Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

Kees Cook
2016-06-23T18:00:08Z

On Thu, Jun 23, 2016 at 10:52 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Jun 23, 2016 at 10:44 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> The thread_info->tsk pointer, that was one of the most critical issues
>> and the main raison d'être of the thread_info, has been replaced on
>> x86 by just using the per-cpu "current_task". Yes,.there are probably
>> more than a few "ti->task" users left for legacy reasons, harking back
>> to when the thread-info was cheaper to access, but it shouldn't be a
>> big deal.
>
> Ugh. Looking around at this, it turns out that a great example of this
> kind of legacy issue is the debug_mutex stuff.
>
> It uses "struct thread_info *" as the owner pointer, and there is _no_
> existing reason for it. In fact, in every single place it actually
> wants the task_struct, and it does task_thread_info(task) just to
> convert it to the thread-info, and then converts it back with
> "ti->task".

Heh, yeah, that looks like a nice clean-up.

> So the attached patch seems to be the right thing to do regardless of
> this whole discussion.

Why does __mutex_lock_common() have "task" as a stack variable? It's
only assigned at the start, and is always "current". (I only noticed
from the patch changing "current_thread_info()" and
"task_thread_info(task)" both to "task".)

-Kees

--
Kees Cook
Chrome OS & Brillo Security