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Subject: Re: [PATCH 0/2][v5] Add the ability to do BPF directed error
 injection
To:     Josef Bacik <josef@toxicpanda.com>, <rostedt@goodmis.org>,
        <mingo@redhat.com>, <davem@davemloft.net>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <ast@kernel.org>, <kernel-team@fb.com>, <daniel@iogearbox.net>,
        Peter Zijlstra <peterz@infradead.org>
References: <1510086523-8859-1-git-send-email-josef@toxicpanda.com>
From:   Alexei Starovoitov <ast@fb.com>
Message-ID: <04fd674a-6921-d2ea-8e89-856397ab297d@fb.com>
Date:   Wed, 8 Nov 2017 06:43:25 +0900
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On 11/8/17 5:28 AM, Josef Bacik wrote:
> I'm sending this through Dave since it'll conflict with other BPF changes in his
> tree, but since it touches tracing as well Dave would like a review from
> somebody on the tracing side.
>
> v4->v5:
> - disallow kprobe_override programs from being put in the prog map array so we
>   don't tail call into something we didn't check.  This allows us to make the
>   normal path still fast without a bunch of percpu operations.

looks great to me.
Peter,
could you please review x86 bits?

> v3->v4:
> - fix a build error found by kbuild test bot (I didn't wait long enough
>   apparently.)
> - Added a warning message as per Daniels suggestion.
>
> v2->v3:
> - added a ->kprobe_override flag to bpf_prog.
> - added some sanity checks to disallow attaching bpf progs that have
>   ->kprobe_override set that aren't for ftrace kprobes.
> - added the trace_kprobe_ftrace helper to check if the trace_event_call is a
>   ftrace kprobe.
> - renamed bpf_kprobe_state to bpf_kprobe_override, fixed it so we only read this
>   value in the kprobe path, and thus only write to it if we're overriding or
>   clearing the override.
>
> v1->v2:
> - moved things around to make sure that bpf_override_return could really only be
>   used for an ftrace kprobe.
> - killed the special return values from trace_call_bpf.
> - renamed pc_modified to bpf_kprobe_state so bpf_override_return could tell if
>   it was being called from an ftrace kprobe context.
> - reworked the logic in kprobe_perf_func to take advantage of bpf_kprobe_state.
> - updated the test as per Alexei's review.
>
> - Original message -
>
> A lot of our error paths are not well tested because we have no good way of
> injecting errors generically.  Some subystems (block, memory) have ways to
> inject errors, but they are random so it's hard to get reproduceable results.
>
> With BPF we can add determinism to our error injection.  We can use kprobes and
> other things to verify we are injecting errors at the exact case we are trying
> to test.  This patch gives us the tool to actual do the error injection part.
> It is very simple, we just set the return value of the pt_regs we're given to
> whatever we provide, and then override the PC with a dummy function that simply
> returns.
>
> Right now this only works on x86, but it would be simple enough to expand to
> other architectures.  Thanks,
>
> Josef
>


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