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SubjectRe: [PATCH v3 1/2] kretprobe: produce sane stack traces
On Sat, 3 Nov 2018 13:30:21 -0400
Steven Rostedt <rostedt@goodmis.org> 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

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