Detect tarbombs while extracting

* src/common.h (one_top_level_option): New global.
(one_top_level): New global.
* src/extract.c (extr_init): If one_top_level_option is set, determine
the name one_top_level that might have to be prepended.
(extract_archive): If one_top_level_option is set, prepend one_top_level
to all names that don't already start with it.
* src/tar.c (ONE_TOP_LEVEL_OPTION): New contant.
(options): New option --one-top-level.
(parse_opt): Handle this option.
(decode_options): Make it conflict with --absolute-names.

src/common.h

@@ -235,6 +235,10 @@ GLOBAL bool numeric_owner_option;
 
 GLOBAL bool one_file_system_option;
 
+/* Create a top-level directory for extracting based on the archive name.  */
+GLOBAL bool one_top_level_option;
+GLOBAL char *one_top_level;
+
 /* Specified value to be put into tar file in place of stat () results, or
    just null and -1 if such an override should not take place.  */
 GLOBAL char const *owner_name_option;

src/extract.c

@@ -191,6 +191,35 @@ extr_init (void)
       umask (newdir_umask);	/* restore the kernel umask */
       current_umask = newdir_umask;
     }
+
+  /* If the user wants to guarantee that everything is under one directory,
+     determine its name now and let it be created later.  */
+  if (one_top_level_option)
+    {
+      int i;
+      char *base = base_name (archive_name_array[0]);
+
+      for (i = strlen (base) - 1; i > 2; i--)
+        if (!strncmp (base + i - 3, ".tar", 4) ||
+	    !strncmp (base + i - 3, ".taz", 4) ||
+	    !strncmp (base + i - 3, ".tbz", 4) ||
+	    !strncmp (base + i - 3, ".tb2", 4) ||
+	    !strncmp (base + i - 3, ".tgz", 4) ||
+	    !strncmp (base + i - 3, ".tlz", 4) ||
+	    !strncmp (base + i - 3, ".txz", 4)) break;
+
+      if (i <= 3)
+        {
+	  one_top_level_option = false;
+	  free (base);
+	  return;
+	}
+
+      one_top_level = xmalloc (i - 2);
+      strncpy (one_top_level, base, i - 3);
+      one_top_level[i - 3] = '\0';
+      free (base);
+    }
 }
 
 /* Use fchmod if possible, fchmodat otherwise.  */
@@ -1578,6 +1607,33 @@ prepare_to_extract (char const *file_name, int typeflag, tar_extractor_t *fun)
   return 1;
 }
 
+void
+maybe_prepend_name (char **file_name)
+{
+  int i;
+
+  for (i = 0; i < strlen (*file_name); i++)
+    if (!ISSLASH ((*file_name)[i]) && (*file_name)[i] != '.') break;
+
+  if (i == strlen (*file_name))
+    return;
+
+  if (!strncmp (*file_name + i, one_top_level, strlen (one_top_level)))
+    {
+      int pos = i + strlen (one_top_level);
+      if (ISSLASH ((*file_name)[pos]) || (*file_name)[pos] == '\0') return;
+    }
+
+  char *new_name = xmalloc (strlen (one_top_level) + strlen (*file_name) + 2);
+
+  strcpy (new_name, one_top_level);
+  strcat (new_name, "/");
+  strcat (new_name, *file_name);
+
+  free (*file_name);
+  *file_name = new_name;
+}
+
 /* Extract a file from the archive.  */
 void
 extract_archive (void)
@@ -1628,6 +1684,9 @@ extract_archive (void)
   typeflag = sparse_member_p (&current_stat_info) ?
                   GNUTYPE_SPARSE : current_header->header.typeflag;
 
+  if (one_top_level_option)
+    maybe_prepend_name (&current_stat_info.file_name);
+
   if (prepare_to_extract (current_stat_info.file_name, typeflag, &fun))
     {
       if (fun && (*fun) (current_stat_info.file_name, typeflag)

src/tar.c

@@ -319,6 +319,7 @@ enum
   OCCURRENCE_OPTION,
   OLD_ARCHIVE_OPTION,
   ONE_FILE_SYSTEM_OPTION,
+  ONE_TOP_LEVEL_OPTION,
   OVERWRITE_DIR_OPTION,
   OVERWRITE_OPTION,
   OWNER_OPTION,
@@ -489,6 +490,9 @@ static struct argp_option options[] = {
   {"keep-directory-symlink", KEEP_DIRECTORY_SYMLINK_OPTION, 0, 0,
    N_("preserve existing symlinks to directories when extracting"),
    GRID+1 },
+  {"one-top-level", ONE_TOP_LEVEL_OPTION, 0, 0,
+   N_("create a subdirectory to avoid having loose files extracted"),
+   GRID+1 },
 #undef GRID
 
 #define GRID 40
@@ -1441,6 +1445,10 @@ parse_opt (int key, char *arg, struct argp_state *state)
       one_file_system_option = true;
       break;
 
+    case ONE_TOP_LEVEL_OPTION:
+      one_top_level_option = true;
+      break;
+
     case 'l':
       check_links_option = 1;
       break;
@@ -2393,6 +2401,9 @@ decode_options (int argc, char **argv)
 		      subcommand_string (subcommand_option)));
     }
 
+  if (one_top_level_option && absolute_names_option)
+    USAGE_ERROR ((0, 0, _("--one-top-level cannot be used with --absolute-names")));
+
   if (archive_names == 0)
     {
       /* If no archive file name given, try TAPE from the environment, or