unofficial mirror of bug-gnu-emacs@gnu.org 
 help / color / mirror / code / Atom feed
From: Paul Eggert <eggert@cs.ucla.edu>
To: rms@gnu.org
Cc: 8545@debbugs.gnu.org
Subject: bug#8545: issues with recent doprnt-related changes
Date: Tue, 03 May 2011 13:24:16 -0700	[thread overview]
Message-ID: <4DC06470.4050902@cs.ucla.edu> (raw)
In-Reply-To: <E1QH37h-0001yM-HR@fencepost.gnu.org>

>>     There are similar reliable tests for the other arithmetic operations.
> 
> Is this documented somewhere?  Is there a list of the standard ways?

CERT has something, here:

https://www.securecoding.cert.org/confluence/display/seccode/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow

Although the principles in that memo are OK, the actual code is
hard to read and its multiplication overflow checking is buggy.

Here's something better, which I just now wrote.  Also, please see
Emacs Bug#8611 <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8611>;
its patch uses code like the following.


#include <limits.h>

int
add_overflow (int a, int b)
{
  return (b < 0
	  ? a < INT_MIN - b
	  : INT_MAX - b < a);
}

int
subtract_overflow (int a, int b)
{
  return (b < 0
	  ? INT_MAX + b < a
	  : a < INT_MIN + b);
}

int
unary_minus_overflow (int a)
{
  return a < -INT_MAX;
}

int
multiply_overflow (int a, int b)
{
  return (b < 0
	  ? (a < 0
	     ? a < INT_MAX / b
	     : b != -1 && INT_MIN / b < a)
	  : (b != 0
	     && (a < 0
		 ? a < INT_MIN / b
		 : INT_MAX / b < a)));
}

int
quotient_overflow (int a, int b)
{
  /* This does not check for division by zero.  Add that if you like.  */
  return a < -INT_MAX && b == -1;
}

int
remainder_overflow (int a, int b)
{
  /* Mathematically the remainder should never overflow, but on x86-like
     hosts INT_MIN % -1 traps, and the C standard permits this.  */
  return quotient_overflow (a, b);
}





  parent reply	other threads:[~2011-05-03 20:24 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-04-25  5:46 bug#8545: issues with recent doprnt-related changes Paul Eggert
2011-04-25  9:00 ` Eli Zaretskii
2011-04-25 13:37   ` Stefan Monnier
2011-04-26 20:25     ` Paul Eggert
2011-04-27  1:14       ` Stefan Monnier
2011-04-26  6:02   ` Paul Eggert
2011-04-27 19:34     ` Eli Zaretskii
2011-04-27 23:51       ` Paul Eggert
2011-04-28  1:32         ` Juanma Barranquero
2011-04-28  3:11           ` Paul Eggert
2011-04-28  3:42             ` Juanma Barranquero
2011-04-28  5:06               ` Paul Eggert
2011-04-28  5:15             ` Eli Zaretskii
2011-04-28  5:29               ` Paul Eggert
2011-04-28  6:10                 ` Eli Zaretskii
2011-04-28  6:42                   ` Paul Eggert
2011-04-28  7:26                     ` Eli Zaretskii
2011-04-28  7:54                       ` Paul Eggert
2011-04-28 11:14                         ` Eli Zaretskii
2011-04-29 12:28             ` Richard Stallman
2011-04-29 19:56               ` Eli Zaretskii
2011-04-29 23:49               ` Paul Eggert
2011-04-30 21:03                 ` Richard Stallman
2011-05-01  5:41                   ` Paul Eggert
2011-05-01 23:59                     ` Richard Stallman
2011-05-02  0:23                       ` Paul Eggert
     [not found]                         ` <E1QH37h-0001yM-HR@fencepost.gnu.org>
2011-05-03 20:24                           ` Paul Eggert [this message]
2011-05-01  4:25                 ` Jason Rumney
2011-05-01  5:56                   ` Paul Eggert
2011-05-01  8:12                     ` Jason Rumney
2011-05-01 11:02                       ` Andreas Schwab
2011-04-28  5:02           ` Eli Zaretskii
2011-04-28  5:50         ` Eli Zaretskii
     [not found]           ` <4DB9146D.2040702@cs.ucla.edu>
     [not found]             ` <E1QFQVO-0004Dq-6o@fencepost.gnu.org>
     [not found]               ` <4DB9E5FF.9020506@cs.ucla.edu>
2011-04-29 11:16                 ` Eli Zaretskii
2011-04-29 14:41                   ` Paul Eggert
2011-04-29 19:35                     ` Eli Zaretskii
2011-04-29 20:32                       ` Paul Eggert
2011-04-30  8:59                         ` Eli Zaretskii
2011-05-04  7:28                   ` Paul Eggert
2011-05-04  9:52                     ` Eli Zaretskii
2011-05-04 14:56                       ` Paul Eggert
     [not found]                       ` <4DC1692B.1090101@cs.ucla.edu>
2011-05-05 20:36                         ` Eli Zaretskii
     [not found]                         ` <83ei4cnau6.fsf@gnu.org>
2011-05-06 13:33                           ` Stefan Monnier
     [not found]                           ` <jwvsjss2bz3.fsf-monnier+emacs@gnu.org>
2011-05-06 14:41                             ` Paul Eggert
2011-05-06 15:03                             ` Eli Zaretskii
     [not found]                             ` <83vcxnlvl9.fsf@gnu.org>
2011-05-06 17:13                               ` Stefan Monnier
     [not found]                               ` <jwv8vuj21q0.fsf-monnier+emacs@gnu.org>
2011-05-06 19:57                                 ` Eli Zaretskii
     [not found]                                 ` <83k4e3lhzp.fsf@gnu.org>
2011-05-07  3:18                                   ` Stefan Monnier
     [not found]                                   ` <jwvr58byz9s.fsf-monnier+emacs@gnu.org>
2011-05-07  7:55                                     ` Eli Zaretskii
  -- strict thread matches above, loose matches on Subject: below --
2011-05-01 18:19 bug#8601: * 2 -> * 4 typo fix in detect_coding_charset Paul Eggert
2011-05-01 19:06 ` Andreas Schwab
2011-05-01 19:25   ` Paul Eggert
2011-05-06  7:29 ` bug#8601: Merged fixes for 8600, 8601, 8602, and (partially) for 8545 Paul Eggert
2020-09-14 12:37   ` bug#8545: " Lars Ingebrigtsen
2020-09-14 18:41     ` Eli Zaretskii
2020-09-16  2:01       ` Paul Eggert

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: https://www.gnu.org/software/emacs/

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=4DC06470.4050902@cs.ucla.edu \
    --to=eggert@cs.ucla.edu \
    --cc=8545@debbugs.gnu.org \
    --cc=rms@gnu.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

	https://git.savannah.gnu.org/cgit/emacs.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).