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

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Message-ID: <b472dca0-9bd0-caf3-efab-586b6ff32765@infradead.org>
Date:   Wed, 20 Jun 2018 15:36:44 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     "Tobin C. Harding" <me@...in.cc>
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 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.  :)

thanks,
-- 
~Randy