Re: [RFC] _FORTIFY_SOURCE strictness

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 09/04/2022 02:37, Paul Eggert wrote:
> On 4/7/22 23:02, Siddhesh Poyarekar wrote:
>> Hmm, I think I conflated it with something other str* function.  
>> You're right, strncpy probably doesn't fall into this category.
> 
> Ouch, I made the same mistake.
> 
> As for wcrtomb, unfortunately the standard's wording appears to allow 
> you to pass an output buffer smaller than MB_CUR_MAX if you know that 
> the multibyte character will fit into the smaller buffer. So I guess 
> this is an example of a function where __FORTIFY_LEVEL > 2 doesn't 
> conform to the standard.
> 
> I don't know whether the standard's authors intended this.
> 

Thomas (in cc, he maintains ncurses) found that the manual documents[1] 
the fact that the glibc implementation of wcrtomb assumes the 
destination buffer to have at least MB_CUR_MAX bytes, so this looks more 
like a situation where we *deliberately* deviate from the standard.

If we decide to comply with the standard now we would incur an 
additional copy from an internal buffer to the destination with in 
addition to wrappers to pass the object size from the checking variant 
whenever available.  The question then is whether that's the direction 
we want to take in glibc.

Siddhesh

[1] 
https://www.gnu.org/software/libc/manual/html_node/Converting-a-Character.html