On Sat, 3 Nov 2018 13:30:21 -0400
Steven Rostedt [off-list ref] wrote:

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.

And since the most common case is a single function to call, instead of
using a count, we can take advantage that kernel functions are negative
numbers and do:

	[original_return_address]
	[function_A]

	----

	long count;

	count = pop_shadow_stack();
	if (count < 0) {
		func = (void *)count;
		func();
	} else {
		for (i = 0; i < count; i++) {
			[...]

The unwinder will just need to know how to handle all this :-)

-- Steve