bug#28264: Accessing source directory through symlink produces false warnings

[Paul Eggert <eggert@cs.ucla.edu>]

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Michael Albinus wrote:

> This special problem does not seem to affect Tramp, tramp-tests tell.

The problem I was thinking of does not seem to be covered by Tramp tests. If, 
for example, I do these shell commands:

$ ln -s "../penguin:motd" /tmp/foo
$ ls -l /tmp/foo
lrwxrwxrwx 1 eggert eggert 15 Aug 29 19:00 /tmp/foo -> ../penguin:motd

then (file-truename "/tmp/foo") returns "/penguin:motd" which is not /tmp/foo's 
true name as far as Emacs file-oriented commands are concerned. Admittedly this 
is an improvement over Emacs 25.2 where the same file-truename call ssh'es into 
penguin to resolve the name, which is a clear security issue. Still, it doesn't 
seem right, if file-truename is expected to quote its result if necessary.

Sorry about all this confusion, but I do not know the general principle that 
Emacs is supposed to be using with file names, and to some extent I fear that 
there isn't one alas.

> However, the following code is unclear to me:
> 
>> +	(concat "/:" file))
> 
> What, if file is already quoted? Shouldn't this be
> 
>          (file-name-quote file)

Quite possibly, and I'll take your word for it. I installed the attached.

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From 0eb9f21ded1b6aefcf0ac33f393b07ab6a543a05 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Tue, 29 Aug 2017 19:17:54 -0700
Subject: [PATCH] Prefer file-name-quote to concat "/:"

Suggested by Michael Albinus (Bug#28264#13).
* lisp/files.el (files--splice-dirname-file): Use file-name-quote
rather than attempting to do it by hand.
---
 lisp/files.el | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lisp/files.el b/lisp/files.el
index 5f55aa7..7754be2 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -1156,16 +1156,16 @@ files--name-absolute-system-p
 (defun files--splice-dirname-file (dirname file)
   "Splice DIRNAME to FILE like the operating system would.
 If FILENAME is relative, return DIRNAME concatenated to FILE.
-Otherwise return FILE, quoted with `/:' if DIRNAME and FILE have
+Otherwise return FILE, quoted as needed if DIRNAME and FILE have
 different handlers; although this quoting is dubious if DIRNAME
-is remote, it is not clear what would be better.  This function
+is magic, it is not clear what would be better.  This function
 differs from `expand-file-name' in that DIRNAME must be a
 directory name and leading `~' and `/:' are not special in FILE."
   (if (files--name-absolute-system-p file)
       (if (eq (find-file-name-handler dirname 'file-symlink-p)
 	      (find-file-name-handler file 'file-symlink-p))
 	  file
-	(concat "/:" file))
+	(file-name-quote file))
     (concat dirname file)))
 
 (defun file-truename (filename &optional counter prev-dirs)
-- 
2.7.4