[PATCH v2] getpw: Get rid of alloca

[Adhemerval Zanella Netto]

On 29/08/23 05:03, Florian Weimer via Libc-alpha wrote:
> * Joe Simmons-Talbott via Libc-alpha:
> 
>> Since _SC_GETPW_R_SIZE_MAX will be always NSS_BUFLEN_PASSWD, use a fixed
>> sized array rather than alloca to avoid potential stack overflow.
> 
> _SC_GETPW_R_SIZE_MAX is not very well-named, it is the initial suggested
> buffer size.  The code should use the usual scratch buffer retry loop.

I though about that on initial revision, however this will change the function
semantic and the code below:

  long int sz = sysconf (_SC_GETPW_R_SIZE_MAX);
  char *buf = NULL;
  while (1) {
    buf = realloc (buf,  sz);
    r = getpw (uid, buf);
    if (r != -1 || errno != ENOMEM)
      break;
    sz *= 2;
  }

will start to trigger a buffer overrun.  This is an old tricky interface
where I would prefer if we continue to keep same semantic, and maybe 
deprecate and move it to compat symbol.