Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

Randy Dunlap
2017-12-05T21:09:09Z

On 11/30/2017 02:38 AM, David Laight wrote:
> From: Kees Cook
>> Sent: 29 November 2017 22:28
>> On Wed, Nov 29, 2017 at 2:07 AM, David Laight <David.Laight@aculab.com> wrote:
>>> From: Linus Torvalds
>>>> Sent: 29 November 2017 02:29
>>>>
>>>> On Tue, Nov 28, 2017 at 6:05 PM, Tobin C. Harding <me@tobin.cc> wrote:
>>>>>
>>>>> Let's add specifier %px as a
>>>>> clear, opt-in, way to print a pointer and maintain some level of
>>>>> isolation from all the other hex integer output within the Kernel.
>>>>
>>>> Yes, I like this model. It's easy and it's obvious ("'x' for hex"),
>>>> and it gives people a good way to say "yes, I really want the actual
>>>> address as hex" for if/when the hashed pointer doesn't work for some
>>>> reason.
>>>
>>> Remind me to change every %p to %px on kernels that support it.
>>>
>>> Although the absolute values of pointers may not be useful, knowing
>>> that two pointer differ by a small amount is useful.
>>> It is also useful to know whether pointers are to stack, code, static
>>> data or heap.
>>>
>>> This change to %p is going to make debugging a nightmare.
>>
>> In the future, maybe we could have a knob: unhashed, hashed (default),
>> or zeroed.
>
> Add a 4th, hashed_page+offset.
>
> Isn't there already a knob for %pK, bits in the same value could be used.
> That would make it easy to ensure that %pK is more restructive than %p.

(yeah, I'm kind of behind on this thread.)

This kind of option (with default hashed) is what I was just thinking of
after having seen a few unhelpful traces. But then the knob might not be
changed in time for the traces either. :(

--
~Randy