linux-kernel - Re: [PATCHv3] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH

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Date:   Thu, 9 Feb 2017 19:01:10 +0000
From:   Abel Vesa <abelvesa@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Abel Vesa <abelvesa@...ux.com>, mingo@...hat.com,
        viro@...iv.linux.org.uk, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, jjhiblot@...phandler.com,
        pmladek@...e.com, robin.murphy@....com, zhouchengming1@...wei.com,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCHv3] arm: ftrace: Adds support for
 CONFIG_DYNAMIC_FTRACE_WITH_REGS

On Thu, Feb 09, 2017 at 01:14:52PM -0500, Steven Rostedt wrote:
> 
> [ sending again with Masami Cc'd ]
> 
> On Thu, 9 Feb 2017 13:14:14 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Thu, 9 Feb 2017 18:06:44 +0000
> > Russell King - ARM Linux <linux@...linux.org.uk> wrote:
> > 
> > > On Thu, Feb 09, 2017 at 12:13:22PM -0500, Steven Rostedt wrote:  
> > > > Then came along live kernel patching, which I believe this series is
> > > > trying to support. What is needed by pt_regs is a way to "hijack" the
> > > > function being called to instead call the patched function. That is,
> > > > ftrace is not being used for tracing, but in reality, being used to
> > > > modify the running kernel. It is being used to change what function
> > > > gets called. ftrace is just a hook for that mechanism.    
> > > 
> > > So, would I be correct to assume that the only parts of pt_regs that
> > > would be touched are those which contain arguments to the function,
> > > and the register which would contain the return value?
> > >   
> > 
> > For live kernel patching, perhaps.
> > 
> > But for kprobes, I think they can touch anything. Matters what the
> > creater of the kprobe wanted to do.
> > 
Thing is, by saving all of them is the easiest way to ensure that the
whole context is the same when the replacing function gets called, as
I said before.

We can't be sure that while __ftrace_ops_list_func is executing, any of
the regs will have the value they had when the function-to-be-replaced
was called. That's the reason I say we need to save them all.
> > -- Steve
>