Re: gcc-12.0.0-0.4.fc36 in rawhide

["Richard W.M. Jones" <rjones@xxxxxxxxxx>]

There is a new warning in gcc-12.0.0-0.4.fc36.x86_64.  In this code:

  int
  guestfs_int_create_socketname (guestfs_h *g, const char *filename,
                                 char (*sockpath)[UNIX_PATH_MAX])
  {
    if (guestfs_int_lazy_make_sockdir (g) == -1)
      return -1;
  
    if (strlen (g->sockdir) + 1 + strlen (filename) > UNIX_PATH_MAX-1) {
      error (g, _("socket path too long: %s/%s"), g->sockdir, filename);
      return -1;
    }
  
    snprintf (*sockpath, UNIX_PATH_MAX, "%s/%s", g->sockdir, filename);
  
    return 0;
  }

[https://github.com/libguestfs/libguestfs/blob/d1e7e1a323619d8f1e913a7833d07009f02a2d33/lib/launch.c#L324]

the new warning is:

  launch.c: In function ‘guestfs_int_create_socketname’:
  launch.c:336:43: error: ‘%s’ directive output may be truncated writing up to 106 bytes into a region of size between 1 and 107 [-Werror=format-truncation=]
    336 |   snprintf (*sockpath, UNIX_PATH_MAX, "%s/%s", g->sockdir, filename);
        |                                           ^~
  In file included from /usr/include/stdio.h:894,
                   from launch.c:30:
  In function ‘snprintf’,
      inlined from ‘guestfs_int_create_socketname’ at launch.c:336:3:
/usr/include/bits/stdio2.h:71:10: note: ‘__snprintf_chk’ output between 2 and 2  14 bytes into a destination of size 108
     71 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     72 |                                    __glibc_objsize (__s), __fmt,
        |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     73 |                                    __va_arg_pack ());
        |                                    ~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

*sockpath is a fixed buffer of size UNIX_PATH_MAX == 108.  We check
that strlen (g->sockdir) + strlen (filename) + 1 (for the '/'
character) > UNIX_PATH_MAX - 1 (for the terminating '\0').

The check seems correct as far as I can tell.  I don't think I'm
making a fencepost error here.  Why does GCC 12 think there should be
a warning when GCC 11 didn't?

I've attached a standalone test case.

  $ gcc -O2 -Wall sp.c -o sp
  sp.c: In function ‘create_sockpath’:
  sp.c:12:43: warning: ‘%s’ directive output may be truncated writing up to 106 bytes into a region of size between 1 and 107 [-Wformat-truncation=]
     12 |   snprintf (*sockpath, UNIX_PATH_MAX, "%s/%s", sockdir, filename);
        |                                           ^~
  sp.c:12:3: note: ‘snprintf’ output between 2 and 214 bytes into a destination of size 108
     12 |   snprintf (*sockpath, UNIX_PATH_MAX, "%s/%s", sockdir, filename);
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(No warning with gcc-11.2.1-1.fc35.x86_64)

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

/* gcc -O2 -Wall sp.c -o sp */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <linux/un.h>

void
create_sockpath (const char *sockdir, const char *filename,
                 char (*sockpath)[UNIX_PATH_MAX])
{
  if (strlen (sockdir) + 1 + strlen (filename) > UNIX_PATH_MAX - 1)
    abort ();
  snprintf (*sockpath, UNIX_PATH_MAX, "%s/%s", sockdir, filename);
}

int
main (int argc, char *argv[])
{
  char sockpath[UNIX_PATH_MAX];

  if (argc != 3) {
    fprintf (stderr, "%s sockdir filename\n", argv[0]);
    exit (EXIT_FAILURE);
  }

  create_sockpath (argv[1], argv[2], &sockpath);
  printf ("sockpath = %s\n", sockpath);
  exit (EXIT_SUCCESS);
}

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