On Sat, 3 Nov 2018 09:13:41 -0400
Steven Rostedt [off-list ref] wrote:

On Sat, 3 Nov 2018 22:00:12 +0900
Masami Hiramatsu [off-list ref] wrote:

quoted

On Fri, 2 Nov 2018 12:13:07 -0400
Steven Rostedt [off-list ref] wrote:

quoted

quoted

Because that means if function graph tracer is active, then you can't
do a kretprobe, and vice versa. I'd really like to have it working for
multiple users, then we could trace different graph functions and store
them in different buffers. It would also allow for perf to use function
graph tracer too.  

Steve, how woul you allow multiple users on it? Something like this?

ret_trampoline_multiple(){
   list_for_each(handler, &shadow_entry[i].handlers, list)
	handler(shadow_entry[i]);
   restore_retval_and_jump_to(shadow_entry[i].orig);
}

Something like that. But since it's not a single mapping between shadow
entries and handlers, that is we have multiple tasks with multiple
shadow entries and multiple handlers, we can't use a link list (two
different parents per handler).

Yes, I understand it.

I was thinking of a bitmask that represents the handlers, and use that
to map which handler gets called for which shadow entry for a
particular task.

Hmm, I doubt that is too complicated and not scalable. I rather like to see
the open shadow entry...

entry: [[original_retaddr][function][modified_retaddr]]

So if there are many users on same function, the entries will be like this 

[[original_return_address][function][trampoline_A]]
[[trampline_A][function][trampoline_B]]
[[trampline_B][function][trampoline_C]]

And on the top of the stack, there is trampline_C instead of original_return_address.
In this case, return to trampoline_C(), it jumps back to trampline_B() and then
it jumps back to trampline_A(). And eventually it jumps back to
original_return_address.

This way, we don't need allocate another bitmap/pages for the shadow stack.
We only need a shadow stack for each task.
Also, unwinder can easily find the trampline_C from the shadow stack and restores
original_return_address. (of course trampline_A,B,C must be registered so that
search function can skip it.)

Thank you,

-- 
Masami Hiramatsu [off-list ref]