Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace?

Esben Nielsen
2006-02-16T19:07:44Z

I just jump into a thread somewhere to ask my question :-)

Why does the list have to be in userspace?

As I see it there can only be a problem when some thread has done
FUTEX_WAIT and is blocked. If no task is blocked (or on it's way to
being blocked) there is no problem.

The solution I could imagine was the FUTEX_WAIT operation adds the
waiting task to a list of waiters attached to the mutex owner's task_t
(which is known by it's pid in the userspace flag) just before calling
schedule(). This list needs to be protected by a spinlock, ofcourse.
When a task dies it can wake up the waiters on it's list without relying
on the userspace.

What race conditions have I missed?

Esben

On Thu, 16 Feb 2006, Daniel Walker wrote:

> On Thu, 2006-02-16 at 18:24 +0100, Ingo Molnar wrote:
> > * Daniel Walker <dwalker@mvista.com> wrote:
> >
> > > Another thing I noticed was that futex_offset on the surface looks
> > > like a malicious users dream variable .. [...]
> >
> > i have no idea what you mean by that - could you explain whatever threat
> > you have in mind, in more detail?
>
> As I said, "on the surface" you could manipulate the futex_offset to
> access memory unrelated to the futex structure . That's all I'm
> referring too ..
>
> Daniel
>
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