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Date: Tue, 5 Nov 2019 14:05:39 -0800
From: Sami Tolvanen <>
To: Mark Rutland <>
Cc: Will Deacon <>, Catalin Marinas <>, 
	Steven Rostedt <>, Masami Hiramatsu <>, 
	Ard Biesheuvel <>, Dave Martin <>, 
	Kees Cook <>, Laura Abbott <>, 
	Marc Zyngier <>, Nick Desaulniers <>, Jann Horn <>, 
	Miguel Ojeda <>, 
	Masahiro Yamada <>, 
	clang-built-linux <>, 
	Kernel Hardening <>, 
	linux-arm-kernel <>, LKML <>
Subject: Re: [PATCH v4 11/17] arm64: disable function graph tracing with SCS

On Tue, Nov 5, 2019 at 11:55 AM Mark Rutland <> wrote:
> On Mon, Nov 04, 2019 at 03:44:39PM -0800, Sami Tolvanen wrote:
> > Sure, I'll add a better description in v5. In this case, the return
> > address is modified in the kernel stack, which means the changes are
> > ignored with SCS.
> Ok, that makes sense to me. I'd suggest something like:
> | The graph tracer hooks returns by modifying frame records on the
> | (regular) stack, but with SCS the return address is taken from the
> | shadow stack, and the value in the frame record has no effect. As we
> | don't currently have a mechanism to determine the corresponding slot
> | on the shadow stack (and to pass this through the ftrace
> | infrastructure), for now let's disable the graph tracer when SCS is
> | enabled.
> ... as I suspect with some rework of the trampoline and common ftrace
> code we'd be able to correctly manipulate the shadow stack for this.
> Similarly, if clang gained -fpatchable-funciton-etnry, we'd get that for
> free.

That sounds good to me. Thanks, Mark.


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