Messages in this thread | | | Date | Wed, 25 May 2022 09:58:45 -0400 | From | Steven Rostedt <> | 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.
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