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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
