Re: [PATCH 0/2] perf/hw_breakpoint: Fix breakpoint overcommit issue

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Jul 10, 2019 at 04:04:21PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 09, 2019 at 03:48:19PM +0200, Frederic Weisbecker wrote:
> > 
> > Syzbot has found a breakpoint overcommit issue:
> > 
> > https://lore.kernel.org/lkml/000000000000639f6a0584d11b82@google.com/
> > 
> > It took me a long time to find out what the actual root problem was. Also
> > its reproducer only worked on a few month old kernel but it didn't feel like
> > the issue was actually solved.
> > 
> > I eventually cooked a reproducer that works with latest upstream, see in
> > the end of this message.
> > 
> > The fix is just a few liner but implies to shut down the context swapping
> > optimization for contexts containing breakpoints.
> > 
> > Also I feel like uprobes may be concerned as well as it seems to make use
> > of event.hw->target after pmu::init().
> 
> Can't we simply swizzle event.hw->target along too?

You mean remove it? But it's still needed by breakpoint code during all the event
lifecycle (init, destroy and anytime in-between).

I wish we could use event->ctx->task instead but on pmu::init() there is no ctx yet (we could pass
the task in parameter though) and on event->destroy() it's TASK_TOMBSTONE and retrieving the task
at that time would be non trivial.