Re: [PATCH v8 4/4] vsprintf: Add command line option debug_boot_weak_hash

Randy Dunlap
2018-06-21T04:10:08Z

On 06/20/2018 08:15 PM, Tobin C. Harding wrote:
> On Wed, Jun 20, 2018 at 04:38:05PM -0700, Randy Dunlap wrote:
>> On 06/20/2018 04:22 PM, Tobin C. Harding wrote:
>>> On Wed, Jun 20, 2018 at 03:36:44PM -0700, Randy Dunlap wrote:
>>>> On 06/20/2018 03:30 PM, Tobin C. Harding wrote:
>>>>> On Wed, Jun 20, 2018 at 09:09:49AM -0700, Randy Dunlap wrote:
>>>>>> On 06/19/2018 09:20 PM, Tobin C. Harding wrote:
>>>>>>> Currently printing [hashed] pointers requires enough entropy to be
>>>>>>> available. Early in the boot sequence this may not be the case
>>>>>>> resulting in a dummy string '(____ptrval____)' being printed. This
>>>>>>> makes debugging the early boot sequence difficult. We can relax the
>>>>>>> requirement to use cryptographically secure hashing during debugging.
>>>>>>> This enables debugging while keeping development/production kernel
>>>>>>> behaviour the same.
>>>>>>>
>>>>>>> If new command line option debug_boot_weak_hash is enabled use
>>>>>>> cryptographically insecure hashing and hash pointer value immediately.
>>>>>>>
>>>>>>> Signed-off-by: Tobin C. Harding <me@tobin.cc>
>>>>>>> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
>>>>>>> ---
>>>>>>> Documentation/admin-guide/kernel-parameters.txt | 9 +++++++++
>>>>>>> lib/vsprintf.c | 17 +++++++++++++++++
>>>>>>> 2 files changed, 26 insertions(+)
>>>>>>>
>>>>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>>>>>> index 638342d0a095..a116fc0366b0 100644
>>>>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>>>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>>>>>> @@ -748,6 +748,15 @@
>>>>>>>
>>>>>>> debug [KNL] Enable kernel debugging (events log level).
>>>>>>>
>>>>>>> + debug_boot_weak_hash
>>>>>>> + [KNL] Enable printing pointers early in the boot
>>>>>>> + sequence. If enabled, we use a weak hash instead of
>>>>>>> + siphash to hash pointers. Use this option if you need
>>>>>>> + to see pointer values during early boot (i.e you are
>>>>>>
>>>>>> maybe:
>>>>>> to see hashed pointer values
>>>>>> i.e., not raw pointers.
>>>>>
>>>>> You cannot see 'raw pointers' anyways?
>>>>
>>>> only if using %px ?
>>>>
>>>> Maybe it's just terminology. I don't consider a hashed value as a pointer value.
>>>> It's just a key or handle or some other number, but it's not a pointer.
>>>>
>>>>>>
>>>>>>> + seeing instances of '(___ptrval___)').
>>>>>>> + Cryptographically insecure, please do not use on
>>>>>>> + production kernels.
>>>>>
>>>>> thanks for the review, I don't quiet see how to use your suggestion to
>>>>> make the text clearer. If you still feel this change is needed perhaps
>>>>> you could write so I understand i.e 'Use this option if ...'
>>>>
>>>>
>>>> OK, if you are good with it, I am too. :)
>>>
>>> I get you know. I agree, how about this
>>>
>>> [KNL] Enable printing pointers early in the boot
>>> sequence. If enabled, we use a weak hash instead of
>>> siphash to hash pointers. Use this option if you need
>>> to print pointers with %px during early boot
>>> (i.e you are seeing instances of '(___ptrval___)').
>>> Cryptographically insecure, please do not use on
>>> production kernels.
>>
>> Sorry, I'm still confused by this paragraph. It seems to say two different
>> things.
>
> My bad, I got totally confused myself. After all this time you would
> think I knew which specifier hashed and which didn't. My apologies,
> how about this:
>
> [KNL] Enable printing [hashed] pointers early in
> the boot sequence. If enabled, we use a weak hash
> instead of siphash to hash pointers. Use this option if
> you are seeing instances of '(___ptrval___)') and need
> to see a value (hashed pointer) instead. Cryptographically
> insecure, please do not use on production kernels.
>
>
> thanks for your patience,
> Tobin.

Yes, that's good. Thanks.

--
~Randy