bug#45245: 28.0.50; Feature request: tramp sudo autosaves/backups shouldn't be exposed without right config

[Vandrus Zoltán]

It's mentioned in (tramp)Auto-save and Backup that root owned file could 
be exposed, but it would be more newbie friendly if emacs did the right 
thing without configuration. The defaults for backups are fine, but for 
autosaves are not. In emacs -Q after:

C-x C-f /sudo::/tmp/secretfile
M-x do-auto-save

There is a file '/tmp/#!sudo:root@hostname:!tmp!secretfile#' owned by 
the user.

Even if the defaults are fixed, there are problems. Protecting root 
owned files is somewhat complicated. For example the user might not use 
tramp from the beginning, but littering directories with backups and 
autosaves files are easily seen and can be annoying enough to look for a 
solution. Looking on the net the suggested code is some variant of

    (setq auto-save-file-name-transforms
          '((".*" ,auto-save-dir t)))

    (setq backup-directory-alist
         '("." ,backup-dir))

And then they are fine, until they start to use tramp, because the 
autosaves/backups will be owned by the normal user even for sudo and su 
methods.
For backups following the tramp manual is easy:

     (customize-set-variable
      'tramp-backup-directory-alist backup-directory-alist)

But the user could have forgotten already about the problem and never 
look there. For autosaves there is not even info on how to achieve 
something sensible.

I suggest, that tramp could refuse exposing root-owned files or there 
could be an easier switch to put all autosaves/backup in the same 
directory which also deals with tramp.

There is also a comparably minor problem of exposing the file name in 
the autosave files.

-------------

In GNU Emacs 28.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 
3.24.24, cairo version 1.17.4)
of 2020-12-14
Repository revision: b857ea24f7bc5288faa920e6c3174cf1ee958b70
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Arch Linux

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