(backup-buffer): Bind local var MODES.

Don't use renaming for a suid or sgid file.
Use backup-buffer-copy to do copying.
(backup-buffer-copy): New subroutine.
Clear suid and sgid bits for the copy.

1 files changed, 19 insertions, 21 deletions

diff --git a/lisp/files.el b/lisp/files.el
index 4a495aab30c..6a893d958db 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -2365,12 +2365,15 @@ BACKUPNAME is the backup file name, which is the old file renamed."
                             (or (eq delete-old-versions t) (eq delete-old-versions nil))
                             (or delete-old-versions
                                 (y-or-n-p (format "Delete excess backup versions of %s? "
-                                                  real-file-name))))))
+                                                  real-file-name)))))
+                      (modes (file-modes buffer-file-name)))
                   ;; Actually write the back up file.
                   (condition-case ()
                       (if (or file-precious-flag
     ;                         (file-symlink-p buffer-file-name)
                               backup-by-copying
+                              ;; Don't rename a suid or sgid file.
+                              (< 0 (logand modes #o6000))
                               (and backup-by-copying-when-linked
                                    (> (file-nlinks real-file-name) 1))
                               (and (or backup-by-copying-when-mismatch
@@ -2382,19 +2385,10 @@ BACKUPNAME is the backup file name, which is the old file renamed."
                                                    (<= (nth 2 attr) backup-by-copying-when-privileged-mismatch)))
                                           (or (nth 9 attr)
                                               (not (file-ownership-preserved-p real-file-name)))))))
-                          (condition-case ()
-                              (copy-file real-file-name backupname t t)
-                            (file-error
-                             ;; If copying fails because file BACKUPNAME
-                             ;; is not writable, delete that file and try again.
-                             (if (and (file-exists-p backupname)
-                                      (not (file-writable-p backupname)))
-                                 (delete-file backupname))
-                             (copy-file real-file-name backupname t t)))
+                          (backup-buffer-copy real-file-name backupname modes)
                         ;; rename-file should delete old backup.
                         (rename-file real-file-name backupname t)
-                        (setq setmodes
-                              (cons (file-modes backupname) backupname)))
+                        (setq setmodes (cons modes backupname)))
                     (file-error
                      ;; If trouble writing the backup, write it in ~.
                      (setq backupname (expand-file-name
@@ -2403,15 +2397,7 @@ BACKUPNAME is the backup file name, which is the old file renamed."
                      (message "Cannot write backup file; backing up in %s"
                               (file-name-nondirectory backupname))
                      (sleep-for 1)
-                     (condition-case ()
-                         (copy-file real-file-name backupname t t)
-                       (file-error
-                        ;; If copying fails because file BACKUPNAME
-                        ;; is not writable, delete that file and try again.
-                        (if (and (file-exists-p backupname)
-                                 (not (file-writable-p backupname)))
-                            (delete-file backupname))
-                        (copy-file real-file-name backupname t t)))))
+                     (backup-buffer-copy real-file-name backupname modes)))
                   (setq buffer-backed-up t)
                   ;; Now delete the old versions, if desired.
                   (if delete-old-versions
@@ -2423,6 +2409,18 @@ BACKUPNAME is the backup file name, which is the old file renamed."
                   setmodes)
             (file-error nil))))))
 
+(defun backup-buffer-copy (from-name to-name modes)
+  (condition-case ()
+      (copy-file from-name to-name t t)
+    (file-error
+     ;; If copying fails because file TO-NAME
+     ;; is not writable, delete that file and try again.
+     (if (and (file-exists-p to-name)
+              (not (file-writable-p to-name)))
+         (delete-file to-name))
+     (copy-file from-name to-name t t)))
+  (set-file-modes to-name (logand modes #o1777)))
+
 (defun file-name-sans-versions (name &optional keep-backup-version)
   "Return file NAME sans backup versions or strings.
 This is a separate procedure so your site-init or startup file can