Re: [PATCH v3 19/23] arm64: mte: Add PTRACE_{PEEK,POKE}MTETAGS support

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Apr 29, 2020 at 11:27:10AM +0100, Kevin Brodsky wrote:
> On 21/04/2020 15:25, Catalin Marinas wrote:
> > diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
> > index bd51ea7e2fcb..45be04a8c73c 100644
> > --- a/arch/arm64/lib/mte.S
> > +++ b/arch/arm64/lib/mte.S
> > @@ -5,6 +5,7 @@
> >   #include <linux/linkage.h>
> >   #include <asm/assembler.h>
> > +#include <asm/mte.h>
> >   /*
> >    * Compare tags of two pages
> > @@ -44,3 +45,52 @@ SYM_FUNC_START(mte_memcmp_pages)
> >   	ret
> >   SYM_FUNC_END(mte_memcmp_pages)
> > +
> > +/*
> > + * Read tags from a user buffer (one tag per byte) and set the corresponding
> > + * tags at the given kernel address. Used by PTRACE_POKEMTETAGS.
> > + *   x0 - kernel address (to)
> > + *   x1 - user buffer (from)
> > + *   x2 - number of tags/bytes (n)
> > + * Returns:
> > + *   x0 - number of tags read/set
> > + */
> > +SYM_FUNC_START(mte_copy_tags_from_user)
> > +	mov	x3, x1
> > +1:
> > +USER(2f, ldtrb	w4, [x1])
> 
> Here we are making either of the following assumptions:
> 1. The __user pointer (here `from`) actually points to user memory, not
> kernel memory (and we have set_fs(USER_DS) in place).
> 2. CONFIG_ARM64_UAO is enabled and the hardware implements UAO.
> 
> 1. is currently true because these functions are only used for the new
> ptrace requests, which indeed pass pointers to user memory. However, future
> users of these functions may not know about this requirement.
> 2. is not necessarily true because ARM64_MTE does not depend on ARM64_UAO.
> 
> It is unlikely that future users of these functions actually need to pass
> __user pointers to kernel memory, so adding a comment spelling out the first
> assumption is probably fine.

I found it easier to add uao_user_alternative rather than writing a
comment ;).

Thanks.

-- 
Catalin