linux-kernel - Re: [PATCH v2] tty: ttynull: implement console write

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Date:   Wed, 15 Feb 2023 12:37:36 +0100
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Michael Thalmeier <michael.thalmeier@...e.at>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        <john.ogness@...utronix.de>, <pmladek@...e.com>
Subject: Re: [PATCH v2] tty: ttynull: implement console write

+ Cc: John, Petr

On Tue, Feb 14, 2023 at 12:59:21PM +0100, Michael Thalmeier wrote:
> Since commit a699449bb13b ("printk: refactor and rework printing logic"),
> con->write is called without checking if it is implemented in the console
> driver. This does make some sense, because for all "normal" consoles it
> is vital to have a write function.
> As the ttynull console driver does not need any console output the write
> function was not implemented. This caused a "unable to handle kernel NULL
> pointer dereference" when the write function is called now.
> 
> To fix this issue, implement an empty write function.
> 
> Fixes: a699449bb13b ("printk: refactor and rework printing logic")
> Cc: stable@...r.kernel.org
> Signed-off-by: Michael Thalmeier <michael.thalmeier@...e.at>

Looking at the referenced commit, the only place I see it calling
con->write() is from call_console_driver(), which is in turn only called
from console_emit_next_record().  And console_flush_all(), the only
caller of console_emit_next_record(), checks that con->write is not NULL
using console_is_usable() before calling console_emit_next_record().

What am I missing?  Which code path in the referenced commit calls
con->write without checking for NULL?