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

Randy Dunlap
2018-06-20T22:36:54Z

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. :)

thanks,
--
~Randy