> 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. https://onlinelibrary.wiley.com/doi/abs/10.1002/spe.2746 https://dl.acm.org/doi/pdf/10.1145/3062341.3062344 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. https://dyninst.org/ > 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 > lttng-dev@xxxxxxxxxxxxxxx > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
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