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

Tobin C. Harding
2018-06-20T23:22:41Z

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.

thanks for clarifying,
Tobin.