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

Randy Dunlap
2018-06-20T23:38:27Z

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.

(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