[lttng-dev] Function tracer question

[Martin, Wilson]

So, this is mainly for a kernel function. Details below:
unsigned int get_random_int(void);

lttng enable-event get_random_int --kernel --function get_random_int

get_random_int_entry: { cpu_id = 0 }, { pid = 22596, tid = 22596 }, { ip = 0xFFFFFFFF8152B8B0, parent_ip = 0xFFFFFFFF8106F8B5 }
get_random_int_return: { cpu_id = 0 }, { pid = 22596, tid = 22596 }, { ip = 0xFFFFFFFF8152B8B0, parent_ip = 0xFFFFFFFF8126CC6A }

I don’t get the return value from the trace. So, how do I capture that?
And, for other kernel functions, how do I capture the function arguments?

Regarding the kernels built-in ftrace system, you can capture any functions arguments and return value. The ftrace syscall capture is a good example: (NR=0 -> sys_read)
sshd-20065 [000] 1987131.702871: sys_enter:            NR 0 (b, 30f64380, 4000, 8, 0, 0)
sshd-20065 [000] 1987131.702871: sys_exit:             NR 0 = 2

Example from lttng syscall capture, with args and return:
syscall_entry_read: { cpu_id = 0 }, { pid = 22596, tid = 22596 }, { fd = 3, count = 832 }
syscall_exit_read: { cpu_id = 0 }, { pid = 22596, tid = 22596 }, { ret = 832, buf = 140722751121336 }

So, let’s also say that the built-in capabilities just don’t support what I want.
What if I were to develop a custom kernel probe using the LTTNG_* macros?
How would I go about capturing some named function (non-tracepoint)? And, its arguments and return value?
How would I go about making a custom syscall probe that adds additional arguments to the context?

I guess I could code my own ftrace hook function, put a custom tracepoint in the code, and then use the macros to push into lttng ring buffers. Maybe?

Thanks,
wilson