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

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Message-ID: <CAKv+Gu-Gkf2jJ+Nn65pT=HDH6S8wceuXU_RrDpatc=biNx9ROA@mail.gmail.com>
Date:   Fri, 5 Oct 2018 16:57:57 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux Kernel Mailing List <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>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH 1/9] kernel: add support for patchable function pointers

On 5 October 2018 at 16:14, Peter Zijlstra <peterz@...radead.org> wrote:
> 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?

This is the generic implementation, and in this case, it is just a
function pointer which gets dereferenced and called indirectly at each
call site.

In the arch specific implementations (for ones where it matters, see
2/9), it is more like a PLT entry, which gets called from each call
site, and which can be patched to point to another function.

So we replace a single indirect call with a direct call plus a direct jump.