linux-kernel - Re: [syzbot] [kernel?] divide error in comedi_inc_scan

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Message-ID: <07e71db1-d6ea-4cb6-8342-86da3c11773e@mev.co.uk>
Date: Mon, 20 Oct 2025 18:29:14 +0100
From: Ian Abbott <abbotti@....co.uk>
To: Жандарович Никита Игоревич <n.zhandarovich@...tech.ru>,
 "syzbot+af53dea94b16396bc646@...kaller.appspotmail.com"
 <syzbot+af53dea94b16396bc646@...kaller.appspotmail.com>,
 H Hartley Sweeten <hsweeten@...ionengravers.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>
Subject: Re: [syzbot] [kernel?] divide error in comedi_inc_scan_progress

On 20/10/2025 14:36, Жандарович Никита Игоревич wrote:
> Hi Ian,
> 
> Thank you for your response!
> 
>>>       ...
>>>       if (!(s->subdev_flags & SDF_PACKED) && (cmd->chanlist_len != 0)) {
>>> 	async->cur_chan += comedi_bytes_to_samples(s, num_bytes);
>>> 	async->cur_chan %= cmd->chanlist_len;
>>>       }
>>>       ...
>>>
>>> Any suggestions are greatly appreciated!
>>
>> I have a plan to deal with these unexpected interrupts, at least within the
>> Comedi core functions.  The basic idea would be to for the Comedi core
>> functions called by the ISRs to check that the subdevice is in the running state
>> and increment a reference counter (either a refcount_t or a struct kref) if it is
>> safe to proceed.  Then it will be allowed to assume that the struct
>> comedi_async contents are reasonable until it decrements the reference
>> counter.  Some other task may be calling
>> do_become_nonbusy() in parallel with the above.  After
>> do_become_nonbusy() clears the COMEDI_SRF_RUNNING flag to mark the
>> subdevice as not being in the running state, it should wait until it is safe to
>> continue before calling comedi_buf_reset().  This will make use of the
>> reference counter and a struct completion.  The reference counter and struct
>> completion can be stored as members of struct comedi_async.
>>
>> There may be other parts of individual driver ISRs that use struct
>> comedi_async directly and may need changing to do similar checks.  The
>> checking and final decrement can done by a couple of new exported
>> functions:
>>
>> bool comedi_get_is_subdevice_running(struct comedi_subdevice *s); void
>> comedi_put_is_subdevice_running(struct comedi_subdevice *s);
>>
>> If comedi_get_is_subdevice_running(s) returns true, the ISR can safely
>> access s->async and then should call comedi_put_is_subdevice_running(s)
>> when it has finished.
>>
>>
> 
> I'll try to tackle this idea when I get the chance, thanks for your suggestion!

I'm trying to implement it at the moment.

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