user kprobes vs debuggers

[Stone, Joshua I]

Vara Prasad wrote:
> Let us say if there is no user space probes involved debugger
> remembers the original instruction before replacing it is with
> breakpoint instruction. It single steps the original instruction that
> is stored in the userspace. Let us say if there is no debugger and
> there is only userspace probes userspace pobes does the same single
> stepping but in the kernel space. Another important distinction to
> remember here is each of them have their own handlers to run when the
> break point is hit. Based on the above background there is a global
> registry of the breakpoints in the kernel that is only used to notify
> who all would like to handle this breakpoint but it is up to each of
> the owners to run their own handlers and as well as handle single
> stepping. 

Sure, I see no issue when only a debugger or a userspace probe
instruments a given address - they just single-step it themselves.  The
issue I was referring to is when a debugger and a userspace probe try to
instrument the same address.  In that scenario it's not clear who should
single-step the original instruction and how.

It's made worse if probes & debuggers are registered & removed in an
interleaved fashion - e.g. probe A registers; debugger B registers; A is
removed; what happens to B?

> I think it would have been o.k to disallow but due to common uses like
> strace people might object to userspace probes  feature.

You only have to disallow probes & debugger breakpoints coexisting at
the same address - if they're at different addresses then there's no
problem.  If that's still too restrictive, then we need to resolve the
issues above.

Josh