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List:       linux-kernel
Subject:    Re: [PATCH] tty: add the option to have a tty reject a new ldisc
From:       Jiri Slaby <jirislaby () kernel ! org>
Date:       2024-04-24 6:25:12
Message-ID: b914dbad-858c-42eb-80be-337e69f189e4 () kernel ! org
[Download RAW message or body]

On 23. 04. 24, 18:33, Linus Torvalds wrote:
> ... and use it to limit the virtual terminals to just N_TTY.  They are
> kind of special, and in particular, the "con_write()" routine violates
> the "writes cannot sleep" rule that some ldiscs rely on.
> 
> This avoids the
> 
>     BUG: sleeping function called from invalid context at kernel/printk/printk.c:2659
> 
> when N_GSM has been attached to a virtual console, and gsmld_write()
> calls con_write() while holding a spinlock, and con_write() then tries
> to get the console lock.
..> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -545,6 +545,12 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
>   		goto out;
>   	}
>   
> +	if (tty->ops->ldisc_ok) {




> +		retval = tty->ops->ldisc_ok(tty, disc);
> +		if (retval)
> +			goto out;

This caught my eye as it looks like:
if (ldisc_ok)
   die;

Perhaps dub it ldisc_check()? Alternatively, make it return a bool and 
invert the logic here? That would make the ugly ternary in 
con_ldisc_ok() go away too.

But whatever, it finally evades that group of bugs.

Now, I am not sure what the problem with attaching some weird ldisc to a 
pty was. I will take a look next week (I am unavailable ATM).

thanks,
-- 
js
suse labs


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