[Mesa-dev] [PATCH] RFC: Workaround for pthread_setaffinity_np() seccomp filtering

[Marc-André Lureau]

Hi

On Fri, Mar 1, 2019 at 12:13 PM Mathias Fröhlich
<Mathias.Froehlich at gmx.net> wrote:
>
> On Friday, 1 March 2019 12:15:08 CET Eero Tamminen wrote:
> > Hi,
> >
> > On 1.3.2019 11.12, Michel Dänzer wrote:
> > > On 2019-02-28 8:41 p.m., Marek Olšák wrote:
> > >>> On Thu, Feb 28, 2019 at 1:37 PM Eero Tamminen <eero.t.tamminen at intel.com>
> > >>>> Why distro versions of Qemu filter sched_setaffinity() syscall?
> > >>>
> > >>> (https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1815889)
> > >>>
> > >>> Daniel Berrange (berrange) wrote on 2019-02-27: #19
> > >>>
> > >>> "IMHO that mesa change is not valid. It is settings its affinity to
> > >>> run on all threads which is definitely *NOT* something we want to be
> > >>> allowed. Management applications want to control which CPUs QEMU runs
> > >>> on, and as such Mesa should honour the CPU placement that the QEMU
> > >>> process has.
> > >>>
> > >>> This is a great example of why QEMU wants to use seccomp to block
> > >>> affinity changes to prevent something silently trying to use more CPUs
> > >>> than are assigned to this QEMU."
> > >>>
> > >>
> > >> Mesa uses thread affinity to optimize memory access performance on some
> > >> CPUs (see util_pin_thread_to_L3). Other places in Mesa need to restore the
> > >> original thread affinity for some child threads. Additionally, if games
> > >> limit the thread affinity, Mesa needs to restore the full thread affinity
> > >> for some of its child threads.
> > >
> > > The last part sounds like Mesa clearly overstepping its authority.
> > >
> > >
> > >> In essence, the thread affinity should only be considered a hint for the
> > >> kernel for optimal performance. There is no reason to kill the process if
> > >> it's disallowed. Just ignore the call or modify the thread mask to make it
> > >> legal.
> > >
> > > The fundamental issue here is that Mesa is using the thread affinity API
> > > for something else than it's intended for. If there was an API for what
> > > Mesa wants (encouraging certain sets of threads to run on topologically
> > > close cores), there should be no need to block that.
> >
> > Why such process needs to be killed instead the request being masked
> > suitably, is there some program that breaks subtly if affinity request
> > is masked (and that being worse than the program being killed)?
>
> But that is still a situation that could be nicely handled with a
> EPERM error return. Way better than just kill a process.
> That 'badly affected' program still can call abort then.
> But nicely working programs don't get just killed then!!


Returning an error seems less secure that prohibiting it completely.
And it may lead to subtle bugs in rarely tested code paths.

It's legitimate that QEMU and management layers want to prevent
arbitrary code from changing resource allocation etc.

There are no easy way I can think of for mesa (and other libraries) to
probe the seccomp filters and associated action.

So we need a way to tell mesa not to call setaffinity() (and other
syscalls). MESA_NO_THREAD_AFFINITY or MESA_NO_SYSCALLS=setaffinity,...
seem like a relatively easy way to go.

thanks


-- 
Marc-André Lureau