Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear current_kprobe and enable preempt in kprobe

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Masami Hiramatsu wrote:
> Clear current_kprobe and enable preemption in kprobe
> even if pre_handler returns !0.
> 
> This simplifies function override using kprobes.
> 
> Jprobe used to require to keep the preemption disabled and
> keep current_kprobe until it returned to original function
> entry. For this reason kprobe_int3_handler() and similar
> arch dependent kprobe handers checks pre_handler result
> and exit without enabling preemption if the result is !0.
> 
> After removing the jprobe, Kprobes does not need to
> keep preempt disabled even if user handler returns !0
> anymore.

I think the reason jprobes did it that way is to address architecture 
specific requirements when changing a function. So, without that 
infrastructure, I am not sure if we will be able to claim support for 
over-riding functions with kprobes. I am not sure if we want to claim 
that, but this is something we need to be clear on.

For powerpc, the current function override in error-inject works fine 
since the new function does nothing. But, if anyone wants to do more 
work in the replacement function, it won't work with the current 
approach.

> 
> But since the function override handler in error-inject
> and bpf is also returns !0 if it overrides a function,
> to balancing the preempt count, it enables preemption
> and reset current kprobe by itself.
> 
> That is a bad design that is very buggy. This fixes
> such unbalanced preempt-count and current_kprobes setting
> in kprobes, bpf and error-inject.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>  arch/arc/kernel/kprobes.c          |    5 +++--
>  arch/arm/probes/kprobes/core.c     |   10 +++++-----
>  arch/arm64/kernel/probes/kprobes.c |   10 +++++-----
>  arch/ia64/kernel/kprobes.c         |   13 ++++---------
>  arch/mips/kernel/kprobes.c         |    4 ++--
>  arch/powerpc/kernel/kprobes.c      |    7 +++++--

I think you should also update arch/powerpc/kernel/kprobes-ftrace.c

- Naveen