Re: [PATCH] time: strftime_l: Use malloc rather than an unbounded alloca.

[Paul Eggert <eggert@cs.ucla.edu>]

On 5/16/23 12:28, Adhemerval Zanella Netto via Libc-alpha wrote:
> Do we have a practical maximum size for the abbreviate timezone name?  The
> internal tz_rule 'name' field is just a pointer, but I think all timezones
> uses a maximum name size.

In current TZDB data the longest abbreviation has four bytes (e.g., 
"AKST", "NZST"). Before TZDB release 2016g (2016-09-13) the longest was 
48 bytes, for the Factory zone's "Local time zone must be set--see zic 
manual page" abbreviation.

Although the TZif file format's limit is 2**32 - 1 bytes, the 'zic' 
command won't process TZif files with abbreviations longer than about 
2000 bytes, due to its internal input line buffer.

There is a limit even if you use long strings in TZ environment 
variables, as glibc strftime is buggy when a buffer contains more than 
INT_MAX bytes (it uses 'int' to store byte counts). This means you can't 
effectively use a time zone abbreviation longer than about 2**31 - 1 
bytes with glibc, even on 64-bit platforms. For example, a program like 
the one shown below won't work with glibc.

For what it's worth, _POSIX_TZNAME_MAX is 6, that is, every POSIX 
platform must support at least 6 bytes.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int
main ()
{
   size_t Xs = (1LL << 32) + 1;
   char *tzbuf = malloc (sizeof "TZ=" - 1 + Xs + sizeof "0");
   if (!tzbuf)
     return perror ("malloc tzbuf"), 1;
   size_t strftimebufsize = Xs + 2;
   char *strftimebuf = malloc (strftimebufsize);
   if (!strftimebuf)
     return perror ("malloc strftimebuf"), 1;
   strcpy (tzbuf, "TZ=");
   memset (tzbuf + 3, 'X', Xs);
   strcpy (tzbuf + 3 + Xs, "0");
   if (putenv (tzbuf))
     return perror ("putenv"), 1;
   time_t t0 = 0;
   struct tm *tm = localtime (&t0);
   if (!tm)
     return perror ("localtime"), 1;
   size_t nbytes = strftime (strftimebuf, strftimebufsize, "%Z", tm);
   if (!nbytes)
     return perror ("strftime"), 1;
   if (puts (strftimebuf) < 0)
     return perror ("puts"), 1;
   if (fclose (stdout) < 0)
     return perror ("fclose"), 1;
   return 0;
}