linux-kernel - Re: [RFC PATCH 1/9] kernel: add support for patchable function pointers

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Date:   Fri, 5 Oct 2018 16:14:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-kernel@...r.kernel.org,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Samuel Neves <sneves@....uc.pt>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Richard Weinberger <richard@....at>,
        linux-crypto@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 1/9] kernel: add support for patchable function
 pointers

On Fri, Oct 05, 2018 at 10:13:25AM +0200, Ard Biesheuvel wrote:
> diff --git a/include/linux/ffp.h b/include/linux/ffp.h
> new file mode 100644
> index 000000000000..8fc3b4c9b38f
> --- /dev/null
> +++ b/include/linux/ffp.h
> @@ -0,0 +1,43 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __LINUX_FFP_H
> +#define __LINUX_FFP_H
> +
> +#include <linux/types.h>
> +#include <linux/compiler.h>
> +
> +#ifdef CONFIG_HAVE_ARCH_FFP
> +#include <asm/ffp.h>
> +#else
> +
> +struct ffp {
> +	void	(**fn)(void);
> +	void	(*default_fn)(void);
> +};
> +
> +#define DECLARE_FFP(_fn, _def)						\
> +	extern typeof(_def) *_fn;					\
> +	extern struct ffp const __ffp_ ## _fn
> +
> +#define DEFINE_FFP(_fn, _def)						\
> +	typeof(_def) *_fn = &_def;					\
> +	struct ffp const __ffp_ ## _fn					\
> +		= { (void(**)(void))&_fn, (void(*)(void))&_def };	\
> +	EXPORT_SYMBOL(__ffp_ ## _fn)
> +
> +static inline void ffp_set_target(const struct ffp *m, void *new_fn)
> +{
> +	WRITE_ONCE(*m->fn, new_fn);
> +}
> +
> +static inline void ffp_reset_target(const struct ffp *m)
> +{
> +	WRITE_ONCE(*m->fn, m->default_fn);
> +}
> +
> +#endif
> +
> +#define SET_FFP(_fn, _new)	ffp_set_target(&__ffp_ ## _fn, _new)
> +#define RESET_FFP(_fn)		ffp_reset_target(&__ffp_ ## _fn)
> +
> +#endif

I don't understand this interface. There is no wrapper for the call
site, so how are we going to patch all call-sites when you update the
target?