Re: [PATCH v9 3/3] arm64: Relax Documentation/arm64/tagged-pointers.rst

From: Dave Martin <Dave.Martin@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arch@vger.kernel.org, linux-doc@vger.kernel.org,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>,
	Dave Hansen <dave.hansen@intel.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v9 3/3] arm64: Relax Documentation/arm64/tagged-pointers.rst
Date: Fri, 23 Aug 2019 17:32:49 +0100	[thread overview]
Message-ID: <20190823163247.GG27757@arm.com> (raw)
In-Reply-To: <20190823161912.GJ29387@arrakis.emea.arm.com>

On Fri, Aug 23, 2019 at 05:19:13PM +0100, Catalin Marinas wrote:
> On Thu, Aug 22, 2019 at 05:37:23PM +0100, Dave P Martin wrote:
> > On Thu, Aug 22, 2019 at 04:55:32PM +0100, Catalin Marinas wrote:
> > > On Wed, Aug 21, 2019 at 07:46:51PM +0100, Dave P Martin wrote:

[...]

> > > > sigaltstack() is interesting, since we don't support tagged stacks.
> > > 
> > > We should support tagged SP with the new ABI as they'll be required for
> > > MTE. sigaltstack() and clone() are the two syscalls that come to mind
> > > here.
> > > 
> > > > Do we keep the ss_sp tag in the kernel, but squash it when delivering
> > > > a signal to the alternate stack?
> > > 
> > > We don't seem to be doing any untagging, so we just just use whatever
> > > the caller asked for. We may need a small test to confirm.
> > 
> > If we want to support tagged SP, then I guess we shouldn't be squashing
> > the tag anywhere.  A test for that would be sensible to have.
> 
> I hacked the sas.c kselftest to use a tagged stack and works fine, the
> SP register has a tagged address on the signal handler.

Cool...

[...]

> > > > There is no foolproof rule, unless we can rewrite history...
> > > 
> > > I would expect the norm to be the preservation of tags with a few
> > > exceptions. The only ones I think where we won't preserve the tags are
> > > mmap, mremap, brk (apart from the signal stuff already mentioned in the
> > > current tagged-pointers.rst doc).
> > > 
> > > So I can remove this paragraph altogether and add a note in part 3 of
> > > the tagged-address-abi.rst document that mmap/mremap/brk do not preserve
> > > the tag information.
> > 
> > Deleting text is always a good idea ;)
> 
> I'm going this route ;).

[reply deleted]

Cheers
---Dave