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

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Sep 06, 2022 at 05:18:54PM +0200, Ard Biesheuvel wrote:
> 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().

We could right now, but I'm intending to fix some noinstr issues (and make
debugging easier) by splitting the alternatives sanity-checking & patching into
distinct initcalls (alnog with some extra debug), and having the structure for
the common definition is quite nice to avoid open-coding the start and end
value in a bunch of places.

So I'd prefer to keep it for now, but I can follow up and delete it if the
above doesn't turn out to need it, if that's ok?

Mark.

> > 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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