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/[email protected]/ > > > > 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.
