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From:   Thomas Garnier <thgarnie@google.com>
Date:   Tue, 5 Jun 2018 09:56:14 -0700
Message-ID: <CAJcbSZGc7PjLTaoosZFPHpzfDGteE0fWrYS9g_GMBxMAegjfwQ@mail.gmail.com>
Subject: Re: [PATCH v4 21/27] x86/ftrace: Adapt function tracing for PIE support
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Francis Deslauriers <francis.deslauriers@efficios.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Guenter Roeck <linux@roeck-us.net>,
        nixiaoming <nixiaoming@huawei.com>,
        James Hogan <jhogan@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jun 4, 2018 at 2:44 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Mon, 4 Jun 2018 14:06:03 -0700
> Thomas Garnier <thgarnie@google.com> wrote:
>
> > On Mon, Jun 4, 2018 at 1:16 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > On Tue, 29 May 2018 15:15:22 -0700
> > > Thomas Garnier <thgarnie@google.com> wrote:
> > >
> > > > When using -fPIE/PIC with function tracing, the compiler generates a
> > > > call through the GOT (call *__fentry__@GOTPCREL). This instruction
> > > > takes 6 bytes instead of 5 on the usual relative call.
> > > >
> > > > If PIE is enabled, replace the 6th byte of the GOT call by a 1-byte nop
> > > > so ftrace can handle the previous 5-bytes as before.
> > > >
> > > > Position Independent Executable (PIE) support will allow to extend the
> > > > KASLR randomization range 0xffffffff80000000.
> > >
> > > I thought you were going to write a update to recordmcount.c to handle
> > > this at compile time?
> >
> > I can correctly calculate the start of the call instruction with
> > recordmcount (no need for addr-1) but I still need to handle the
> > different size of the instructions. I don't think I can completely
> > replace the GOT call with a relative call. Maybe I am missing
> > something on the way recordmcount is used? Should it replace all
> > mcount locations with a nop slide? Why is it done at runtime too then?
>
> Because we need to figure out the "ideal nop" thus we need to change it
> regardless.

I see what you mean looking at the different ideal_nops based on configurations.

>
> We could have recordmcount.c replace everything with the default nop
> (I've thought of that before), and then we could update with the ideal
> nop at run time, if that helps with this.

I don't think that's necessary. In proposed implementation of PIE,
kernel modules would not use a GOT call. In the current implementation
the __fentry__ call is always GOT based (6-bytes). I will simplify the
runtime implementation in the next patch set to just swap the expected
size and ideal_nop when PIE is enabled.

>
> -- Steve


-- 
Thomas