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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Fri, 5 Oct 2018 17:24:03 +0200
Message-ID: <CAKv+Gu9z503ZJXcsG+9Ys240BZiB-+GB=kNY4VGWpjvdzN4JtA@mail.gmail.com>
Subject: Re: [RFC PATCH 1/9] kernel: add support for patchable function pointers
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        Eric Biggers <ebiggers@kernel.org>,
        Samuel Neves <sneves@dei.uc.pt>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Richard Weinberger <richard@nod.at>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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On 5 October 2018 at 17:08, Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>> On Oct 5, 2018, at 7:14 AM, Peter Zijlstra <peterz@infradead.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 =3D &_def;                    \
>>> +    struct ffp const __ffp_ ## _fn                    \
>>> +        =3D { (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?
>
> I=E2=80=99m also confused.
>
> Anyway, we have patchable functions on x86. They=E2=80=99re called PVOPs,=
 and they=E2=80=99re way overcomplicated.
>
> I=E2=80=99ve proposed a better way that should generate better code, be m=
ore portable, and be more maintainable.  It goes like this.
>
> To call the function, you literally just call  the default implementation=
.  It *might* be necessary to call a nonexistent wrapper to avoid annoying =
optimizations. At build time, the kernel is built with relocations, so the =
object files contain relocation entries for the call. We collect these entr=
ies into a table. If we=E2=80=99re using the =E2=80=9Cnonexistent wrapper=
=E2=80=9D approach, we can link in a .S or linker script to alias them to t=
he default implementation.
>
> To patch them, we just patch them. It can=E2=80=99t necessarily be done c=
oncurrently because nothing forces the right alignment. But we can do it at=
 boot time and module load time. (Maybe we can patch at runtime on architec=
tures with appropriate instruction alignment.  Or we ask gcc for an extensi=
on to align calls to a function.)
>
> Most of the machinery already exists: this is roughly how the module load=
er resolves calls outside of a module.

Yeah nothing is ever simple on x86 :-(

So are you saying the approach i use in patch #2 (which would
translate to emitting a jmpq instruction pointing to the default
implementation, and patching it at runtime to point elsewhere) would
not fly on x86?