On Wed, 23 Oct 2024 at 11:29, David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 23.10.24 11:18, Lorenzo Stoakes wrote: > > On Wed, Oct 23, 2024 at 11:13:47AM +0200, David Hildenbrand wrote: > >> On 23.10.24 11:06, Vlastimil Babka wrote: > >>> On 10/23/24 10:56, Dmitry Vyukov wrote: > >>>>> > >>>>> Overall while I sympathise with this, it feels dangerous and a pretty major > >>>>> change, because there'll be something somewhere that will break because it > >>>>> expects faults to be swallowed that we no longer do swallow. > >>>>> > >>>>> So I'd say it'd be something we should defer, but of course it's a highly > >>>>> user-facing change so how easy that would be I don't know. > >>>>> > >>>>> But I definitely don't think a 'introduce the ability to do cheap PROT_NONE > >>>>> guards' series is the place to also fundmentally change how user access > >>>>> page faults are handled within the kernel :) > >>>> > >>>> Will delivering signals on kernel access be a backwards compatible > >>>> change? Or will we need a different API? MADV_GUARD_POISON_KERNEL? > >>>> It's just somewhat painful to detect/update all userspace if we add > >>>> this feature in future. Can we say signal delivery on kernel accesses > >>>> is unspecified? > >>> > >>> Would adding signal delivery to guard PTEs only help enough the ASAN etc > >>> usecase? Wouldn't it be instead possible to add some prctl to opt-in the > >>> whole ASANized process to deliver all existing segfaults as signals instead > >>> of -EFAULT ? > >> > >> Not sure if it is an "instead", you might have to deliver the signal in > >> addition to letting the syscall fail (not that I would be an expert on > >> signal delivery :D ). > >> > >> prctl sounds better, or some way to configure the behavior on VMA ranges; > >> otherwise we would need yet another marker, which is not the end of the > >> world but would make it slightly more confusing. > >> > > > > Yeah prctl() sounds sensible, and since we are explicitly adding a marker > > for guard pages here we can do this as a follow up too without breaking any > > userland expectations, i.e. 'new feature to make guard pages signal' is not > > going to contradict the default behaviour. > > > > So all makes sense to me, but I do think best as a follow up! :) > > Yeah, fully agreed. And my gut feeling is that it might not be that easy > ... :) > > In the end, what we want is *some* notification that a guard PTE was > accessed. Likely the notification must not necessarily completely > synchronous (although it would be ideal) and it must not be a signal. > > Maybe having a different way to obtain that information from user space > would work. For bug detection tools (like GWP-ASan [1]) it's essential to have useful stack traces. As such, having this signal be synchronous would be more useful. I don't see how one could get a useful stack trace (or other information like what's stashed away in ucontext like CPU registers) if this were asynchronous. [1] https://arxiv.org/pdf/2311.09394
