Messages in this thread | | | Date | Wed, 15 Feb 2023 17:38:23 +0100 (CET) | From | Michael Thalmeier <> | Subject | Re: [PATCH v2] tty: ttynull: implement console write |
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Hi Petr,
----- On 15 Feb, 2023, at 15:33, Petr Mladek pmladek@suse.com wrote: > 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.
You are right. I have encountered this problem with the RT-patchset. We currently are using the latest v5.15-rt kernel which had this problem. > 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.
Unfortunately I have not yet tested with the current RT or non-RT versions.
> > Best Regards, > Petr
Regards, Michael
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