[PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Dmitry Vyukov]

On Fri, 22 Jul 2022 at 13:03, Will Deacon <will at 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.