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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 09:06:33 -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@xxxxxxxxxxxxxxxxxx, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>, Christoph Lameter <cl@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Chris Metcalf <cmetcalf@xxxxxxxxxxxx>, 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@xxxxxxxxxxxxxxxxxxxx, linux-sparse@xxxxxxxxxxxxxxx, Matthias Kaehlcke <mka@xxxxxxxxxxxx>, 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@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@xxxxxxxxxxxxxxx, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>, "Rafael J . Wysocki" <rjw@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, "Luis R . Rodriguez" <mcgrof@xxxxxxxxxx>, Lukas Wunner <lukas@xxxxxxxxx>, 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 13:06:49 +0000
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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?
-- Steve
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