649057814029879720636142810833722202669061030484458925649047021859875730105532595466846094449735851132072400310565642
t =
6864797660130609714981900799081393217269435300143305409394463459185543183397655394245057746333217197532963996371363321113864768612440380340372808892707005449
ttt =
3578942983869995381409279381784954092773488221200587338717407747084074518600438737913956360066709506626656635118978526559637095289631190671071178702312877654

but ttt should be
5247183143879283565064056089869553515440215252945038779070795247972433237341511292686962488915110675351673985513318500576097977116134885013371639187873069514


I give the code to reproduce the bug in attachment.

GMP is built with the options:
CC=/home/titouan/Softs/gcc-arm-none-eabi-10-2020-q4-major-x86_64-linux/gcc-arm-none-eabi-10-2020-q4-major/bin/arm-none-eabi-gcc
../configure CFLAGS="-nostartfiles --specs=nosys.specs -mcpu=cortex-m4"
--host=arm-none-eabi --disable-assembly --prefix=YOUR_CHOICE

You also must change the PATHS variables at the beggining of the Makefile.

Then classically burn on the stm32 board using openocd...


The bug appears only with this size of number, based on arithmetic over
the nist521 EC.


Best regards,


Titouan Coladon



-------------- next part --------------
A non-text attachment was scrubbed...
Name: Bug_gmp.tar.xz
Type: application/x-xz
Size: 2751084 bytes
Desc: not available
URL: <https://gmplib.org/list-archives/gmp-bugs/attachments/20210121/20a735ee/attachment-0001.bin>