Date: Mon, 4 Jun 2018 14:06:03 -0700 From: Thomas Garnier <thgarnie@...gle.com> To: Steven Rostedt <rostedt@...dmis.org> Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, "H . Peter Anvin" <hpa@...or.com>, "the arch/x86 maintainers" <x86@...nel.org>, Francis Deslauriers <francis.deslauriers@...icios.com>, Greg KH <gregkh@...uxfoundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, Guenter Roeck <linux@...ck-us.net>, nixiaoming <nixiaoming@...wei.com>, James Hogan <jhogan@...nel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v4 21/27] x86/ftrace: Adapt function tracing for PIE support On Mon, Jun 4, 2018 at 1:16 PM Steven Rostedt <rostedt@...dmis.org> wrote: > > On Tue, 29 May 2018 15:15:22 -0700 > Thomas Garnier <thgarnie@...gle.com> wrote: > > > When using -fPIE/PIC with function tracing, the compiler generates a > > call through the GOT (call *__fentry__@...PCREL). 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? > > -- Steve > > > > > Signed-off-by: Thomas Garnier <thgarnie@...gle.com> > > --- > > -- Thomas
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