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Subject: Re: [BUGFIX PATCH bpf-next] error-injection: Fix to prohibit jump
 optimization
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, linux-btrfs@vger.kernel.org,
        darrick.wong@oracle.com, Josef Bacik <josef@toxicpanda.com>,
        Akinobu Mita <akinobu.mita@gmail.com>
References: <152084884886.1536.4464134247603910765.stgit@devbox>
 <20180312192738.77061ec53f9437d960b437d9@kernel.org>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <c5e08b3e-069c-5160-a1d2-10c2dbac229e@iogearbox.net>
Date:   Mon, 12 Mar 2018 11:44:21 +0100
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Hi Masami,

On 03/12/2018 11:27 AM, Masami Hiramatsu wrote:
> 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 for the fix! I presume this should go via bpf instead of bpf-next
tree since 4b1a29a7f542 ("error-injection: Support fault injection framework")
is in Linus' tree as well. Unless there are objection I would rather route
it that way so it would be for 4.16.

Thanks,
Daniel

> 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);
>>  	}
>>
> 
>