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.