Re: [PATCH v2] tty: n_gsm: restrict tty devices to attach

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	 Andrew Morton <akpm@linux-foundation.org>,
	"Starke, Daniel" <daniel.starke@siemens.com>,
	 LKML <linux-kernel@vger.kernel.org>,
	 linux-security-module <linux-security-module@vger.kernel.org>
Subject: Re: [PATCH v2] tty: n_gsm: restrict tty devices to attach
Date: Sat, 20 Apr 2024 11:02:48 -0700	[thread overview]
Message-ID: <CAHk-=wjzqaqcicTtWfBtXyytJs1nqjJNved2JFsLVsVLYgVkuQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wjEZvnn51dhhLqBKUd=cuFhbYA47_OyfUOPB-0zKToL7Q@mail.gmail.com>

On Sat, 20 Apr 2024 at 10:34, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Alternatively, we could go the opposite way, and have some flag in the
> line discipline that says "I can be a console", and just check that in
> tty_set_ldisc() for the console.

Actually, I take that back. It's not /dev/console that is the problem,
that just happened to be the one oops I looked at.

Most other normal tty devices just expect ->write() to be called in
normal process context, so if we do a line discipline flag, it would
have to be something like "I'm ok with being called with interrupts
disabled", and then the n_gsm ->open function would just check that.

So it would end up being just another form of that

  +     if (tty->ops->set_serial == NULL)
  +             return -EINVAL;

check - but maybe more explicit and prettier.

Because a real serial driver might not be ok with it either, if it
uses a semaphore or something.

Whatever. I think the 'set_serial' test would at least be an improvement.

            Linus