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From:   Thomas Garnier <thgarnie@google.com>
Date:   Mon, 4 Jun 2018 14:06:03 -0700
Message-ID: <CAJcbSZG1dVBVeOjEDeTm4mwXS60wMi-5ej_FB+YyS7iwXoebOQ@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 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?

>
> -- Steve
>
> >
> > Signed-off-by: Thomas Garnier <thgarnie@google.com>
> > ---
> >


-- 
Thomas