linux-kernel - Re: [PATCH 0/7] Uprobes: Support SDT markers having reference count (semaphore)

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Message-Id: <6225481e-fa00-a242-2b57-223559e3c488@linux.ibm.com>
Date:   Mon, 11 Jun 2018 10:01:58 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     oleg@...hat.com, srikar@...ux.vnet.ibm.com, rostedt@...dmis.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,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH 0/7] Uprobes: Support SDT markers having reference count
 (semaphore)

Hi Masami,

>>> Hmm, it sounds simple... maybe we can increment refctr in install_breakpoint/
>>> remove_breakpoint?
>>
>> Not really, it would be simpler if I can put it inside install_breakpoint().
>> Consider an mmap() case. Probed instruction resides in the text section whereas
>> reference counter resides in the data section. These sections gets mapped using
>> separate mmap() calls. So, when process mmaps the text section we will change the
>> instruction, but section holding the reference counter may not have been mapped
>> yet in the virtual memory. If so, we will fail to update the reference counter.
> 
> Got it. 
> In such case, maybe we can hook the target page mmapped and do install_breakpoint()
> at that point. Since the instruction is protected by a refctr, unless mmap the
> page on where the refctr is, the program doesn't reach the tracepoint. Is that right?
> 

You mean, when mmap(text) happens, save the target page somewhere and when
mmap(data) happens, update both instruction and ref_ctr?

This sounds feasible. Let me think on it.

Thanks for suggestion,
Ravi