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Date:   Thu, 12 Apr 2018 09:52:20 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     Vince Weaver <vincent.weaver@maine.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: perf: fuzzer leads to trace_kprobe: Could not insert message
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* Song Liu <songliubraving@fb.com> wrote:

> > spamming the kernel log ...
> 
> Yeah, the new API allows non-root user to trigger this message. We should only 
> allow root to create kprobe with perf_event_open(). 
> 
> On the other hand, do we need to fix this for root? In fact, a simple bash loop 
> can create something similar through the text interface (with root):
> 
> root@virt-test:~# for x in {0..5} ; do echo p:xx xx+$x >> /sys/kernel/debug/tracing/kprobe_events ; done
> -bash: echo: write error: No such file or directory
> -bash: echo: write error: No such file or directory
> -bash: echo: write error: No such file or directory
> -bash: echo: write error: No such file or directory
> -bash: echo: write error: No such file or directory
> -bash: echo: write error: No such file or directory
> root@virt-test:~# dmesg | tail -n 5
> [  664.208374] trace_kprobe: Could not insert probe at xx+1: -2
> [  664.237882] trace_kprobe: Could not insert probe at xx+2: -2
> [  664.268067] trace_kprobe: Could not insert probe at xx+3: -2
> [  664.297395] trace_kprobe: Could not insert probe at xx+4: -2
> [  664.327614] trace_kprobe: Could not insert probe at xx+5: -2
> 
> This happens before the new API is introduced. 
> 
> The following patch does capable(CAP_SYS_ADMIN) for perf_kprobe and 
> perf_uprobe at an earlier stage, so non-root user cannot trigger 
> this error message. Please let me know whether we need to fix this 
> for root. 

That's two bugs then, and yes, I think we should fix the log spamming: what's the 
point? We already get an error code from the write.

I'll apply your CAP_SYS_ADMIN fix.

Thanks,

	Ingo