Subject: Re: [PATCH v7 29/29] arm64: mte: Add Memory Tagging Extension documentation

From: Catalin Marinas <[email-masked]>
Date: Sat, 22 Aug 2020 12:31:38 +0100

On Thu, Aug 20, 2020 at 10:27:43AM -0700, Paul Eggert wrote:
> On 8/20/20 9:43 AM, Szabolcs Nagy wrote:
> > the compat issue with this is existing code
> > using pointer top bits which i assume faults
> > when dereferenced with the mte checks enabled.
> > (although this should be very rare since
> > top byte ignore on deref is aarch64 specific.)
> 
> Does anyone know of significant aarch64-specific application code that
> depends on top byte ignore? I would think it's so rare (nonexistent?) as to
> not be worth worrying about.

Apart from the LLVM hwasan feature, I'm not aware of code relying on the
top byte ignore. There were discussions in the past to use it with some
JITs but I'm not sure they ever materialised.

I think the Mozilla JS engine uses (used?) additional bits on top of a
pointer but they are masked out before the access.

> Even in the bad old days when Emacs used pointer top bits for typechecking,
> it carefully removed those bits before dereferencing. Any other
> reasonably-portable application would have to do the same of course.

I agree.

-- 
Catalin

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