Date: Tue, 5 Jun 2018 14:19:40 -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 Tue, Jun 5, 2018 at 9:56 AM Thomas Garnier <thgarnie@...gle.com> wrote: > > On Mon, Jun 4, 2018 at 2:44 PM Steven Rostedt <rostedt@...dmis.org> wrote: > > > > On Mon, 4 Jun 2018 14:06:03 -0700 > > Thomas Garnier <thgarnie@...gle.com> wrote: > > > > > 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? > > > > 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. Actually moving the logic from 5-bytes to 6-bytes is much more complicated, that's why I went with this approach before. I don't think it can be improved much more beyond creating a nop slide in mrecordcount but that's a different approach. I will clean-up the code a bit for the next iteration but that's about it. Let me know what you think. > > > > > -- Steve > > > > -- > Thomas -- Thomas
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