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

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 5 Oct 2017 09:01:14 -0700
Thomas Garnier <thgarnie@google.com> wrote:

> On Thu, Oct 5, 2017 at 6:06 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Wed,  4 Oct 2017 14:19:56 -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.
> >>
> >> 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.

-- Steve