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

[Randy Dunlap]

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@xxxxxxxx>
>>>>> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
>>>>> ---
>>>>>  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.

(a) If [this option] is enabled, we use a weak hash instead of siphash.
(b) Use this option to print pointers with %px [during early boot].

Maybe they aren't contradictory?  Is (b) in effect until there is enough
entropy, then (a) takes over?


and nit:  s/i.e/i.e./

thanks,
-- 
~Randy