On Fri, Nov 19, 2021 at 10:35:44AM -0800, Josh Poimboeuf wrote: > On Fri, Nov 19, 2021 at 10:29:47AM +0100, Peter Zijlstra wrote: > > On Thu, Nov 18, 2021 at 06:04:27PM -0800, Josh Poimboeuf wrote: > > > On Thu, Nov 18, 2021 at 01:11:09PM +0100, Peter Zijlstra wrote: > > > > > > I now have the below, the only thing missing is that there's a > > > > user_mode() call on a stack based regs. Now on x86_64 we can > > > > __get_kernel_nofault() regs->cs and call it a day, but on i386 we have > > > > to also fetch regs->flags. > > > > > > > > Is this really the way to go? > > > > > > Please no. Can we just add a check in unwind_start() to ensure the > > > caller did try_get_task_stack()? > > > > I tried; but at best it's fundamentally racy and in practise its worse > > because init_task doesn't seem to believe in refcounts and kthreads are > > odd for some raisin. Now those are fixable, but given the fundamental > > races, I don't see how it's ever going to be reliable. > > I'm probably out of the loop here, but I wonder what races you're > referring to. We can do the warn as you suggest, however, it can become 0 right after we test and then still make the unwder explode. That is, the test is not sufficient. > And I assume 'stack_refcount > 0' only needs to be asserted when > unwinding other tasks, not current. So it shouldn't affect unwinds > during boot, or oopses. > > Yes, the unwinder has to be rock solid, but if the caller can't even > ensure the given task's memory exists, it sounds like a bug in the > caller that needs a warning. Well, yes. Still it would be nice if the unwinder would not itself burn the house, even in that case.
