Re: [PATCH 5/9] arm64: alternatives: make alt_region const

[Ard Biesheuvel <ardb@kernel.org>]

Hi Mark,

On Thu, 1 Sept 2022 at 17:14, Mark Rutland <mark.rutland@arm.com> wrote:
>
> We never alter a struct alt_region after creation, and we open-code the
> bounds of the kernel alternatives region in two functions.
>
> This patch adds a shared struct alt_region, and marks the alt regions as
> const to prevent unintentional modification.
>

What is the point of struct alt_region? Can we just get rid of it
entirely? It seems its only purpose is to carry a <start, end> tuple
that could easily be converted into start and end arguments to
__apply_alternatives().

> There should be no functional change as a result of this patch.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: James Morse <james.morse@arm.com>
> Cc: Joey Gouly <joey.gouly@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kernel/alternative.c | 26 ++++++++++++--------------
>  1 file changed, 12 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index d94d97cb4a0bf..2e18c9c0f612b 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -133,8 +133,9 @@ static void clean_dcache_range_nopatch(u64 start, u64 end)
>         } while (cur += d_size, cur < end);
>  }
>
> -static void __nocfi __apply_alternatives(struct alt_region *region, bool is_module,
> -                                unsigned long *feature_mask)
> +static void __nocfi __apply_alternatives(const struct alt_region *region,
> +                                        bool is_module,
> +                                        unsigned long *feature_mask)
>  {
>         struct alt_instr *alt;
>         __le32 *origptr, *updptr;
> @@ -190,17 +191,17 @@ static void __nocfi __apply_alternatives(struct alt_region *region, bool is_modu
>         }
>  }
>
> +static const struct alt_region kernel_alternatives = {
> +       .begin  = (struct alt_instr *)__alt_instructions,
> +       .end    = (struct alt_instr *)__alt_instructions_end,
> +};
> +
>  /*
>   * We might be patching the stop_machine state machine, so implement a
>   * really simple polling protocol here.
>   */
>  static int __apply_alternatives_multi_stop(void *unused)
>  {
> -       struct alt_region region = {
> -               .begin  = (struct alt_instr *)__alt_instructions,
> -               .end    = (struct alt_instr *)__alt_instructions_end,
> -       };
> -
>         /* We always have a CPU 0 at this point (__init) */
>         if (smp_processor_id()) {
>                 while (!all_alternatives_applied)
> @@ -213,7 +214,8 @@ static int __apply_alternatives_multi_stop(void *unused)
>                                   ARM64_NPATCHABLE);
>
>                 BUG_ON(all_alternatives_applied);
> -               __apply_alternatives(&region, false, remaining_capabilities);
> +               __apply_alternatives(&kernel_alternatives, false,
> +                                    remaining_capabilities);
>                 /* Barriers provided by the cache flushing */
>                 all_alternatives_applied = 1;
>         }
> @@ -236,17 +238,13 @@ void __init apply_alternatives_all(void)
>   */
>  void __init apply_boot_alternatives(void)
>  {
> -       struct alt_region region = {
> -               .begin  = (struct alt_instr *)__alt_instructions,
> -               .end    = (struct alt_instr *)__alt_instructions_end,
> -       };
> -
>         /* If called on non-boot cpu things could go wrong */
>         WARN_ON(smp_processor_id() != 0);
>
>         pr_info("applying boot alternatives\n");
>
> -       __apply_alternatives(&region, false, &boot_capabilities[0]);
> +       __apply_alternatives(&kernel_alternatives, false,
> +                            &boot_capabilities[0]);
>  }
>
>  #ifdef CONFIG_MODULES
> --
> 2.30.2
>

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