From: Paul Eggert <eggert@cs.ucla.edu>
Date: Mon, 13 Jul 2015 16:53:00 +0000 (-0700)
Subject: tar: fix symlink race and symlink transform bug
X-Git-Url: https://git.dogcows.com/gitweb?p=chaz%2Ftar;a=commitdiff_plain;h=fe3b106cb3b0bc4d50230ed263bf5ccbc8bf1ea7

tar: fix symlink race and symlink transform bug

Problem reported by Tobias Stoeckmann in:
http://lists.gnu.org/archive/html/bug-tar/2015-07/msg00004.html
* gnulib.modules: Add areadlinkat-with-size.
* src/create.c: Include areadlink.h.
(dump_file0): Use areadlinkat_with_size, rather than trying to do
it by hand, incorrectly.  This also avoids assumption that
the symlink contents fit on the stack.  Also, use the transformed
link name, not the original link name, when deciding whether the
name is long enough to require writing a long link.
---

diff --git a/gnulib.modules b/gnulib.modules
index ec3dc90..fe5ab73 100644
--- a/gnulib.modules
+++ b/gnulib.modules
@@ -19,6 +19,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 alloca
+areadlinkat-with-size
 argmatch
 argp
 argp-version-etc
diff --git a/src/create.c b/src/create.c
index 1b08e0b..7cdc978 100644
--- a/src/create.c
+++ b/src/create.c
@@ -22,6 +22,7 @@
 
 #include <system.h>
 
+#include <areadlink.h>
 #include <quotearg.h>
 
 #include "common.h"
@@ -1114,7 +1115,7 @@ dump_dir0 (struct tar_stat_info *st, char const *directory)
     return;
 
   info_attach_exclist (st);
-  
+
   if (incremental_option && archive_format != POSIX_FORMAT)
     blk->header.typeflag = GNUTYPE_DUMPDIR;
   else /* if (standard_option) */
@@ -1198,7 +1199,7 @@ dump_dir0 (struct tar_stat_info *st, char const *directory)
 	    char const *entry;
 	    size_t entry_len;
 	    size_t name_len;
-	    
+
 	    name_buf = xstrdup (st->orig_file_name);
 	    name_size = name_len = strlen (name_buf);
 
@@ -1837,22 +1838,17 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
 #ifdef HAVE_READLINK
   else if (S_ISLNK (st->stat.st_mode))
     {
-      char *buffer;
-      int size;
-      size_t linklen = st->stat.st_size;
-      if (linklen != st->stat.st_size || linklen + 1 == 0)
-	xalloc_die ();
-      buffer = (char *) alloca (linklen + 1);
-      size = readlinkat (parentfd, name, buffer, linklen + 1);
-      if (size < 0)
+      st->link_name = areadlinkat_with_size (parentfd, name, st->stat.st_size);
+      if (!st->link_name)
 	{
+	  if (errno == ENOMEM)
+	    xalloc_die ();
 	  file_removed_diag (p, top_level, readlink_diag);
 	  return;
 	}
-      buffer[size] = '\0';
-      assign_string (&st->link_name, buffer);
       transform_name (&st->link_name, XFORM_SYMLINK);
-      if (NAME_FIELD_SIZE - (archive_format == OLDGNU_FORMAT) < size)
+      if (NAME_FIELD_SIZE - (archive_format == OLDGNU_FORMAT)
+	  < strlen (st->link_name))
 	write_long_link (st);
 
       xattrs_selinux_get (parentfd, name, st, 0);