Re: [Xen-devel] [RFC v3 20/27] x86/ftrace: Adapt function tracing for PIE support

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Thu, Oct 5, 2017 at 9:11 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Thu, 5 Oct 2017 09:01:14 -0700
> Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>
>> On Thu, Oct 5, 2017 at 6:06 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> > On Wed,  4 Oct 2017 14:19:56 -0700
>> > Thomas Garnier <thgarnie@xxxxxxxxxx> 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.
>> >>
>> >> 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.
>>
>
> Then I think a better idea is to simply nop them out at compile time,
> and have the code that updates them to nops to know about it.
>
> See scripts/recordmcount.c
>
> Could we simply add a 5 byte nop followed by a 1 byte nop, and treat it
> the same as if it didn't exist? This code can be a little complex, and
> can cause really nasty side effects if things go wrong. I would like to
> keep from adding more variables to the changes here.

Sure, I will simplify it for the next iteration.

Thanks for the feedback.

>
> -- Steve



-- 
Thomas

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