linux-kernel - Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear current

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Message-Id: <1527763450.f65xiu4yuz.naveen@linux.ibm.com>
Date:   Thu, 31 May 2018 16:25:38 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Alexei Starovoitov <ast@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, Josef Bacik <jbacik@...com>,
        James Hogan <jhogan@...nel.org>,
        Laura Abbott <labbott@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tony Luck <tony.luck@...el.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Will Deacon <will.deacon@....com>, x86@...nel.org
Subject: Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear
 current_kprobe and enable preempt in kprobe

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@...nel.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