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Date: Thu, 5 Oct 2017 09:01:14 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Herbert Xu <herbert@...dor.apana.org.au>, "David S . Miller" <davem@...emloft.net>, 
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Subject: Re: [RFC v3 20/27] x86/ftrace: Adapt function tracing for PIE support

On Thu, Oct 5, 2017 at 6:06 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed,  4 Oct 2017 14:19:56 -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.
>>
>> With this change, function tracing supports 6 bytes on traceable
>> function and can still replace relative calls on the ftrace assembly
>> functions.
>>
>> Position Independent Executable (PIE) support will allow to extended the
>> KASLR randomization range below the -2G memory limit.
>
> Question: This 6 bytes is only the initial call that gcc creates. When
> function tracing is enabled, the calls are back to the normal call to
> the ftrace trampoline?

That is correct.

>
> -- Steve
>



-- 
Thomas

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