CCing Martin.
On Sat, Jan 15, 2022 at 09:04:35AM +0000, Richard W.M. Jones wrote:
> 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);
> }
Jakub
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