Re: [PATCH v2] uaccess: add noop untagged_addr definition

[Andrey Konovalov]

On Tue, Jun 4, 2019 at 2:28 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Tue, Jun 04, 2019 at 02:04:47PM +0200, Andrey Konovalov wrote:
> > Architectures that support memory tagging have a need to perform untagging
> > (stripping the tag) in various parts of the kernel. This patch adds an
> > untagged_addr() macro, which is defined as noop for architectures that do
> > not support memory tagging. The oncoming patch series will define it at
> > least for sparc64 and arm64.
> >
> > Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Reviewed-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> >  include/linux/mm.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 0e8834ac32b7..dd0b5f4e1e45 100644
> > +++ b/include/linux/mm.h
> > @@ -99,6 +99,17 @@ extern int mmap_rnd_compat_bits __read_mostly;
> >  #include <asm/pgtable.h>
> >  #include <asm/processor.h>
> >
> > +/*
> > + * Architectures that support memory tagging (assigning tags to memory regions,
> > + * embedding these tags into addresses that point to these memory regions, and
> > + * checking that the memory and the pointer tags match on memory accesses)
> > + * redefine this macro to strip tags from pointers.
> > + * It's defined as noop for arcitectures that don't support memory tagging.
> > + */
> > +#ifndef untagged_addr
> > +#define untagged_addr(addr) (addr)
>
> Can you please make this a static inline instead of this macro? Then
> we can actually know what the input/output types are supposed to be.
>
> Is it
>
> static inline unsigned long untagged_addr(void __user *ptr) {return ptr;}
>
> ?
>
> Which would sort of make sense to me.

Hm, I'm not sure. arm64 specifically defines this as a macro that
works on different kinds of pointer compatible types to avoid casting
everywhere it's used:

https://elixir.bootlin.com/linux/v5.1.7/source/arch/arm64/include/asm/memory.h#L214

>
> Jason