From: ahmadkhorrami <ahmadkhorrami@ut.ac.ir>
To: Michel Dagenais <michel.dagenais@polymtl.ca>
Cc: Steven Rostedt <rostedt@goodmis.org>,
linux-trace-users-owner@vger.kernel.org,
linux-trace-users <linux-trace-users@vger.kernel.org>,
lttng-dev <lttng-dev@lists.lttng.org>,
Namhyung Kim <namhyung@kernel.org>
Subject: Re: [lttng-dev] Capturing User-Level Function Calls/Returns
Date: Thu, 16 Jul 2020 20:50:16 +0430 [thread overview]
Message-ID: <29db598e77d0c2f04dabd6881aae097e@ut.ac.ir> (raw)
In-Reply-To: <489547987.230950.1594861561764.JavaMail.zimbra@polymtl.ca>
<p>Hi Michel,</p>
<p>Thanks for the detailed answer! DBI tools are really interesting but
I want to do this during normal execution and on multiple programs
running simultaneously. I mean this is not supposed to
be conventional tracing with multiple re-executions. I want to
extract some information about the execution-state at runtime and inform
the lower levels in the software stack to make smarter choices.
Fortunately, there are only a few functions that need to be traced. But
any reduction in the wasted cycles is helpful, specially if it is caused
by privilege level transitions.</p>
<p>Regards.</p>
<p> </p>
<p>On 2020-07-16 05:36, Michel Dagenais wrote:</p>
<blockquote><!-- html ignored --><!-- head ignored --><!-- meta ignored
-->
<div class="pre"><br />
<blockquote>Without recompiling, how would that be
implemented?</blockquote>
<br /> As you mentioned, this is possible when "jump patching" 5 bytes
instructions. Fast tracepoints in GDB and in kprobe do it. Kprobe goes
further and patches sequences of instructions (because the target
instruction is less than 5 bytes) if there is no incoming branch into
the middle of the sequence. You can go even further, for instance using
3 bytes jumps to a trampoline installed in alignment nops. If you
combine different strategies like this, you can eventually reach almost
100% success rate for "jump patching" tracepoints. This gets quite hairy
though. However, the short story is that there is currently no tool as
far as I know that does that easily and reliably in user space.<br /><br
/><a href="https://onlinelibrary.wiley.com/doi/abs/10.1002/spe.2746"
target="_blank" rel="noopener
noreferrer">https://onlinelibrary.wiley.com/doi/abs/10.1002/spe.2746</a><br
/><a href="https://dl.acm.org/doi/pdf/10.1145/3062341.3062344"
target="_blank" rel="noopener
noreferrer">https://dl.acm.org/doi/pdf/10.1145/3062341.3062344</a><br
/><br /> If you can afford a more invasive tool, that requires a lot of
memory and stops your application for quite some time, you can look at
approaches like dyninst that decompile the binary, insert
instrumentation code and reassemble the code.<br /><br /><a
href="https://dyninst.org/" target="_blank" rel="noopener
noreferrer">https://dyninst.org/</a><br /><br />
<blockquote>You would need to insert a jump on top of code, and still be
able to<br /> preserve that code. What a trap does, is to insert a int3,
that will<br /> trap into the kernel, it would then emulate the code
that the int3 was<br /> on, and also call some code that can trace the
current state.<br /><br /> To do it in user land, you would need to find
way to replace the code<br /> at the location you want to trace, with a
jump to the tracing<br /> infrastructure, that will also be able to
emulate the code that the<br /> jump was inserted on top of. As on x86,
that jump will need to be 5<br /> bytes long (covering 5 bytes of text
to emulate), where as a int3 is a<br /> single byte.<br /><br /> Thus,
you either recompile and insert nops where you want to place your<br />
jumps, or you trap using int3 that can do the work from within the<br />
kernel.<br /><br /> -- Steve<br />
_______________________________________________<br /> lttng-dev mailing
list<br /><a
href="mailto:lttng-dev@lists.lttng.org">lttng-dev@lists.lttng.org</a><br
/><a href="https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev"
target="_blank" rel="noopener
noreferrer">https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev</a></blockquote>
</div>
</blockquote>
<p> </p>
<div id="_rc_sig"> </div>
next prev parent reply other threads:[~2020-07-16 16:20 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-15 16:07 Capturing User-Level Function Calls/Returns ahmadkhorrami
2020-07-15 18:28 ` Steven Rostedt
2020-07-15 18:45 ` Mathieu Desnoyers
2020-07-15 21:39 ` ahmadkhorrami
2020-07-15 21:48 ` Steven Rostedt
2020-07-15 22:25 ` ahmadkhorrami
2020-07-16 1:06 ` [lttng-dev] " Michel Dagenais
2020-07-16 1:49 ` Frank Ch. Eigler
2020-07-16 16:26 ` ahmadkhorrami
2020-07-16 16:20 ` ahmadkhorrami [this message]
2020-07-16 16:34 ` ahmadkhorrami
2020-07-16 1:04 ` Namhyung Kim
2020-07-16 16:07 ` ahmadkhorrami
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