Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

On Wed, Feb 11, 2015 at 11:58 PM, Hekuang [off-list ref] wrote:

quoted

quoted

eBPF is very flexible, which means it is bound to have someone use it
in a way you never dreamed of, and that will be what bites you in the
end (pun intended).

understood :)
let's start slow then with bpf+syscall and bpf+kprobe only.

I think BPF + system calls/kprobes can meet our use case
(https://lkml.org/lkml/2015/2/6/44), but there're some issues to be
improved.

I suggest that you can improve bpf+kprobes when attached to function
headers(or TRACE_MARKERS), make it converts pt-regs to bpf_ctx->arg1,
arg2.., then top models and architectures can be separated by bpf.

BPF bytecode is cross-platform, but what we can get by using bpf+kprobes
is a 'regs->rdx' kind of information, such information is both
architecture and kernel version related.

for kprobes in the middle of the function, kernel cannot
convert pt_regs into argN. Placement was decided by compiler
and can only be found in debug info.
I think bpf+kprobe will be using it when it is available.
When there is no debug info, kprobes will be limited
to function entry and mapping of regs/stack into
argN can be done by user space depending on architecture.
So user tracing scripts in some higher level language
can be kernel/arch independent when 'perf probe+bpf'
is loading them on the fly on the given machine.

We hope to establish some models for describing kernel procedures such
as IO and network, which requires that it does not rely on architecture
and does not rely to a specific kernel version as much as possible.

That's obviously a goal, but it requires a new approach to tracepoints.
I think a lot of great ideas were discussed in this thread, so I'm
hopeful that we'll come up with solution that will satisfy even
strictest Peter's requirements :)