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

[Marc Zyngier]

On 2021-01-25 12:54, Ard Biesheuvel wrote:
> On Mon, 25 Jan 2021 at 11:53, Marc Zyngier <maz@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Acked-by: David Brazdil <dbrazdil@xxxxxxxxxx>
> > ---
> >  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! ;-).

Thanks,

        M.

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...