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

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

On Mon, May 18, 2020 at 02:12:24PM -0300, Luis Machado wrote:
> On 5/18/20 1:47 PM, Dave Martin wrote:
> > Wrinkle: just because MTE is "off", pages might still be mapped with
> > PROT_MTE and have arbitrary tags set on them, and the debugger perhaps
> > needs a way to know that.  Currently grubbing around in /proc is the
> > only way to discover that.  Dunno whether it matters.
> 
> That is the sort of thing that may confused the debugger.
> 
> If MTE is "off" (and thus the debugger doesn't need to validate tags), then
> the pages mapped with PROT_MTE that show up in /proc/<pid>/smaps should be
> ignored?

There is no such thing as global MTE "off". If the HWCAP is present, a
user program can map an address with PROT_MTE and access tags. Maybe it
uses it for extra storage, you never know, doesn't have to be heap
allocation related.

> I'm looking for a precise way to tell if MTE is being used or not for a
> particular process/thread. This, in turn, will tell debuggers when to look
> for PROT_MTE mappings in /proc/<pid>/smaps and when to validate tagged
> addresses.
> 
> So far my assumption was that MTE will always be "on" when HWCAP2_MTE is
> present. So having HWCAP2_MTE means we have the NT_ARM_MTE regset and that
> PROT_MTE pages have to be checked.

Yes. I haven't figured out what to put in the regset yet, most likely
the prctl value as it has other software-only controls like the tagged
address ABI.

-- 
Catalin