On Wed, Jul 10, 2019 at 05:34:07PM +0200, Frederic Weisbecker wrote: > 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).
No, I meant flip hw->target when we flip the context. It would mean iterating the events, which I suppose would suck. > 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) Right, that should be fairly easy. > and on event->destroy() it's TASK_TOMBSTONE and retrieving the task at > that time would be non trivial. Well, right, we can maybe make TOMBSTONE be the LSB instead of the whole word, then we can recover the task pointer... *yuck* though.
