Re: Bug#794947: manpages-dev: printf(3) example: possible integer overflow

["Stéphane Aulery" <saulery-lkSrsyIBln0dnm+yROfE0A@public.gmane.org>]

Hello Walter,

Le 17/02/2016 22:13, walter harms a écrit :
>>
>> Jakub Wilk reported a possible integer overflow in make_message example :
>>
>>> The example in the printf(3) manpages looks like this (with boring parts
>>> omitted):
>>>
>>> int n;
>>> /* ... */
>>>    n = vsnprintf(p, size, fmt, ap);
>>>     /* ... */
>>>     if (n < 0) {
>>>         /* ... */
>>>         return NULL;
>>>     }
>>>     /* ... */
>>>     size = n + 1;
>>>
>>>
>>> But vsnprintf could return INT_MAX, which would then cause "n + 1" to
>>> overflow.
>>>
>>> (AFAICS, the glibc vsnprintf implementation never returns INT_MAX, but
>>> it could in principle.)
>>>
>>> I'd suggest changing "n < 0" to "n < 0 || n == INT_MAX".
>>
>
> the bug is real, the type of size should be size_t (in my original post it was int)
> That would make the error check useless, so we would need to store
> the vsnprintf return value in an int.
>
> The problem is that the idea was to have a simple example and cluttering
> it with error checks will make it hard to read. How many people would
> notice that size_t is unsigned and n is signed ? (i added an comment).
>
> IMHO we should simply add a sentence that "examples are examples and
> will not check for every possible error condition."


I agree with the general idea: the examples must remain so. They must 
also be correct. Tough choice!

I will not put a note on this page about it, nor on the other, too much 
for so little.

man-pages.7 specifically requests:

      Example programs shoulds be fairly short (preferably less than 100 
lines;
      Ideally less than 50 lines).

      Example programs shoulds do error checking after-system calls and
      library function calls.

So I will do a patch with your new corrected version that is very readable.

Thanks a lot for your help.

Regards,

-- 
Stéphane Aulery
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