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Date: Tue, 29 Oct 2019 10:45:22 -0700
From: Sami Tolvanen <>
To: Mark Rutland <>, Nick Desaulniers <>
Cc: Will Deacon <>, Catalin Marinas <>, 
	Steven Rostedt <>, Masami Hiramatsu <>, 
	Ard Biesheuvel <>, Dave Martin <>, 
	Kees Cook <>, Laura Abbott <>, Jann Horn <>, 
	Miguel Ojeda <>, 
	Masahiro Yamada <>, 
	clang-built-linux <>, 
	Kernel Hardening <>, 
	linux-arm-kernel <>, LKML <>
Subject: Re: [PATCH v2 09/17] arm64: disable function graph tracing with SCS

On Fri, Oct 25, 2019 at 4:03 AM Mark Rutland <> wrote:
> I'm guessing it's difficult to always figure out the SCS slot for an
> instrumented callsite unless we pass this explicitly from the ftrace
> entry code, so we'd probably have to change some common infrastructure
> for that.
> We have a similar issue with pointer authentication, and we're solving
> that with -fpatchable-function-entry, which allows us to hook the
> callsite before it does anything with the return address. IIUC we could
> use the same mechanism here (and avoid introducing a third).
> Are there plans to implement -fpatchable-function-entry on the clang
> side?

I'm not sure if there are plans at the moment, but if this feature is
needed for PAC, adding it to clang shouldn't be a problem. Nick, did
you have any thoughts on this?


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