On Thu, 5 Oct 2017 09:01:14 -0700 Thomas Garnier <thgar...@google.com> wrote:
> On Thu, Oct 5, 2017 at 6:06 AM, Steven Rostedt <rost...@goodmis.org> wrote: > > On Wed, 4 Oct 2017 14:19:56 -0700 > > Thomas Garnier <thgar...@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 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
