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

Tobin C. Harding
2018-06-21T03:15:14Z

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.