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Date:   Wed, 25 May 2022 09:58:45 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Mark Rutland <mark.rutland@arm.com>
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Subject: Re: [RFC PATCH -next v2 0/4] arm64/ftrace: support dynamic
 trampoline
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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.