tar-1.13.25: seg fault in get_directory_contents for EPERM dirs

[John E Hein]

There is a potential seg. fault in tar-1.13.25/src/incremen.c

If savedir fails (at line 180 in get_directory_contents), which might
 happen if, say, the directory cannot be opened due to permissions issues,
 it will return a NULL.  This NULL pointer might be dereferenced, later,
 however.

Recreate this by running with --listed-incremental on a dir for which
 you have no read perms:

mkdir /tmp/noperm
chmod a-rwx /tmp/noperm
tar-1.13.25/src/tar --create --directory /tmp --listed-incremental /tmp/list 
--file /dev/null /tmp/noperm

I am running FreeBSD 4.3, but this should be a problem on other systems,
 too.

Here's a patch.  It prevents the seg. fault.  I don't know the gnu tar
 code that well, so I don't know if it might cause an undesired effect
 elsewhere.  In fact, to be more efficient, it'd probably be good to exit
 from get_directory_contents earlier in the function if there's nothing
 that can be done (rather than check for a NULL pointer each time through
 the loop).


--- incremen.c.orig     Wed Aug 29 12:20:19 2001
+++ incremen.c  Fri Oct 19 09:19:02 2001
@@ -204,7 +204,7 @@
 
     if (children != NO_CHILDREN)
       for (entry = dirp;
-          (entrylen = strlen (entry)) != 0;
+          entry && (entrylen = strlen (entry)) != 0;
           entry += entrylen + 1)
        {
          if (name_buffer_size <= entrylen + name_length)