Re: Constexpr evaluation of heavy stuff

From	Ben <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c++
Subject	Re: Constexpr evaluation of heavy stuff
Date	2022-05-20 11:25 +0100
Organization	A noiseless patient Spider
Message-ID	<87o7zs5wlt.fsf@bsb.me.uk> (permalink)
References	(5 earlier) <b4c80897-17a3-4951-a4a5-f3ca95c2f5d5n@googlegroups.com> <t65v09$bd9$1@dont-email.me> <727d656f-7e08-4be2-a0c9-c2dc9d29b00cn@googlegroups.com> <t67jvc$1q94$1@gioia.aioe.org> <978b4a0b-0740-4b01-96b0-4b9e52e0de1an@googlegroups.com>

Show all headers | View raw

Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:

> On Friday, 20 May 2022 at 09:37:19 UTC+1, Juha Nieminen wrote:
>> Malcolm McLean <malcolm.ar...@gmail.com> wrote: 
>> > The snag is that to use it, you have to be able to put the compiler into a 
>> > mode where it traps on overflow instead of silently wrapping. Most 
>> > compilers don't have such a mode.
>> I'm not entirely sure what the point of that would be. What exactly should 
>> happen in such a program if an unsigned arithmetic operation overflows or 
>> underflows? Should the program end? Should the code throw an exception? 
>> (What would you do if such an exception happens?) How should the program 
>> behave in such a situation? 
>> 
>> Also, should bit-shifting to the left trap if a 1-bit gets shifted out? 
>> Ostensibly multiplying by a power of 2 should trap if it overflows. 
>> Should bit-shifting to the left also do so (given that it's effectively 
>> the exact same thing)? 
>> 
>> Maybe it should behave like a few other programming languages which switch 
>> from hardware-register-sized integers to software multiprecision integers 
>> on any overflow? (But even then, what about underflow?) 
>> 
>> If you want integers that grow as needed, just use GMP?
>>
> Say we've got this code.
>
> int width = getinteger():
> int height = getinteger();
> int Npixels = width* height;
>
> There's a potential vulnerability there if Npixels wraps. An attacker
> might be able to construct an input file that causes the program to
> access memory out of bounds in such a way as to cause arbitrary code
> to be executed.

I found the suggestion (not from you) that UB is an advantage a very odd
one.  I would consider it too unpredictable to be any practical help.

But here, why not just make all the sizes unsigned?  You have to do some
bounds checking since the data come from a file which can be maliciously
constructed, so provided you do that check, a wrapped Npixels will
presumably just produce a junk result.

And, if you decide to add an overflow check, it's no harder using
unsigned.

-- 
Ben.

Thread

Constexpr evaluation of heavy stuff Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-18 16:17 -0700
  Re: Constexpr evaluation of heavy stuff "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-05-19 09:19 +0200
    Re: Constexpr evaluation of heavy stuff David Brown <david.brown@hesbynett.no> - 2022-05-19 11:59 +0200
      Re: Constexpr evaluation of heavy stuff "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-05-19 12:37 +0200
        Re: Constexpr evaluation of heavy stuff David Brown <david.brown@hesbynett.no> - 2022-05-19 14:49 +0200
          Re: Constexpr evaluation of heavy stuff Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-05-19 06:19 -0700
            Re: Constexpr evaluation of heavy stuff Paavo Helde <eesnimi@osa.pri.ee> - 2022-05-19 20:32 +0300
              Re: Constexpr evaluation of heavy stuff Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-05-20 01:12 -0700
                Re: Constexpr evaluation of heavy stuff Juha Nieminen <nospam@thanks.invalid> - 2022-05-20 08:37 +0000
                Re: Constexpr evaluation of heavy stuff Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-05-20 02:21 -0700
                Re: Constexpr evaluation of heavy stuff Ben <ben.usenet@bsb.me.uk> - 2022-05-20 11:25 +0100
                Re: Constexpr evaluation of heavy stuff David Brown <david.brown@hesbynett.no> - 2022-05-20 12:47 +0200
                Re: Constexpr evaluation of heavy stuff Ben <ben.usenet@bsb.me.uk> - 2022-05-20 12:13 +0100
                Re: Constexpr evaluation of heavy stuff David Brown <david.brown@hesbynett.no> - 2022-05-20 14:27 +0200
                Re: Constexpr evaluation of heavy stuff Öö Tiib <ootiib@hot.ee> - 2022-05-20 06:33 -0700
                Re: Constexpr evaluation of heavy stuff Paavo Helde <eesnimi@osa.pri.ee> - 2022-05-20 17:15 +0300
                Re: Constexpr evaluation of heavy stuff Christian Gollwitzer <auriocus@gmx.de> - 2022-05-20 18:57 +0200
                Re: Constexpr evaluation of heavy stuff Paavo Helde <eesnimi@osa.pri.ee> - 2022-05-20 22:31 +0300
                Re: Constexpr evaluation of heavy stuff Manfred <noname@add.invalid> - 2022-05-21 21:56 +0200
                Re: Constexpr evaluation of heavy stuff Öö Tiib <ootiib@hot.ee> - 2022-05-21 23:24 -0700
                Re: Constexpr evaluation of heavy stuff Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-05-22 04:56 -0700
                Re: Constexpr evaluation of heavy stuff Öö Tiib <ootiib@hot.ee> - 2022-05-22 06:15 -0700
                Re: Constexpr evaluation of heavy stuff Manfred <noname@add.invalid> - 2022-05-22 21:46 +0200
                Re: Constexpr evaluation of heavy stuff "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-05-22 23:23 +0200
                Re: Constexpr evaluation of heavy stuff Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-05-22 16:26 -0700
                Re: Constexpr evaluation of heavy stuff Manfred <noname@add.invalid> - 2022-05-23 17:24 +0200
                Re: Constexpr evaluation of heavy stuff Richard Damon <Richard@Damon-Family.org> - 2022-05-23 19:41 -0400
                Re: Constexpr evaluation of heavy stuff scott@slp53.sl.home (Scott Lurndal) - 2022-05-24 16:15 +0000
                Re: Constexpr evaluation of heavy stuff Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-05-24 05:37 -0700
                Re: Constexpr evaluation of heavy stuff Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-05-20 04:19 -0700
                Re: Constexpr evaluation of heavy stuff Ben <ben.usenet@bsb.me.uk> - 2022-05-20 12:32 +0100
                Re: Constexpr evaluation of heavy stuff Juha Nieminen <nospam@thanks.invalid> - 2022-05-20 10:40 +0000
                Re: Constexpr evaluation of heavy stuff David Brown <david.brown@hesbynett.no> - 2022-05-20 12:31 +0200
                Re: Constexpr evaluation of heavy stuff Juha Nieminen <nospam@thanks.invalid> - 2022-05-20 10:43 +0000
                Re: Constexpr evaluation of heavy stuff David Brown <david.brown@hesbynett.no> - 2022-05-20 14:52 +0200
          Re: Constexpr evaluation of heavy stuff "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-05-19 15:34 +0200
    Re: Constexpr evaluation of heavy stuff "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-05-19 17:03 +0200
  Re: Constexpr evaluation of heavy stuff Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-05-19 08:53 -0700
    Re: Constexpr evaluation of heavy stuff Marcel Mueller <news.5.maazl@spamgourmet.org> - 2022-05-19 18:33 +0200

csiph-web