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

Tobin C. Harding
2018-06-21T05:40:58Z

On Wed, Jun 20, 2018 at 09:09:49PM -0700, Randy Dunlap wrote:
> 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.

Awesome, v9 on it's way :)

thanks,
Tobin.