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

From: Peter Zijlstra <[email-masked]>
Date: Fri, 5 Oct 2018 16:14:33 +0200

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?