Re: copying a string with truncation (was: [PATCH] resolv: add IPv6 support to inet_net_pton())

[Alejandro Colomar <alx.manpages@gmail.com>]

[-- Attachment #1.1: Type: text/plain, Size: 2864 bytes --]

On 12/22/22 19:28, Alejandro Colomar wrote:>> 1) Use strncpy() instead of strlcpy()
> 
> Would someone please add a function to glibc that truncates a string, while 
> still producing a string (as opposed to a null-padded fixed-width character 
> sequence)?
> 
> Here goes an extract of the yet-unreleased strncpy(3) manual page from the Linux 
> man-pages master branch:
> 
> 
> DESCRIPTION
>         These functions copy the string pointed to by src  into  a  null‐padded
>         character sequence at the fixed‐width buffer pointed to by dst.  If the
>         destination buffer, limited by its size, isn’t large enough to hold the
>         copy,  the  resulting character sequence is truncated.  For the differ‐
>         ence between the two functions, see RETURN VALUE.
> 
>         An implementation of these functions might be:
> 
>             char *
>             stpncpy(char *restrict dst, const char *restrict src, size_t sz)
>             {
>                 bzero(dst, sz);
>                 return mempcpy(dst, src, strnlen(src, sz));
>             }
> 
>             char *
>             strncpy(char *restrict dst, const char *restrict src, size_t sz)
>             {
>                 stpncpy(dst, src, sz);
>                 return dst;
>             }
> 
>         [...]
> 
> CAVEATS
>         The name of these functions is confusing.  These  functions  produce  a
>         null‐padded character sequence, not a string (see string_copying(7)).
> 
>         It’s  impossible  to  distinguish truncation by the result of the call,
>         from a character sequence that just fits the destination buffer;  trun‐
>         cation  should  be detected by comparing the length of the input string
>         with the size of the destination buffer.
> 
> I'll be releasing the a new man-pages version very soon (a week at most), so 
> that this page and also the new string_copying(7) overview are widely available.

I released a moment ago.  So, I'd suggest either adding strlcpy(3)&strlcat(3) to 
glibc, or stpecpy(3) (defined here: 
<http://www.alejandro-colomar.es/src/alx/alx/libstp.git/tree/src/stp/stpe/stpecpy.c>). 
  Or even both, since each of them serves a purpose: strlcpy(3)&strlcat(3) are 
for simpler code where performance and truncation are not a concern, and 
stpecpy(3) does the same but faster with just a few more bytes of code (and 
detecting truncation is even simpler).

I'll send an actual patch for adding stpecpy(3), to discuss with some actual code.

Cheers,

Alex


-- 
<http://www.alejandro-colomar.es/>

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]