Hello,
We don't have such device, but we saw in the commit log that "use it to limit the virtual terminals to just N_TTY. They are kind of special" and thought that it may help prevent other unexpected ldisc. So, we submitted this patch for review. If it's improper we will stop here for both 5.15 and 5.10.
-----Original Message----- From: Greg KH gregkh@linuxfoundation.org Sent: Monday, May 12, 2025 22:10 To: Ren, Jianqi (Jacky) (CN) Jianqi.Ren.CN@windriver.com Cc: stable@vger.kernel.org; patches@lists.linux.dev; linux-kernel@vger.kernel.org; penguin-kernel@i-love.sakura.ne.jp; akpm@linux-foundation.org; daniel.starke@siemens.com; torvalds@linux-foundation.org Subject: Re: [PATCH 5.15.y] tty: add the option to have a tty reject a new ldisc
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On Fri, May 09, 2025 at 05:14:54PM +0800, jianqi.ren.cn@windriver.com wrote:
From: Linus Torvalds torvalds@linux-foundation.org
[ Upstream commit 6bd23e0c2bb6c65d4f5754d1456bc9a4427fc59b ]
... 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.
WHy do you want this in 5.15 and older kernels? You have already disabledf n_gsm from your kernels already so this isn't an issue, right? Unless you have this hardware, and explicitly know what is talking to it, that is the recommendation for this code.
And how was this and the 5.10.y backport tested? Did you see the above "BUG:" line without it?
thanks,
greg k-h