linux-kernel - Re: [PATCH v7 16/17] arm64: disable recordmcount with DYNAMIC_FTRACE_WITH

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Date:   Mon, 30 Nov 2020 11:59:03 +0000
From:   Will Deacon <will@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v7 16/17] arm64: disable recordmcount with
 DYNAMIC_FTRACE_WITH_REGS

On Wed, Nov 18, 2020 at 02:07:30PM -0800, Sami Tolvanen wrote:
> DYNAMIC_FTRACE_WITH_REGS uses -fpatchable-function-entry, which makes
> running recordmcount unnecessary as there are no mcount calls in object
> files, and __mcount_loc doesn't need to be generated.
> 
> While there's normally no harm in running recordmcount even when it's
> not strictly needed, this won't work with LTO as we have LLVM bitcode
> instead of ELF objects.
> 
> This change selects FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY, which
> disables recordmcount when patchable function entries are used instead.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/arm64/Kconfig | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 1515f6f153a0..c7f07978f5b6 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -158,6 +158,8 @@ config ARM64
>  	select HAVE_DYNAMIC_FTRACE
>  	select HAVE_DYNAMIC_FTRACE_WITH_REGS \
>  		if $(cc-option,-fpatchable-function-entry=2)
> +	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
> +		if DYNAMIC_FTRACE_WITH_REGS

I don't really understand why this is in the arch header file, rather
than have the core code check for "fpatchable-function-entry=2" and expose
a CC_HAS_PATCHABLE_FUNCTION_ENTRY, but in the interest of making some
progress on this series:

Acked-by: Will Deacon <will@...nel.org>

Will