Re: [PATCH v5 18/21] arm64: Move "nokaslr" over to the early cpufeature infrastructure

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 25 Jan 2021 at 14:54, Marc Zyngier <maz@kernel.org> wrote:
>
> On 2021-01-25 12:54, Ard Biesheuvel wrote:
> > On Mon, 25 Jan 2021 at 11:53, Marc Zyngier <maz@kernel.org> wrote:
> >>
> >> Given that the early cpufeature infrastructure has borrowed quite
> >> a lot of code from the kaslr implementation, let's reimplement
> >> the matching of the "nokaslr" option with it.
> >>
> >> Signed-off-by: Marc Zyngier <maz@kernel.org>
> >> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
> >> Acked-by: David Brazdil <dbrazdil@google.com>
> >> ---
> >>  arch/arm64/kernel/idreg-override.c | 15 +++++++++++++
> >>  arch/arm64/kernel/kaslr.c          | 36
> >> ++----------------------------
> >>  2 files changed, 17 insertions(+), 34 deletions(-)
> >>
> >> diff --git a/arch/arm64/kernel/idreg-override.c
> >> b/arch/arm64/kernel/idreg-override.c
> >> index cbb8eaa48742..3ccf51b84ba4 100644
> >> --- a/arch/arm64/kernel/idreg-override.c
> >> +++ b/arch/arm64/kernel/idreg-override.c
> >> @@ -31,8 +31,22 @@ static const struct ftr_set_desc mmfr1 __initdata =
> >> {
> >>         },
> >>  };
> >>
> >> +extern struct arm64_ftr_override kaslr_feature_override;
> >> +
> >> +static const struct ftr_set_desc kaslr __initdata = {
> >
> > This should be __initconst not __initdata (below too)
> >
> >> +       .name           = "kaslr",
> >> +#ifdef CONFIG_RANDOMIZE_BASE
> >> +       .override       = &kaslr_feature_override,
> >> +#endif
> >> +       .fields         = {
> >> +               { "disabled", 0 },
> >> +               {}
> >> +       },
> >> +};
> >> +
> >>  static const struct ftr_set_desc * const regs[] __initdata = {
> >>         &mmfr1,
> >> +       &kaslr,
> >>  };
> >>
> >>  static const struct {
> >> @@ -41,6 +55,7 @@ static const struct {
> >>  } aliases[] __initdata = {
> >>         { "kvm-arm.mode=nvhe",          "id_aa64mmfr1.vh=0" },
> >>         { "kvm-arm.mode=protected",     "id_aa64mmfr1.vh=0" },
> >> +       { "nokaslr",                    "kaslr.disabled=1" },
> >>  };
> >>
> >
> > This struct now takes up
> > - ~100 bytes for the characters themselves (which btw are not emitted
> > into __initdata or __initconst)
> > - 6x8 bytes for the char pointers
> > - 6x24 bytes for the RELA relocations that annotate these pointers as
> > quantities that need to be relocated at boot (on a kernel built with
> > KASLR)
> >
> > I know it's only a drop in the ocean, but in this case, where the
> > struct is statically declared and defined only once, and in the same
> > place, we could easily turn this into
> >
> > static const struct {
> >    char alias[24];
> >    char param[20];
> > };
> >
> > and get rid of all the overhead. The only slightly annoying thing is
> > that the array sizes need to be kept in sync with the largest instance
> > appearing in the array, but this is easy when the struct type is
> > declared in the same place where its only instance is defined.
>
> Fair enough. I personally find the result butt-ugly, but I agree
> that it certainly saves some memory. Does the following work for
> you? I can even give symbolic names to the various constants (how
> generous of me! ;-).
>

To be honest, I was anticipating more of a discussion, but this looks
reasonable to me. Does 'char    feature[80];' really need 80 bytes
though?

> diff --git a/arch/arm64/kernel/idreg-override.c
> b/arch/arm64/kernel/idreg-override.c
> index d1310438d95c..9e7043bdc808 100644
> --- a/arch/arm64/kernel/idreg-override.c
> +++ b/arch/arm64/kernel/idreg-override.c
> @@ -14,15 +14,15 @@
>   #include <asm/setup.h>
>
>   struct ftr_set_desc {
> -       const char                      *name;
> +       char                            name[20];
>         struct arm64_ftr_override       *override;
>         struct {
> -               const char              *name;
> +               char                    name[20];
>                 u8                      shift;
>         }                               fields[];
>   };
>
> -static const struct ftr_set_desc mmfr1 __initdata = {
> +static const struct ftr_set_desc mmfr1 __initconst = {
>         .name           = "id_aa64mmfr1",
>         .override       = &id_aa64mmfr1_override,
>         .fields         = {
> @@ -31,7 +31,7 @@ static const struct ftr_set_desc mmfr1 __initdata = {
>         },
>   };
>
> -static const struct ftr_set_desc pfr1 __initdata = {
> +static const struct ftr_set_desc pfr1 __initconst = {
>         .name           = "id_aa64pfr1",
>         .override       = &id_aa64pfr1_override,
>         .fields         = {
> @@ -40,7 +40,7 @@ static const struct ftr_set_desc pfr1 __initdata = {
>         },
>   };
>
> -static const struct ftr_set_desc isar1 __initdata = {
> +static const struct ftr_set_desc isar1 __initconst = {
>         .name           = "id_aa64isar1",
>         .override       = &id_aa64isar1_override,
>         .fields         = {
> @@ -54,7 +54,7 @@ static const struct ftr_set_desc isar1 __initdata = {
>
>   extern struct arm64_ftr_override kaslr_feature_override;
>
> -static const struct ftr_set_desc kaslr __initdata = {
> +static const struct ftr_set_desc kaslr __initconst = {
>         .name           = "kaslr",
>   #ifdef CONFIG_RANDOMIZE_BASE
>         .override       = &kaslr_feature_override,
> @@ -65,7 +65,7 @@ static const struct ftr_set_desc kaslr __initdata = {
>         },
>   };
>
> -static const struct ftr_set_desc * const regs[] __initdata = {
> +static const struct ftr_set_desc * const regs[] __initconst = {
>         &mmfr1,
>         &pfr1,
>         &isar1,
> @@ -73,9 +73,9 @@ static const struct ftr_set_desc * const regs[]
> __initdata = {
>   };
>
>   static const struct {
> -       const char      *alias;
> -       const char      *feature;
> -} aliases[] __initdata = {
> +       char    alias[30];
> +       char    feature[80];
> +} aliases[] __initconst = {
>         { "kvm-arm.mode=nvhe",          "id_aa64mmfr1.vh=0" },
>         { "kvm-arm.mode=protected",     "id_aa64mmfr1.vh=0" },
>         { "arm64.nobti",                "id_aa64pfr1.bt=0" },
>
> --
> Jazz is not dead. It just smells funny...