Re: [PATCH] Add shell-quasiquote.

[Taylan Ulrich Bayırlı/Kammer]

Dmitry Gutov <address@hidden> writes:

> On 10/18/2015 12:25 AM, Taylan Ulrich Bayırlı/Kammer wrote:
>
>> Not knowing that there are bugs is not proof that there are no bugs.
>
> If you can't point out a bug, you have no justification to not use the
> standard function.

No, I will *not* let users of my code potentially suffer from arbitrary
code injection attacks, thank you very much.

>>> Either way, please avoid reinventing the wheel.
>>
>> It's not a reinvention because it has very strict semantics with regard
>> to safety guarantees, which shell-quote-argument apparently doesn't.
>
> shell-quote-argument doesn't guarantee quoting the argument?

Apparently, it doesn't.  See Random832's demonstration of an injection
attack on shell-quote-argument when it's used with csh.


My function has a clearly defined domain, and operates correctly within
that domain.  Shell-quote-argument apparently does not even have a
clearly defined domain in which it's supposed to work.

If anyone has a shell argument quoting function that expands the domain
of supported shells in a well-defined manner, without weakening the
safety guarantees, please let me know.  Until then, please accept this
modified patch which clarifies that the library is not supposed to work
with any shells other than ones conforming to POSIX sh.

Taylan

>From f8bee6aeed124f0e14fbc55509597cddba439cbd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Taylan=20Ulrich=20Bay=C4=B1rl=C4=B1/Kammer?=
 <address@hidden>
Date: Sat, 17 Oct 2015 18:32:22 +0200
Subject: [PATCH] Add shell-quasiquote.

---
 packages/shell-quasiquote/shell-quasiquote.el | 152 ++++++++++++++++++++++++++
 1 file changed, 152 insertions(+)
 create mode 100644 packages/shell-quasiquote/shell-quasiquote.el

diff --git a/packages/shell-quasiquote/shell-quasiquote.el 
b/packages/shell-quasiquote/shell-quasiquote.el
new file mode 100644
index 0000000..7f20afd
--- /dev/null
+++ b/packages/shell-quasiquote/shell-quasiquote.el
@@ -0,0 +1,152 @@
+;;; shell-quasiquote.el --- Turn s-expressions into shell command strings.
+
+;; Copyright (C) 2015  Free Software Foundation, Inc.
+
+;; Author: Taylan Ulrich Bayırlı/Kammer <address@hidden>
+;; Keywords: extensions, unix
+;; URL: https://github.com/TaylanUB/emacs-shell-quasiquote
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; "Shell quasiquote" -- turn s-expressions into POSIX shell command strings.
+;;
+;; Shells other than POSIX sh are not supported and WILL make your code subject
+;; to arbitrary code injection attacks.  Otherwise...
+;;
+;; Quoting is automatic and safe against injection.
+;;
+;;   (let ((file1 "file one")
+;;         (file2 "file two"))
+;;     (shqq (cp -r ,file1 ,file2 "My Files")))
+;;       => "cp -r 'file one' 'file two' 'My Files'"
+;;
+;; You can splice many arguments into place with ,@foo.
+;;
+;;   (let ((files (list "file one" "file two")))
+;;     (shqq (cp -r ,@files "My Files")))
+;;       => "cp -r 'file one' 'file two' 'My Files'"
+;;
+;; Note that the quoting disables a variety of shell expansions like ~/foo,
+;; $ENV_VAR, and e.g. {x..y} in GNU Bash.
+;;
+;; You can use ,,foo to escape the quoting.
+;;
+;;   (let ((files "file1 file2"))
+;;     (shqq (cp -r ,,files "My Files")))
+;;       => "cp -r file1 file2 'My Files'"
+;;
+;; And ,,@foo to splice and escape quoting.
+;;
+;;   (let* ((arglist '("-x 'foo bar' -y baz"))
+;;          (arglist (append arglist '("-z 'qux fux'"))))
+;;     (shqq (command ,,@arglist)))
+;;       => "command -x 'foo bar' -y baz -z 'qux fux'"
+;;
+;; Neat, eh?
+
+
+;;; Code:
+
+;;; Like `shell-quote-argument', but much simpler in implementation.
+(defun shqq--quote-string (string)
+  (concat "'" (replace-regexp-in-string "'" "'\\\\''" string) "'"))
+
+(defun shqq--atom-to-string (atom)
+  (cond
+   ((symbolp atom) (symbol-name atom))
+   ((stringp atom) atom)
+   ((numberp atom) (number-to-string atom))
+   (t (error "Bad shqq atom: %S" atom))))
+
+(defun shqq--quote-atom (atom)
+  (shqq--quote-string (shqq--atom-to-string atom)))
+
+(defun shqq--match-comma (form)
+  "Matches FORM against ,foo i.e. (\, foo) and returns foo.
+Returns nil if FORM didn't match.  You can't disambiguate between
+FORM matching ,nil and not matching."
+  (if (and (consp form)
+           (eq '\, (car form))
+           (consp (cdr form))
+           (null (cddr form)))
+      (cadr form)))
+
+(defun shqq--match-comma2 (form)
+  "Matches FORM against ,,foo i.e. (\, (\, foo)) and returns foo.
+Returns nil if FORM didn't match.  You can't disambiguate between
+FORM matching ,,nil and not matching."
+  (if (and (consp form)
+           (eq '\, (car form))
+           (consp (cdr form))
+           (null (cddr form)))
+      (shqq--match-comma (cadr form))))
+
+
+(defmacro shqq (parts)
+  "First, PARTS is turned into a list of strings.  For this,
+every element of PARTS must be one of:
+
+- a symbol, evaluating to its name,
+
+- a string, evaluating to itself,
+
+- a number, evaluating to its decimal representation,
+
+- \",expr\", where EXPR must evaluate to an atom that will be
+  interpreted according to the previous rules,
+
+- \",@list-expr\", where LIST-EXPR must evaluate to a list whose
+  elements will each be interpreted like the EXPR in an \",EXPR\"
+  form, and spliced into the list of strings,
+
+- \",,expr\", where EXPR is interpreted like in \",expr\",
+
+- or \",,@expr\", where EXPR is interpreted like in \",@expr\".
+
+In the resulting list of strings, all elements except the ones
+resulting from \",,expr\" and \",,@expr\" forms are quoted for
+shell grammar.
+
+Finally, the resulting list of strings is concatenated with
+separating spaces."
+  (let ((parts
+         (mapcar
+          (lambda (part)
+            (cond
+             ((atom part) (shqq--quote-atom part))
+             ;; We use the match-comma helpers because pcase can't match ,foo.
+             (t (pcase part
+                  ;; ,,foo i.e. (, (, foo))
+                  ((pred shqq--match-comma2)
+                   (shqq--match-comma2 part))
+                  ;; ,,@foo i.e. (, (,@ foo))
+                  ((and (pred shqq--match-comma)
+                        (let `,@,form (shqq--match-comma part)))
+                   `(mapconcat #'identity ,form " "))
+                  ;; ,foo
+                  ;; Insert redundant 'and x' to work around debbugs#18554.
+                  ((and x (pred shqq--match-comma))
+                   `(shqq--quote-atom ,(shqq--match-comma part)))
+                  ;; ,@foo
+                  (`,@,form
+                   `(mapconcat #'shqq--quote-atom ,form " "))
+                  (_
+                   (error "Bad shqq part: %S" part))))))
+          parts)))
+    `(mapconcat #'identity (list ,@parts) " ")))
+
+(provide 'shell-quasiquote)
+;;; shell-quasiquote.el ends here
-- 
2.5.0