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Re: [Xen-devel] [RFC v3 20/27] x86/ftrace: Adapt function tracing for PIE support
- To: Thomas Garnier <thgarnie@xxxxxxxxxx>
- From: Steven Rostedt <rostedt@xxxxxxxxxxx>
- Date: Thu, 5 Oct 2017 12:11:32 -0400
- Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>, Michal Hocko <mhocko@xxxxxxxx>, Radim Krčmář <rkrcmar@xxxxxxxxxx>, linux-doc@xxxxxxxxxxxxxxx, Daniel Micay <danielmicay@xxxxxxxxx>, Len Brown <len.brown@xxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Christopher Li <sparse@xxxxxxxxxxx>, "Jan H . Schönherr" <jschoenh@xxxxxxxxx>, Alexei Starovoitov <ast@xxxxxxxxxx>, Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, David Howells <dhowells@xxxxxxxxxx>, Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>, Pavel Machek <pavel@xxxxxx>, "H . Peter Anvin" <hpa@xxxxxxxxx>, Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>, Christoph Lameter <cl@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Chris Metcalf <cmetcalf@xxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>, Daniel Borkmann <daniel@xxxxxxxxxxxxx>, Jonathan Corbet <corbet@xxxxxxx>, Peter Foley <pefoley2@xxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, "Rafael J . Wysocki" <rafael.j.wysocki@xxxxxxxxx>, Kyle Huey <me@xxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Sparse Mailing-list <linux-sparse@xxxxxxxxxxxxxxx>, Matthias Kaehlcke <mka@xxxxxxxxxxxx>, linux-arch <linux-arch@xxxxxxxxxxxxxxx>, Waiman Long <longman@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxx>, Alexey Dobriyan <adobriyan@xxxxxxxxx>, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>, Tom Lendacky <thomas.lendacky@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, Linux PM list <linux-pm@xxxxxxxxxxxxxxx>, "H . J . Lu" <hjl.tools@xxxxxxxxx>, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, Jason Baron <jbaron@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Josh Poimboeuf <jpoimboe@xxxxxxxxxx>, Alok Kataria <akataria@xxxxxxxxxx>, Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Dou Liyang <douly.fnst@xxxxxxxxxxxxxx>, Paul Bolle <pebolle@xxxxxxxxxx>, "Paul E . McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Baoquan He <bhe@xxxxxxxxxx>, Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>, kvm list <kvm@xxxxxxxxxxxxxxx>, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>, "Rafael J . Wysocki" <rjw@xxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, "Luis R . Rodriguez" <mcgrof@xxxxxxxxxx>, Lukas Wunner <lukas@xxxxxxxxx>, Linux Crypto Mailing List <linux-crypto@xxxxxxxxxxxxxxx>, Rob Landley <rob@xxxxxxxxxxx>, Jiri Kosina <jkosina@xxxxxxx>, Tejun Heo <tj@xxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, "David S . Miller" <davem@xxxxxxxxxxxxx>, "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>, Joerg Roedel <joro@xxxxxxxxxx>
- Delivery-date: Thu, 05 Oct 2017 16:11:52 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xen.org>
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.
-- Steve
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