linux-kernel - Re: [PATCH v4 21/27] x86/ftrace: Adapt function tracing for PIE support

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Date:   Mon, 4 Jun 2018 16:16:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Garnier <thgarnie@...gle.com>
Cc:     kernel-hardening@...ts.openwall.com,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Francis Deslauriers <francis.deslauriers@...icios.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Guenter Roeck <linux@...ck-us.net>,
        nixiaoming <nixiaoming@...wei.com>,
        James Hogan <jhogan@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 21/27] x86/ftrace: Adapt function tracing for PIE
 support

On Tue, 29 May 2018 15:15:22 -0700
Thomas Garnier <thgarnie@...gle.com> wrote:

> When using -fPIE/PIC with function tracing, the compiler generates a
> call through the GOT (call *__fentry__@...PCREL). 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?

-- Steve

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