Messages in this thread | | | Date | Wed, 15 Feb 2023 15:33:41 +0100 | From | Petr Mladek <> | Subject | Re: [PATCH v2] tty: ttynull: implement console write |
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On Wed 2023-02-15 12:37:36, Vincent Whitchurch wrote: > + 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@vger.kernel.org > > Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.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().
I see the same. The refactoring moved the check of con->write from call_console_driver() to console_is_usable(). It detects the NULL pointer earlier in console_flush_all()...
> What am I missing? Which code path in the referenced commit calls > con->write without checking for NULL?
Vincent, could you please provide log with the backtrace?
I wonder if the problem is in the RT-patchset where console_emit_next_record() is called also from the printk kthread.
That said, the current code is error-prone. The check for non-NULL con->write is too far from the caller.
I would prefer to make it more safe. For example, I would prevent registration of consoles without con->write callback in the first place. It would require, to implement the empty write() callback for ttynull console as done by this patch.
Anyway, I would like to understand if the "unable to handle kernel NULL pointer dereference" is a real problem in the current implementation.
Best Regards, Petr
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