Add 'nofollow' flag to set-file-modes etc.

This avoids some race conditions (Bug#39683).  E.g., if some other
program changes a file to a symlink between the time Emacs creates
the file and the time it changes the file’s permissions, using the
new flag prevents Emacs from inadvertently changing the
permissions of a victim in some completely unrelated directory.
* admin/merge-gnulib (GNULIB_MODULES): Add fchmodat.
* doc/lispref/files.texi (Testing Accessibility, Changing Files):
* doc/lispref/os.texi (File Notifications):
* etc/NEWS:
Adjust documentation accordingly.
* lib/chmodat.c, lib/fchmodat.c, lib/lchmod.c, m4/fchmodat.m4:
* m4/lchmod.m4: New files, copied from Gnulib.
* lib/gnulib.mk.in: Regenerate.
* lisp/dired-aux.el (dired-do-chmod):
* lisp/doc-view.el (doc-view-make-safe-dir):
* lisp/emacs-lisp/autoload.el (autoload--save-buffer):
* lisp/emacs-lisp/bytecomp.el (byte-compile-file):
* lisp/eshell/em-pred.el (eshell-pred-file-mode):
* lisp/files.el (backup-buffer-copy, copy-directory):
* lisp/gnus/mail-source.el (mail-source-movemail):
* lisp/gnus/mm-decode.el (mm-display-external):
* lisp/gnus/nnmail.el (nnmail-write-region):
* lisp/net/tramp-adb.el (tramp-adb-handle-file-local-copy)
(tramp-adb-handle-write-region):
* lisp/net/tramp-sh.el (tramp-do-copy-or-rename-file-directly):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-handle-write-region):
* lisp/net/tramp.el (tramp-handle-write-region)
(tramp-make-tramp-temp-file):
* lisp/server.el (server-ensure-safe-dir):
* lisp/url/url-util.el (url-make-private-file):
When getting or setting file modes, avoid following symbolic links
when the file is not supposed to be a symbolic link.
* lisp/doc-view.el (doc-view-make-safe-dir):
Omit no-longer-needed separate symlink test.
* lisp/gnus/gnus-util.el (gnus-set-file-modes):
* lisp/net/tramp.el (tramp-handle-file-modes):
* lisp/net/tramp-gvfs.el (tramp-gvfs-handle-set-file-modes):
* src/fileio.c (symlink_nofollow_flag): New function.
(Ffile_modes, Fset_file_modes):
Support an optional FLAG arg.  All C callers changed.
* lisp/net/ange-ftp.el (ange-ftp-set-file-modes):
* lisp/net/tramp-adb.el (tramp-adb-handle-set-file-modes):
* lisp/net/tramp-sh.el (tramp-sh-handle-set-file-modes):
* lisp/net/tramp-smb.el (tramp-smb-handle-set-file-modes):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-handle-set-file-modes):
Accept an optional FLAG arg that is currently ignored,
and add a FIXME comment for it.
* m4/gnulib-comp.m4: Regenerate.

1 files changed, 1 insertions, 3 deletions

diff --git a/lisp/doc-view.el b/lisp/doc-view.el
index 3788d797258..8b3d5527f08 100644
--- a/lisp/doc-view.el
+++ b/lisp/doc-view.el
@@ -683,8 +683,6 @@ at the top edge of the page moves to the previous page."
       ;; time-window of loose permissions otherwise.
       (with-file-modes #o0700 (make-directory dir))
     (file-already-exists
-     (when (file-symlink-p dir)
-       (error "Danger: %s points to a symbolic link" dir))
      ;; In case it was created earlier with looser rights.
      ;; We could check the mode info returned by file-attributes, but it's
      ;; a pain to parse and it may not tell you what we want under
@@ -694,7 +692,7 @@ at the top edge of the page moves to the previous page."
      ;; sure we have write-access to the directory and that we own it, thus
      ;; closing a bunch of security holes.
      (condition-case error
-	 (set-file-modes dir #o0700)
+	 (set-file-modes dir #o0700 'nofollow)
        (file-error
 	(error
 	 (format "Unable to use temporary directory %s: %s"