Messages in this thread | | | From | Dmitry Vyukov <> | Date | Fri, 22 Jul 2022 15:41:38 +0200 | Subject | Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting |
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On Fri, 22 Jul 2022 at 13:03, Will Deacon <will@kernel.org> wrote: > > > > > > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote: > > > > > > I'm not immediately sure what would be necessary to support per-task kernel > > > > > > breakpoints, but given a lot of that state is currently per-cpu, I imagine it's > > > > > > invasive. > > > > > > > > > > I would actually like to remove HW_BREAKPOINT completely for arm64 as it > > > > > doesn't really work and causes problems for other interfaces such as ptrace > > > > > and kgdb. > > > > > > > > Will it be a localized removal of code that will be easy to revert in > > > > future? Or will it touch lots of code here and there? > > > > Let's say we come up with a very important use case for HW_BREAKPOINT > > > > and will need to make it work on arm64 as well in future. > > > > > > My (rough) plan is to implement a lower-level abstraction for handling the > > > underlying hardware resources, so we can layer consumers on top of that > > > instead of funneling through hw_breakpoint. So if we figure out how to make > > > bits of hw_breakpoint work on arm64, then it should just go on top. > > > > > > The main pain point for hw_breakpoint is kernel-side {break,watch}points > > > and I think there are open design questions about how they should work > > > on arm64, particularly when considering the interaction with user > > > watchpoints triggering on uaccess routines and the possibility of hitting > > > a kernel watchpoint in irq context. > > > > I see. Our main interest would be break/watchpoints on user addresses > > firing from both user-space and kernel (uaccess), so at least on irqs. > > Interesting. Do other architectures report watchpoint hits on user > addresses from kernel uaccess? It feels like this might be surprising to > some users, and it opens up questions about accesses using different virtual > aliases (e.g. via GUP) or from other entities as well (e.g. firmware, > KVM guests, DMA).
x86 supports this. There is that attr.exclude_kernel flag that requires special permissions: https://elixir.bootlin.com/linux/v5.19-rc7/source/kernel/events/core.c#L12061 https://elixir.bootlin.com/linux/v5.19-rc7/source/kernel/events/core.c#L9323 But if I understand correctly, it only filters out delivery, the HW breakpoint fires even if attr.exclude_kernel is set.
We also wanted to relax this permission check somewhat: https://lore.kernel.org/all/20220601093502.364142-1-elver@google.com/
Yes, if the kernel maps the page at a different virtual address, then the breakpoint won't fire I think. Don't know what are the issues with firmware/KVM.
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