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Date:   Mon, 12 Mar 2018 19:27:38 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Alexei Starovoitov <ast@fb.com>, Josef Bacik <jbacik@fb.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,
        linux-btrfs@vger.kernel.org, darrick.wong@oracle.com,
        Josef Bacik <josef@toxicpanda.com>,
        Akinobu Mita <akinobu.mita@gmail.com>
Subject: Re: [BUGFIX PATCH bpf-next] error-injection: Fix to prohibit jump
 optimization
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On Mon, 12 Mar 2018 19:00:49 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Since the kprobe which was optimized by jump can not change
> the execution path, the kprobe for error-injection must not
> be optimized. To prohibit it, set a dummy post-handler as
> officially stated in Documentation/kprobes.txt.

Note that trace-probe based BPF is not affected, because it
ensures the trace-probe is based on ftrace, which is not
jump optimized.

Thanks,

> 
> Fixes: 4b1a29a7f542 ("error-injection: Support fault injection framework")
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>  kernel/fail_function.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/fail_function.c b/kernel/fail_function.c
> index 21b0122cb39c..1d5632d8bbcc 100644
> --- a/kernel/fail_function.c
> +++ b/kernel/fail_function.c
> @@ -14,6 +14,15 @@
>  
>  static int fei_kprobe_handler(struct kprobe *kp, struct pt_regs *regs);
>  
> +static void fei_post_handler(struct kprobe *kp, struct pt_regs *regs,
> +			     unsigned long flags)
> +{
> +	/*
> +	 * A dummy post handler is required to prohibit optimizing, because
> +	 * jump optimization does not support execution path overriding.
> +	 */
> +}
> +
>  struct fei_attr {
>  	struct list_head list;
>  	struct kprobe kp;
> @@ -56,6 +65,7 @@ static struct fei_attr *fei_attr_new(const char *sym, unsigned long addr)
>  			return NULL;
>  		}
>  		attr->kp.pre_handler = fei_kprobe_handler;
> +		attr->kp.post_handler = fei_post_handler;
>  		attr->retval = adjust_error_retval(addr, 0);
>  		INIT_LIST_HEAD(&attr->list);
>  	}
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>