[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAJcbSZGc7PjLTaoosZFPHpzfDGteE0fWrYS9g_GMBxMAegjfwQ@mail.gmail.com>
Date: Tue, 5 Jun 2018 09:56:14 -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 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.
>
> -- Steve
--
Thomas
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
