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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Wed, 19 Feb 2020 10:01:14 -0800
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Subject: Re: [PATCH v8 04/12] scs: disable when function graph tracing is enabled
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        James Morse <james.morse@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
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On Wed, Feb 19, 2020 at 3:34 AM Mark Rutland <mark.rutland@arm.com> wrote:
> Fangrui Song has implemented `-fpatchable-function-entry` in LLVM (for
> 10.x onwards), so we can support this when DYNAMIC_FTRACE_WITH_REGS is
> selected.
>
> This can be:
>
>         depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
>
> ... and we can update the commit message to something like:
>
> | With SCS the return address is taken from the shadow stack and the
> | value in the frame record has no effect. The mcount based graph tracer
> | hooks returns by modifying frame records on the (regular) stack, and
> | thus is not compatible. The patchable-function-entry graph tracer
> | used for DYNAMIC_FTRACE_WITH_REGS modifies the LR before it is saved
> | to the shadow stack, and is compatible.
> |
> | Modifying the mcount based graph tracer to work with SCS would require
> | a mechanism to determine the corresponding slot on the shadow stack
> | (and to pass this through the ftrace infrastructure), and we expect
> | that everyone will eventually move to the patchable-function-entry
> | based graph tracer anyway, so for now let's disable SCS when the
> | mcount-based graph tracer is enabled.
> |
> | SCS and patchable-function-entry are both supported from LLVM 10.x.
>
> Assuming you're happy with that:
>
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Great, thanks for pointing that out! This looks good to me, I'll use this in v9.

Sami