linux-kernel - Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH

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Message-ID: <20170228114638.GC21222@n2100.armlinux.org.uk>
Date:   Tue, 28 Feb 2017 11:46:38 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Abel Vesa <abelvesa@...il.com>
Cc:     Nicolai Stange <nicstange@...il.com>,
        Abel Vesa <abelvesa@...ux.com>, robin.murphy@....com,
        jjhiblot@...phandler.com, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, pmladek@...e.com,
        mhiramat@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4] arm: ftrace: Adds support for
 CONFIG_DYNAMIC_FTRACE_WITH_REGS

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...

-- 
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