Re: [lttng-dev] Capturing User-Level Function Calls/Returns

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Hi Michel,

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.


On 2020-07-16 05:36, Michel Dagenais wrote:

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

-- Steve
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