linux-kernel - Re: [PATCH v8 4/5] arm64: implement ftrace with regs

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Date:   Wed, 13 Feb 2019 10:30:12 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Torsten Duwe <duwe@....de>, Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v8 4/5] arm64: implement ftrace with regs

Hi Torsten,

On 08/02/2019 15:10, Torsten Duwe wrote:
> Implement ftrace with regs, based on the new gcc flag
> -fpatchable-function-entry (=2)
> 
> Now that gcc8 added 2 NOPs at the beginning of each function, replace
> the first NOP thus generated with a quick LR saver (move it to scratch
> reg x9), so the 2nd replacement insn, the call to ftrace, does not
> clobber the value. Ftrace will then generate the standard stack
> frames.
> 
> Note that patchable-function-entry in GCC disables IPA-RA, which means
> ABI register calling conventions are obeyed and scratch registers such
> as x9 are available.
> 
> Introduce and handle an ftrace_regs_trampoline for module PLTs, right
> after ftrace_trampoline in an ftrace_trampolines[2] array, and double
> the size of the corresponding special section.
> 
> Signed-off-by: Torsten Duwe <duwe@...e.de>
> 
> ---
>  arch/arm64/include/asm/ftrace.h  |   16 ++++
>  arch/arm64/include/asm/module.h  |    3 
>  arch/arm64/kernel/entry-ftrace.S |  125 +++++++++++++++++++++++++++++++++++++--
>  arch/arm64/kernel/ftrace.c       |  117 ++++++++++++++++++++++++++++--------
>  arch/arm64/kernel/module-plts.c  |    3 
>  arch/arm64/kernel/module.c       |    2 
>  6 files changed, 231 insertions(+), 35 deletions(-)

[...]

> --- a/arch/arm64/include/asm/module.h
> +++ b/arch/arm64/include/asm/module.h
> @@ -32,7 +32,8 @@ struct mod_arch_specific {
>  	struct mod_plt_sec	init;
>  
>  	/* for CONFIG_DYNAMIC_FTRACE */
> -	struct plt_entry 	*ftrace_trampoline;
> +	struct plt_entry	*ftrace_trampolines;
> +#define MOD_ARCH_NR_FTRACE_TRAMPOLINES	2

Nit: I'd define this outside of the struct, just below
MODULE_ARCH_VERMAGIC, and use the prefix "MODULE_ARCH" instead of
"MOD_ARCH" as this seem to be already in use for a few other defines.

Otherwise things look good to me:

Reviewed-by: Julien Thierry <julien.thierry@....com>

Cheers,

-- 
Julien Thierry