Re: [RFC PATCH -next v2 0/4] arm64/ftrace: support dynamic trampoline

Steven Rostedt
2022-05-25T14:00:10Z

On Wed, 25 May 2022 13:45:13 +0100
Mark Rutland <mark.rutland@arm.com> wrote:

> ... the compiler places 3 NOPs *before* any BTI, and 2 NOPs *after* any BTI,
> still recording the location of the first NOP. So in the two cases we get:
>
> NOP <--- recorded location
> NOP
> NOP
> __func_without_bti:
> NOP
> NOP
>
> NOP <--- recorded location
> NOP
> NOP
> __func_with_bti:
> BTI
> NOP
> NOP

Are you saying that the above "recorded location" is what we have in
mcount_loc section? If that's the case, we will need to modify it to point
to something that kallsyms will recognize (ie. sym+0 or greater). Because
that will cause set_ftrace_filter to fail as well.

-- Steve

>
> ... so where we want to patch one of the later nops to banch to a pre-function
> NOP, we need to know whether or not the compiler generated a BTI. We can
> discover discover that either by:
>
> * Checking whether the recorded location is at sym+0 (no BTI) or sym+4 (BTI).
>
> * Reading the instruction before the recorded location, and seeing if this is a
> BTI.
>
> ... and depending on how we handle thigns the two cases *might* need different
> trampolines.