linux-kernel - Re: [PATCH v5 09/10] Uprobes/sdt: Document about reference counter

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Date:   Tue, 3 Jul 2018 13:20:29 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     oleg@...hat.com, rostedt@...dmis.org, mhiramat@...nel.org,
        peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, linux-kernel@...r.kernel.org, corbet@....net,
        linux-doc@...r.kernel.org, ananth@...ux.vnet.ibm.com,
        alexis.berlemont@...il.com, naveen.n.rao@...ux.vnet.ibm.com,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
        linux@...linux.org.uk, ralf@...ux-mips.org, paul.burton@...s.com,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH v5 09/10] Uprobes/sdt: Document about reference counter

Hi Srikar,

On 07/02/2018 08:24 PM, Srikar Dronamraju wrote:
> * Ravi Bangoria <ravi.bangoria@...ux.ibm.com> [2018-06-28 10:52:08]:
> 
>> Reference counter gate the invocation of probe. If present,
>> by default reference count is 0. Kernel needs to increment
>> it before tracing the probe and decrement it when done. This
>> is identical to semaphore in Userspace Statically Defined
>> Tracepoints (USDT).
>>
>> Document usage of reference counter.
>>
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
>> Acked-by: Masami Hiramatsu <mhiramat@...nel.org>
> 
> Unlike perf, this mechanism cannot detect ref count and depends on the
> users data. What happens if the user mistakenly provides a wrong location?
> I guess he can corrupt some other data structures?

Yes, if user is giving wrong ref_ctr_offset, uprobe infrastructure will
corrupt some user data.

> 
> Hence I would think twice of advertising this mechanism. i.e keep this
> as an undocumented feature.
> 

I don't mind.

Thanks,
Ravi