Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

From	Russell King - ARM Linux <linux@armlinux.org.uk>
Newsgroups	linux.kernel
Subject	Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS
Date	2017-02-28 13:10 +0100
Message-ID	<tfOBI-3VC-5@gated-at.bofh.it> (permalink)
References	<tex0d-73G-1@gated-at.bofh.it> <tfvSq-7FO-17@gated-at.bofh.it> <tfEiZ-576-13@gated-at.bofh.it> <tfNPj-3Bl-13@gated-at.bofh.it> <tfO8G-3IX-13@gated-at.bofh.it>
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On Tue, Feb 28, 2017 at 11:22:27AM +0000, Abel Vesa wrote:
> On Tue, Feb 28, 2017 at 11:58:49AM +0100, Nicolai Stange wrote:
> > Hi Abel,
> > 
> > On Tue, Feb 28 2017, Abel Vesa wrote:
> > 
> > > On Mon, Feb 27, 2017 at 04:52:06PM +0100, Nicolai Stange wrote:
> > >> On Fri, Feb 24 2017, Abel Vesa wrote:
> > >> Wouldn't it be better (and more consistent with other archs) to have
> > >> 
> > >>   pt_regs->ARM_lr = original lr
> > >>   pt_refs->ARM_pc = current lr
> > >> 
> > >> instead?
> > 
> > The stack would look like this then
> > 
> > @ ...           | ARM_ip | ARM_sp | ARM_lr      | ARM_pc      | ...          |
> > @ 0  4          48       52       56            60            64    68       72
> > @ R0 | R1 | ... | LR     | SP + 4 | original LR | original PC | PSR | OLD_R0 | original LR |
> > 
> > I.e. the pt_regs would capture almost the full context of the
> > instrumented function (except for ip).
> > 
> So basicly what you are saying is:
> - instead of current LR save original LR (previous one saved in instrumented function epilog)
> - instead of current PC save original PC (previous one saved in instrumented function epilog)
> 
> I still don't see the point of saving the actual value of PC since nobody will ever
> restore it. In case of livepatch it will get overwritten anyway. As for LR, I agree,
> it could be the original one in pt_regs.
> 
> I'll look into this sometime today or tomorrow and get back with updates.

Which is exactly what I proposed, with code, on one of the previous
iterations of this patch...

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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[PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS Abel Vesa <abelvesa@linux.com> - 2017-02-25 00:00 +0100
  Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS Nicolai Stange <nicstange@gmail.com> - 2017-02-27 17:00 +0100
    Re: [PATCHv4] arm: ftrace: Adds support for  CONFIG_DYNAMIC_FTRACE_WITH_REGS Abel Vesa <abelvesa@gmail.com> - 2017-02-28 02:00 +0100
      Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS Nicolai Stange <nicstange@gmail.com> - 2017-02-28 12:10 +0100
        Re: [PATCHv4] arm: ftrace: Adds support for  CONFIG_DYNAMIC_FTRACE_WITH_REGS Abel Vesa <abelvesa@gmail.com> - 2017-02-28 12:30 +0100
          Re: [PATCHv4] arm: ftrace: Adds support for  CONFIG_DYNAMIC_FTRACE_WITH_REGS Abel Vesa <abelvesa@gmail.com> - 2017-02-28 13:00 +0100
          Re: [PATCHv4] arm: ftrace: Adds support for  CONFIG_DYNAMIC_FTRACE_WITH_REGS Russell King - ARM Linux <linux@armlinux.org.uk> - 2017-02-28 13:10 +0100

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