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

On Thu, 5 Oct 2017 09:01:14 -0700
Thomas Garnier [off-list ref] wrote:

On Thu, Oct 5, 2017 at 6:06 AM, Steven Rostedt [off-list ref] wrote:

quoted

On Wed,  4 Oct 2017 14:19:56 -0700
Thomas Garnier [off-list ref] wrote:
 

quoted

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