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

[Thomas Garnier]

On Mon, Jun 4, 2018 at 2:44 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Mon, 4 Jun 2018 14:06:03 -0700
> Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>
> > On Mon, Jun 4, 2018 at 1:16 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > >
> > > On Tue, 29 May 2018 15:15:22 -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.
> > > >
> > > > 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?
>
> Because we need to figure out the "ideal nop" thus we need to change it
> regardless.

I see what you mean looking at the different ideal_nops based on configurations.

>
> We could have recordmcount.c replace everything with the default nop
> (I've thought of that before), and then we could update with the ideal
> nop at run time, if that helps with this.

I don't think that's necessary. In proposed implementation of PIE,
kernel modules would not use a GOT call. In the current implementation
the __fentry__ call is always GOT based (6-bytes). I will simplify the
runtime implementation in the next patch set to just swap the expected
size and ideal_nop when PIE is enabled.

>
> -- Steve



-- 
Thomas