linux-kernel - Re: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst atomic

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Date:   Thu, 5 Oct 2023 13:17:56 +0100
From:   Lee Jones <lee@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Starke, Daniel" <daniel.starke@...mens.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Fedor Pchelkin <pchelkin@...ras.ru>,
        Jiri Slaby <jirislaby@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "syzbot+5f47a8cea6a12b77a876@...kaller.appspotmail.com" 
        <syzbot+5f47a8cea6a12b77a876@...kaller.appspotmail.com>
Subject: Re: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst
 atomic

On Thu, 05 Oct 2023, Greg Kroah-Hartman wrote:

> On Thu, Oct 05, 2023 at 12:46:32PM +0100, Lee Jones wrote:
> > On Thu, 05 Oct 2023, Starke, Daniel wrote:
> > 
> > > > > Would something like this tick that box?
> > > > > 
> > > > > diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c index 
> > > > > 1f3aba607cd51..5c1d2fcd5d9e2 100644
> > > > > --- a/drivers/tty/n_gsm.c
> > > > > +++ b/drivers/tty/n_gsm.c
> > > > > @@ -3716,6 +3716,10 @@ static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
> > > > >         if (!gsm)
> > > > >                 return -ENODEV;
> > > > >  
> > > > > +       /* The GSM line discipline does not support binding to console */
> > > > > +       if (strncmp(tty->name, "tty", 3))
> > > > > +               return -EINVAL;
> > > > 
> > > > No, that's not going to work, some consoles do not start with "tty" :(
> > 
> > Ah, you mean there are others that we need to consider?
> > 
> > I was just covering off con_write() from drivers/tty/vt/vt.c.
> > 
> > Does anyone have a counter proposal?
> 
> consoles are dynamically assigned, the device knows if it is a console
> or not, so there is a way to determine this at runtime.  It's not a
> device naming thing at all.

It's not a device naming thing, but it is a ... :)

Okay, here's another uninformed stab in the dark:

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 1f3aba607cd51..ddf00f6a4141d 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -3629,6 +3629,10 @@ static int gsmld_open(struct tty_struct *tty)
        if (tty->ops->write == NULL)
                return -EINVAL;

+       /* The GSM line discipline does not support binding to console */
+       if (tty->driver->type == TTY_DRIVER_TYPE_CONSOLE)
+               return -EINVAL;
+
        /* Attach our ldisc data */
        gsm = gsm_alloc_mux();
        if (gsm == NULL)

This seems to allow for the real serial devices (TTY_DRIVER_TYPE_SERIAL)
suggested by Daniel.

-- 
Lee Jones [李琼斯]