linux-kernel - Re: [PATCH] staging: greybus: Convert timers to use timer

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Message-ID: <CAGXu5jLb_ETq_0OJuS8pHRt2rZ75NRmZXXk6bS8kCh=rk5pCbQ@mail.gmail.com>
Date:   Tue, 24 Oct 2017 07:36:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Bryan O'Donoghue" <pure.logic@...us-software.ie>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>, Alex Elder <elder@...nel.org>,
        greybus-dev@...ts.linaro.org, devel@...verdev.osuosl.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: greybus: Convert timers to use timer_setup()

On Tue, Oct 24, 2017 at 6:30 AM, Bryan O'Donoghue
<pure.logic@...us-software.ie> wrote:
> On 24/10/17 14:14, Kees Cook wrote:
>>
>> On Tue, Oct 24, 2017 at 5:52 AM, Bryan O'Donoghue
>> <pure.logic@...us-software.ie> wrote:
>>>
>>> On 24/10/17 13:47, Kees Cook wrote:
>>>>
>>>>
>>>> On Tue, Oct 24, 2017 at 2:40 AM, Bryan O'Donoghue
>>>> <pure.logic@...us-software.ie> wrote:
>>>>>
>>>>>
>>>>> On 24/10/17 10:35, Bryan O'Donoghue wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 24/10/17 09:25, Kees Cook wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> In preparation for unconditionally passing the struct timer_list
>>>>>>> pointer
>>>>>>> to
>>>>>>> all timer callbacks, switch to using the new timer_setup() and
>>>>>>> from_timer()
>>>>>>> to pass the timer pointer explicitly.
>>>>>>>
>>>>>>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>>>>>> Cc: "Bryan O'Donoghue" <pure.logic@...us-software.ie>
>>>>>>> Cc: Johan Hovold <johan@...nel.org>
>>>>>>> Cc: Alex Elder <elder@...nel.org>
>>>>>>> Cc: greybus-dev@...ts.linaro.org
>>>>>>> Cc: devel@...verdev.osuosl.org
>>>>>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>>>>>> ---
>>>>>>>     drivers/staging/greybus/loopback.c  | 14 ++++----------
>>>>>>>     drivers/staging/greybus/operation.c |  7 +++----
>>>>>>>     2 files changed, 7 insertions(+), 14 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/staging/greybus/loopback.c
>>>>>>> b/drivers/staging/greybus/loopback.c
>>>>>>> index 08e255884206..045aaf81113a 100644
>>>>>>> --- a/drivers/staging/greybus/loopback.c
>>>>>>> +++ b/drivers/staging/greybus/loopback.c
>>>>>>> @@ -572,16 +572,11 @@ static void
>>>>>>> gb_loopback_async_operation_work(struct
>>>>>>> work_struct *work)
>>>>>>>         gb_loopback_async_operation_put(op_async);
>>>>>>>     }
>>>>>>> -static void gb_loopback_async_operation_timeout(unsigned long data)
>>>>>>> +static void gb_loopback_async_operation_timeout(struct timer_list
>>>>>>> *t)
>>>>>>>     {
>>>>>>> -    struct gb_loopback_async_operation *op_async;
>>>>>>> -    u16 id = data;
>>>>>>> +    struct gb_loopback_async_operation *op_async =
>>>>>>> +        from_timer(op_async, t, timer);
>>>>>>> -    op_async = gb_loopback_operation_find(id);
>>>>>>> -    if (!op_async) {
>>>>>>> -        pr_err("operation %d not found - time out ?\n", id);
>>>>>>> -        return;
>>>>>>> -    }
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Hi Kees, you need to add
>>>>>>
>>>>>>        gb_loopback_async_operation_get(op_async); when dropping the
>>>>>> gb_loopback_operation_find() call here.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Actually:
>>>>>
>>>>>           spin_lock_irqsave(&gb_dev.lock, flags);
>>>>>           gb_loopback_async_operation_get(op_async);
>>>>>           spin_unlock_irqrestore(&gb_dev.lock, flags);
>>>>
>>>>
>>>> Shouldn't the get/put follow the lifetime of the timer running
>>>> instead? It shouldnt' be possible to free the op_async while the timer
>>>> is still pending/running.
>>>
>>>
>>> The timeout timer runs for an operation that never completed but on the
>>> regular "everything is good" path you end up doing
>>> del_timer_sync(&op_async->timer); so the timer doesn't run.
>>
>>
>> As far as I can tell, the get/put is already happening external to the
>> timer:
>>
>> gb_loopback_async_operation():
>> ...
>>          op_async = kzalloc(sizeof(*op_async), GFP_KERNEL);
>> ...
>>          INIT_WORK(&op_async->work, gb_loopback_async_operation_work);
>>          kref_init(&op_async->kref);
>> ...
>>          op_async->pending = true;
>> ...
>>          ret = gb_operation_request_send(operation,
>>
>> gb_loopback_async_operation_callback,
>>                                          0,
>>                                          GFP_KERNEL);
>> ...
>>          timer_setup(&op_async->timer,
>> gb_loopback_async_operation_timeout, 0);
>>          op_async->timer.expires = jiffies + gb->jiffy_timeout;
>>          add_timer(&op_async->timer);
>>
>>
>> gb_loopback_async_operation_callback(): (run by operation)
>> ...
>>                  op_async->pending = false;
>>                  del_timer_sync(&op_async->timer);
>>                  gb_loopback_async_operation_put(op_async);
>>
>>
>> gb_loopback_async_operation_work(): (scheduled by timer)
>> ...
>>                  op_async->pending = false;
>>                  gb_loopback_async_operation_put(op_async);
>>
>
>
>> (Doing a get/put in the timer callback itself shouldn't be happening:
>> it must already be pinned in memory for the callback to run.)
>
>
> Both gb_loopback_async_operation_callback() and
> (gb_loopback_async_operation_timeout || gb_loopback_async_operation_work())
> can run in parallel so the get in the timer callback means we can let the
> timer stuff free-run w/r to gb_loopback_async_operation_callback() - take a
> reference indicating we still want that object and then let
> gb_loopback_async_operation_work() run.

Ah right, we're protecting the scheduled worker thread. I'll send a v2, thanks!

-Kees

-- 
Kees Cook
Pixel Security