GNU bug report logs - #57211
29.0.50; generate-new-buffer-name sprintf format overflow warning

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#57211: closed (29.0.50; generate-new-buffer-name sprintf
 format overflow warning)
Date: Sun, 14 Aug 2022 20:55:01 +0000

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Your message dated Sun, 14 Aug 2022 13:54:08 -0700
with message-id <9bd9ae82-8525-1d32-c4a1-5daaf909609a <at> cs.ucla.edu>
and subject line Re: bug#57211: 29.0.50; generate-new-buffer-name sprintf format overflow warning
has caused the debbugs.gnu.org bug report #57211,
regarding 29.0.50; generate-new-buffer-name sprintf format overflow warning
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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57211: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=57211
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From: "Basil L. Contovounesios" <contovob <at> tcd.ie>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.50; generate-new-buffer-name sprintf format overflow warning
Date: Sun, 14 Aug 2022 19:50:03 +0300

Severity: minor

Compiling with gcc (Debian 12.1.0-7) 12.1.0 and -Og, I get the following
-Wformat-overflow warning:

In file included from buffer.c:33:
buffer.c: In function ‘Fgenerate_new_buffer_name’:
buffer.c:1167:46: warning: ‘sprintf’ may write a terminating nul past the end of the destination [-Wformat-overflow=]
 1167 |       AUTO_STRING_WITH_LEN (lnumber, number, sprintf (number, "-%d", i));
      |                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~
lisp.h:5493:36: note: in definition of macro ‘AUTO_STRING_WITH_LEN’
 5493 |         ((&(struct Lisp_String) {{{len, -1, 0, (unsigned char *) (str)}}}), \
      |                                    ^~~
buffer.c:1167:46: note: ‘sprintf’ output between 3 and 9 bytes into a destination of size 8
 1167 |       AUTO_STRING_WITH_LEN (lnumber, number, sprintf (number, "-%d", i));
      |                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~
lisp.h:5493:36: note: in definition of macro ‘AUTO_STRING_WITH_LEN’
 5493 |         ((&(struct Lisp_String) {{{len, -1, 0, (unsigned char *) (str)}}}), \
      |                                    ^~~

Can the upper bound 9 ever be achieved?  If so, how?  If not, is this a
GCC bug?  Either way, is there a way to pacify the warning?

I tried

  snprintf (number, sizeof number, ...)

but got the same warning.

BTW, in the preceding

  int i = r % 1000000;

can the result of % ever exceed INT_MAX?  And do we care either way?

Thanks,

-- 
Basil

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
 of 2022-08-14 built on tia
Repository revision: 1d3fe256907d5e78a4acedd194e55db8ab952952
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[Message part 3 (message/rfc822, inline)]

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: "Basil L. Contovounesios" <contovob <at> tcd.ie>
Cc: 57211-done <at> debbugs.gnu.org
Subject: Re: bug#57211: 29.0.50; generate-new-buffer-name sprintf format
 overflow warning
Date: Sun, 14 Aug 2022 13:54:08 -0700

[Message part 4 (text/plain, inline)]

On 8/14/22 09:50, Basil L. Contovounesios wrote:

> Can the upper bound 9 ever be achieved?  If so, how?  If not, is this a
> GCC bug?  Either way, is there a way to pacify the warning?

It can't be achieved, and it's arguably a GCC bug. I installed the 
attached to pacify GCC.

>    int i = r % 1000000;
> 
> can the result of % ever exceed INT_MAX?

No.


On 8/14/22 11:59, Matt Armstrong wrote:

> Gcc doesn't know that get_random() returns only non-negative numbers,
> and the eassume() call doesn't seem to be enough to convince gcc this
> fact, or gcc does not infer i is also non-negative.

It's worse than that. Even if you add 'eassume (0 <= i && i < 1000000);" 
GCC still doesn't assume that the sprintf is in range.

> Personally, I'd change this code to use a buffer
> 
>    INT_BUFSIZE_BOUND(int) + sizeof "-"

The problem with overallocating buffers is not the memory loss (it's 
trivial, as you say), it's that later readers like me will wonder why 
the buffer is being overallocated, which is maintenance overhead.

I installed the attached to pacify GCC while also attempting to not 
entirely mystify later readers.

[0001-Work-around-Bug-57211.patch (text/x-patch, attachment)]

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