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From: Michel Dagenais <>
To: Steven Rostedt <>
Cc: ahmadkhorrami <>,,
	linux-trace-users <>,
	lttng-dev <>,
	Namhyung Kim <>
Subject: Re: [lttng-dev] Capturing User-Level Function Calls/Returns
Date: Wed, 15 Jul 2020 21:06:01 -0400 (EDT)
Message-ID: <> (raw)
In-Reply-To: <20200715174858.4698803c@oasis.local.home>

> Without recompiling, how would that be implemented?

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.

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.

> You would need to insert a jump on top of code, and still be able to
> preserve that code. What a trap does, is to insert a int3, that will
> trap into the kernel, it would then emulate the code that the int3 was
> on, and also call some code that can trace the current state.
> To do it in user land, you would need to find way to replace the code
> at the location you want to trace, with a jump to the tracing
> infrastructure, that will also be able to emulate the code that the
> jump was inserted on top of. As on x86, that jump will need to be 5
> bytes long (covering 5 bytes of text to emulate), where as a int3 is a
> single byte.
> Thus, you either recompile and insert nops where you want to place your
> jumps, or you trap using int3 that can do the work from within the
> kernel.
> -- Steve
> _______________________________________________
> lttng-dev mailing list

  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-15 16:07 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         ` Michel Dagenais [this message]
2020-07-16  1:49           ` [lttng-dev] " Frank Ch. Eigler
2020-07-16 16:26             ` ahmadkhorrami
2020-07-16 16:20           ` ahmadkhorrami
2020-07-16 16:34           ` ahmadkhorrami
2020-07-16  1:04   ` Namhyung Kim
2020-07-16 16:07     ` ahmadkhorrami

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