linux-kernel - Re: [PATCH v8 4/4] vsprintf: Add command line option debug_boot_weak

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Date:   Thu, 21 Jun 2018 09:22:33 +1000
From:   "Tobin C. Harding" <me@...in.cc>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 4/4] vsprintf: Add command line option
 debug_boot_weak_hash

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@...in.cc>
> >>> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.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.