Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)

[Florian Weimer <fweimer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 07/20/2015 10:07 PM, josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org wrote:
> On Mon, Jul 20, 2015 at 10:41:09AM -0700, Andy Lutomirski wrote:
>> On Mon, Jul 20, 2015 at 8:32 AM, Florian Weimer <fweimer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>>> On 07/20/2015 05:31 PM, Andy Lutomirski wrote:
>>>> On Jul 20, 2015 1:35 AM, "Florian Weimer" <fweimer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>>>>>
>>>>> On 07/18/2015 01:33 AM, Andy Lutomirski wrote:
>>>>>
>>>>>> I think the API should be "set gsbase to x + y*(cpu number)".  On
>>>>>> x86_64, userspace just allocates a big swath of virtual space and
>>>>>> populates it as needed.
>>>>>
>>>>> This will break WINE and similar applications which use %gs today.
>>>>
>>>> Presumably WINE could just opt not to use this facility, just like
>>>> WINE will have to opt out of whatever the enterprise people who want
>>>> WRGSBASE were thinking of doing with it.
>>>
>>> How is this possible if it's process-global attribute and glibc or some
>>> library in the process starts using it?
>>>
>>
>> glibc will have to expose a way to turn it off, I guess. (ELF flag?)
> 
> Or a way to turn it on.

How is this supposed to work?  Who should turn it on?

It totally breaks encapsulation.  We don't need any additional problems
like that.

-- 
Florian Weimer / Red Hat Product Security