Re: [PATCH 2/2] ppc64le live patch: get rid of mini stack frame

On Thu, 2016-03-10 at 14:04 +0100, Torsten Duwe wrote:

On Thu, Mar 10, 2016 at 01:51:16PM +0100, Petr Mladek wrote:

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On Thu 2016-03-10 13:25:08, Petr Mladek wrote:

quoted

On Wed 2016-03-09 18:30:17, Torsten Duwe wrote:

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After the mini stack frame is no longer required for TOC storage, it can
be eliminated iff the functionality of klp_return_helper, which required
a stack frame for the extra return address previously, is carried out
by the replacement function now. This requires _every_ live patch replacement
function to execute the following (or similar) sequence of machine instructions
just before every return to the original caller:

I have thought about it and it is a nono from my point of view.
It is too error prone, especially that there are functions that
call return on several locations.

Yes, that's what I think as well when I look at it.

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BTW: How is this solved in kretprobes? Or is it easier there?

Without any look at the code I assume it uses solution 3. Once
you have a probing framework in place, you can remember the real
return addresses in a data structure. As I wrote, the function
graph tracer does it this way so it would be abvious.

Yeah it has a linked list of struct kretprobe_instance's, each of which stores
the real return address.

I have some ideas for how to fix livepatch, but this week is a bit busy with
merge window prep.

cheers