bug#64281: [Feature Request] Recognize “#” syntax when reading local-variables

bug#64281: [Feature Request] Recognize “#” syntax when reading local-variables

[Shynur Xie]

For example, if I want to use UTF-8-UNIX only for files directly under
directory DIR, and those under DIR/SUBDIR/ or its subdirectories, I
can write this way:

    ((nil . ((subdirs . nil)
               . #1=((coding . utf-8-unix))))
     ("SUBDIR" . ((nil . #1#))))

If “#” can be recognized when reading local-variables, we can write
“((coding . utf-8-unix))” only once.  Thanks.

--
shynur

bug#64281: [Feature Request] Recognize “#” syntax

[Shynur Xie]

It seems that this is a intentional behavior:

<https://www.gnu.org/software/emacs/manual/html_node/elisp/File-Local-Variables.html>:

> For additional safety, read-circle is temporarily bound to nil when
> Emacs reads file-local variables.

No further question; can be closed now.

(I wonder if we can treat ~/.emacs.d/.dir-locals.el specially, or tell
 Emacs which files can be treated specially -- enable read-circle.)

bug#64281: [Feature Request] Recognize “#” syntax

[Eli Zaretskii]

> From: Shynur Xie <one.last.kiss@outlook.com>
> Date: Sun, 25 Jun 2023 13:53:30 +0000
> msip_labels: 
> 
> It seems that this is a intentional behavior:
> 
> <https://www.gnu.org/software/emacs/manual/html_node/elisp/File-Local-Variables.html>:
> 
> > For additional safety, read-circle is temporarily bound to nil when
> > Emacs reads file-local variables.
> 
> No further question; can be closed now.
> 
> (I wonder if we can treat ~/.emacs.d/.dir-locals.el specially, or tell
>  Emacs which files can be treated specially -- enable read-circle.)

Stefan, any comments before we close this?

bug#64281: [Feature Request] Recognize “#” syntax

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

>> It seems that this is a intentional behavior:
>>
>> <https://www.gnu.org/software/emacs/manual/html_node/elisp/File-Local-Variables.html>:
>>
>> > For additional safety, read-circle is temporarily bound to nil when
>> > Emacs reads file-local variables.
>>
>> No further question; can be closed now.
>>
>> (I wonder if we can treat ~/.emacs.d/.dir-locals.el specially, or tell
>>  Emacs which files can be treated specially -- enable read-circle.)
>
> Stefan, any comments before we close this?

file/dir-local variables are dangerous enough as it is, I think.  If you
want to do better, you can use an actual ELisp file.


        Stefan

bug#64281: [Feature Request] Recognize “#” syntax

[Drew Adams]

> >> It seems that this is a intentional behavior:
> >> .../File-Local-Variables.html:
> >>
> >> > For additional safety, read-circle is temporarily bound
> >> > to nil when Emacs reads file-local variables.
> >>
> >> No further question; can be closed now.
> >>
> >> (I wonder if we can treat ~/.emacs.d/.dir-locals.el specially, or tell
> >>  Emacs which files can be treated specially -- enable read-circle.)
> >
> > Stefan, any comments before we close this?
> 
> file/dir-local variables are dangerous enough as it is, I think.  If you
> want to do better, you can use an actual ELisp file.

1. That Elisp manual node (`File Local Variables')
doesn't say what danger is avoided by disabling
`read-circle', but it seems to suggest that that
danger is pointed out in node `Read Syntax for
Circular Objects':

  For additional safety, read-circle is temporarily
  bound to nil when Emacs reads file-local variables
  (see Input Functions). This prevents the Lisp reader
  from recognizing circular and shared Lisp structures
  (see Read Syntax for Circular Objects).

Yes, that's what it does, but why?  What's the
"additional safety" about?  What danger's involved?

Node `Read Syntax for Circular Objects' says nothing
about any possible danger (security/safety problem)
circular objects can introduce.  Can we please say
something about the danger in that node?

2. Wouldn't it make sense (be useful) to let users
and code decide whether to allow this for particular
file-local variables?

A question from ignorance - see #1, above.  Is it
helpful to talk about danger and avoiding it,
without giving readers some idea of what's involved?

bug#64281: [Feature Request] Recognize “#” syntax

[Shynur Xie]

To: Drew

> drew> Node `Read Syntax for Circular Objects' says nothing about any
> drew> possible danger (security/safety problem) circular objects can
> drew> introduce.

I guess that “.dir-locals.el” _isn’t_ read by ELisp syntax, so if you
visit a “.dir-locals.el” file, Emacs will choose “Lisp-Data mode” for
it.  And (info "(elisp)Circular Objects") _is_ about Elisp read
syntax.

> stefan> To: Eli
> stefan> If you want to do better, you can use an actual ELisp file.
                                                ^^^^^^^^^^^^^^^^^^^^

Seems like it is indeed not now, though uses a “.el” postfix.

____________________

To: ALL

> drew> Wouldn't it make sense (be useful) to let users and code
> drew> decide whether to allow this for particular file-local
> drew> variables?

Agree.

There have already been numerous potential dangerous behaviors,
perhaps one more won’t make much of a difference.  I think as long as
users are provided with appropriate warnings, it should suffice.   For
instance, the following code snippet can cause Emacs to crash
instantly, but we still shouldn’t deprive users of their right to
execute it:

    (clear-string (symbol-name t))

Luckily, manual contains a corresponding warning.

--
shynur

bug#64281: [Feature Request] Recognize “#” syntax

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Shynur Xie <one.last.kiss@outlook.com>,  64281@debbugs.gnu.org
> Date: Sun, 25 Jun 2023 11:46:13 -0400
> 
> >> It seems that this is a intentional behavior:
> >>
> >> <https://www.gnu.org/software/emacs/manual/html_node/elisp/File-Local-Variables.html>:
> >>
> >> > For additional safety, read-circle is temporarily bound to nil when
> >> > Emacs reads file-local variables.
> >>
> >> No further question; can be closed now.
> >>
> >> (I wonder if we can treat ~/.emacs.d/.dir-locals.el specially, or tell
> >>  Emacs which files can be treated specially -- enable read-circle.)
> >
> > Stefan, any comments before we close this?
> 
> file/dir-local variables are dangerous enough as it is, I think.  If you
> want to do better, you can use an actual ELisp file.

Agreed.