linux-kernel - Re: [PATCH 4/5] x86: jump_label: switch to jump

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Message-ID: <CAKv+Gu8OuxSWEWA0o+b+DaPLW8tSx-R6K4_KfomZSBKxqSuukA@mail.gmail.com>
Date:   Thu, 28 Jun 2018 11:14:34 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 4/5] x86: jump_label: switch to jump_entry accessors

On 28 June 2018 at 11:11, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Jun 27, 2018 at 06:06:03PM +0200, Ard Biesheuvel wrote:
>> In preparation of switching x86 to use place-relative references for
>> the code, target and key members of struct jump_entry, replace direct
>> references to the struct members with invocations of the new accessors.
>> This will allow us to make the switch by modifying the accessors only.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>
> That just makes horrible code worse.. can't we do something like so
> instead?
>

Sure, I will incorporate that for v2.

>
> --- a/arch/x86/kernel/jump_label.c
> +++ b/arch/x86/kernel/jump_label.c
> @@ -42,52 +42,37 @@ static void __jump_label_transform(struc
>                                    void *(*poker)(void *, const void *, size_t),
>                                    int init)
>  {
> -       union jump_code_union code;
> +       union jump_code_union jmp = {
> +               { .jump = 0xe9,
> +                 .offset = jump_entry_target(entry) -
> +                           (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE), }
> +       };
>         const unsigned char default_nop[] = { STATIC_KEY_INIT_NOP };
>         const unsigned char *ideal_nop = ideal_nops[NOP_ATOMIC5];
> +       const unsigned char *expect, *code;
> +       int line;
>
>         if (type == JUMP_LABEL_JMP) {
>                 if (init) {
> -                       /*
> -                        * Jump label is enabled for the first time.
> -                        * So we expect a default_nop...
> -                        */
> -                       if (unlikely(memcmp((void *)entry->code, default_nop, 5)
> -                                    != 0))
> -                               bug_at((void *)entry->code, __LINE__);
> +                       expect = default_nop; line = __LINE__;
>                 } else {
> -                       /*
> -                        * ...otherwise expect an ideal_nop. Otherwise
> -                        * something went horribly wrong.
> -                        */
> -                       if (unlikely(memcmp((void *)entry->code, ideal_nop, 5)
> -                                    != 0))
> -                               bug_at((void *)entry->code, __LINE__);
> +                       expect = ideal_nop; line = __LINE__;
>                 }
>
> -               code.jump = 0xe9;
> -               code.offset = entry->target -
> -                               (entry->code + JUMP_LABEL_NOP_SIZE);
> +               code = &jmp.code;
>         } else {
> -               /*
> -                * We are disabling this jump label. If it is not what
> -                * we think it is, then something must have gone wrong.
> -                * If this is the first initialization call, then we
> -                * are converting the default nop to the ideal nop.
> -                */
>                 if (init) {
> -                       if (unlikely(memcmp((void *)entry->code, default_nop, 5) != 0))
> -                               bug_at((void *)entry->code, __LINE__);
> +                       expect = default_nop; line = __LINE__;
>                 } else {
> -                       code.jump = 0xe9;
> -                       code.offset = entry->target -
> -                               (entry->code + JUMP_LABEL_NOP_SIZE);
> -                       if (unlikely(memcmp((void *)entry->code, &code, 5) != 0))
> -                               bug_at((void *)entry->code, __LINE__);
> +                       expect = &jmp.code; line = __LINE__;
>                 }
> -               memcpy(&code, ideal_nops[NOP_ATOMIC5], JUMP_LABEL_NOP_SIZE);
> +
> +               code = ideal_nop;
>         }
>
> +       if (memcmp((void *)jump_entry_code(entry), expect, JUMP_LABEL_NOP_SIZE))
> +               bug_at((void *)jump_entry_code(entry), line);
> +
>         /*
>          * Make text_poke_bp() a default fallback poker.
>          *
> @@ -96,11 +81,13 @@ static void __jump_label_transform(struc
>          * always nop being the 'currently valid' instruction
>          *
>          */
> -       if (poker)
> -               (*poker)((void *)entry->code, &code, JUMP_LABEL_NOP_SIZE);
> -       else
> -               text_poke_bp((void *)entry->code, &code, JUMP_LABEL_NOP_SIZE,
> -                            (void *)entry->code + JUMP_LABEL_NOP_SIZE);
> +       if (poker) {
> +               (*poker)((void *)jump_entry_code(entry), &code, JUMP_LABEL_NOP_SIZE);
> +               return;
> +       }
> +
> +       text_poke_bp((void *)jump_entry_code(entry), &code, JUMP_LABEL_NOP_SIZE,
> +                    (void *)jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>  }
>
>  void arch_jump_label_transform(struct jump_entry *entry,