On Sun, 4 Nov 2018 01:34:30 +0900 Masami Hiramatsu mhiramat@kernel.org wrote:
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...
It can scale and not too complex (I already played a little with it). But that said, I'm not committed to it, and using the shadow stack is also an interesting idea.
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.
Where are trampolines A, B, and C made? Do we also need to dynamically create them? If I register multiple function tracing ones, each one will need its own trampoline?
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.)
What I was thinking was to store a count and the functions to be called:
[original_return_address] [function_A] [function_B] [function_C] [ 3 ]
Then the trampoline that processes the return codes for ftrace (and kretprobes and everyone else) can simply do:
count = pop_shadow_stack(); for (i = 0; i < count; i++) { func = pop_shadow_stack(); func(...); } return_address = pop_shadow_stack();
That way we only need to register a function to the return handler and it will be called, without worrying about making trampolines. There will just be a single trampoline that handles all the work.
-- Steve