[PATCH] arm64: fix strlen() with CONFIG_KASAN_HW_TAGS

[Catalin Marinas]

On Mon, Jul 12, 2021 at 11:24:14AM +0200, Marco Elver wrote:
> On Mon, 12 Jul 2021 at 11:00, Mark Rutland <mark.rutland at arm.com> wrote:
> > diff --git a/arch/arm64/lib/strlen.S b/arch/arm64/lib/strlen.S
> > index 35fbdb7d6e1a..1648790e91b3 100644
> > --- a/arch/arm64/lib/strlen.S
> > +++ b/arch/arm64/lib/strlen.S
> > @@ -8,6 +8,7 @@
> >
> >  #include <linux/linkage.h>
> >  #include <asm/assembler.h>
> > +#include <asm/mte-def.h>
> >
> >  /* Assumptions:
> >   *
> > @@ -42,7 +43,16 @@
> >  #define REP8_7f 0x7f7f7f7f7f7f7f7f
> >  #define REP8_80 0x8080808080808080
> >
> > +/*
> > + * When KASAN_HW_TAGS is in use, memory is checked at MTE_GRANULE_SIZE
> > + * (16-byte) granularity, and we must ensure that no access straddles this
> > + * alignment boundary.
> > + */
> > +#ifdef CONFIG_KASAN_HW_TAGS
> > +#define MIN_PAGE_SIZE MTE_GRANULE_SIZE
> > +#else
> >  #define MIN_PAGE_SIZE 4096
> > +#endif
> >
> >         /* Since strings are short on average, we check the first 16 bytes
> >            of the string for a NUL character.  In order to do an unaligned ldp
> 
> Glancing at the code below this hunk, I see some hard coded 16,
> presumably referencing the comment here. Is the algorithm tolerant to
> MIN_PAGE_SIZE<16? I suppose that's only an issue if the granule size
> might become less than 16 for some config in future.

It's probably not tolerant as we have a cmp with MIN_PAGE_SIZE - 16. But
I'm not worried, the architecture won't go below this without some
explicit opt-in. Anyway, we can add an #error MTE_GRANULE_SIZE < 16,
just in case anyone plays with this macro.

-- 
Catalin