linux-kernel - Re: [RFC] debug: add parameters to prevent entering debug mode on errors

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Date:	Thu, 20 Nov 2014 15:54:09 +0530
From:	Kiran Raparthy <kiran.kumar@...aro.org>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...roid.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	kgdb-bugreport@...ts.sourceforge.net,
	Android Kernel Team <kernel-team@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [RFC] debug: add parameters to prevent entering debug mode on errors

Hi,

On 20 November 2014 15:04, Daniel Thompson <daniel.thompson@...aro.org> wrote:
> On 20/11/14 08:18, Kiran Raparthy wrote:
>> Hi Daniel,
>>
>> On 18 November 2014 22:43, Daniel Thompson <daniel.thompson@...aro.org> wrote:
>>> On 18/11/14 12:08, Kiran Kumar Raparthy wrote:
>>>> From: Colin Cross <ccross@...roid.com>
>>>>
>>>> debug: add parameters to prevent entering debug mode on errors
>>>>
>>>> On non-developer devices kgdb prevents CONFIG_PANIC_TIMEOUT from rebooting the
>>>> device after a panic. Add module parameters debug_core.break_on_exception and
>>>> debug_core.break_on_panic to allow skipping debug on panics and exceptions
>>>> respectively.  Both default to true to preserve existing behavior.
>>>
>>> I am a little unsure about break_on_panic.
>>>
>>> It ought to be possible for kgdb/kdb to honour CONFIG_PANIC_TIMEOUT by
>>> tracking how long it takes for the user to attach a debugger (or to run
>>> the first kdb command after the panic). As it happens the timeout value
>>> is already an exported kernel symbol so all the info it there for us to
>>> use...
>>>
>>> Doing so would save us imposing further configuration burden on the user
>>> (although it would be a good deal more code).
>>>
>>> Note that I can't think of an automatic way to handle break_on_exception
>>> so I'm less worried about that one.
>> Alright,so it it okay if we have this mechanism limited to "skip debug
>> on exceptions"?
>> please let me know if i have misunderstood your point.
>
> Spliting it up would certainly stop a review comment from needlessly
> interfering with good stuff being delivered. That's always a good thing.
>
> To be clear though, providing the user a way to prevent kgdb from
> preventing the machine from rebooting after panic seems to me to be a
> useful feature. It is simply that I think the existing panic_timeout
> value could be used to realize it.
Yeah,got it now.
>
>>>> +             return 1;
>>>> +
>>>>       memset(ks, 0, sizeof(struct kgdb_state));
>>>>       ks->cpu                 = raw_smp_processor_id();
>>>>       ks->ex_vector           = evector;
>>>> @@ -821,6 +830,9 @@ static int kgdb_panic_event(struct notifier_block *self,
>>>>                           unsigned long val,
>>>>                           void *data)
>>>>  {
>>>> +     if (!break_on_panic)
>>>> +             return NOTIFY_DONE;
>
> How about simply:
>
>         if (panic_timeout)
>                 return NOTIFY_DONE;
>
> (plus a nice comment explaining why)
>
> This doesn't implement a timeout and so does not prevent a physically
> present user from exploiting kgdb. Nevertheless its an accurate
> interpretation of what the user told us to do and leaves the door open
> to adding a timeout in the future.
>
> Actually it might be a good idea to use panic_timeout to control
> trap-on-oops as well! If the user wants the machine to reboot itself on
> panic they certainly don't want it to hang during an oops.
Okay,I'll resend the patch with suggested modifications.
Thanks for the inputs.
Regards,
Kiran
>
> if (panic_timeout)
>         return NOTIFY_DONE;
>
>>>> +
>>>>       if (dbg_kdb_mode)
>>>>               kdb_prinf("PANIC: %s\n", (char *)data);
>>>>       kgdb_breakpoint();
>>>>
>>>
>
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