[PATCH v9 29/29] arm64: mte: Add Memory Tagging Extension documentation

[Will Deacon]

On Thu, Sep 17, 2020 at 05:15:53PM +0100, Dave Martin wrote:
> On Thu, Sep 17, 2020 at 10:02:30AM +0100, Catalin Marinas wrote:
> > On Thu, Sep 17, 2020 at 09:11:08AM +0100, Will Deacon wrote:
> > > On Fri, Sep 04, 2020 at 11:30:29AM +0100, Catalin Marinas wrote:
> > > > From: Vincenzo Frascino <vincenzo.frascino at arm.com>
> > > > 
> > > > Memory Tagging Extension (part of the ARMv8.5 Extensions) provides
> > > > a mechanism to detect the sources of memory related errors which
> > > > may be vulnerable to exploitation, including bounds violations,
> > > > use-after-free, use-after-return, use-out-of-scope and use before
> > > > initialization errors.
> > > > 
> > > > Add Memory Tagging Extension documentation for the arm64 linux
> > > > kernel support.
> > > > 
> > > > Signed-off-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
> > > > Co-developed-by: Catalin Marinas <catalin.marinas at arm.com>
> > > > Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> > > > Acked-by: Szabolcs Nagy <szabolcs.nagy at arm.com>
> > > 
> > > I'm taking this to mean that Szabolcs is happy with the proposed ABI --
> > > please shout if that's not the case!
> > 
> > I think Szabolcs is still on holiday. To summarise the past threads,
> > AFAICT he's happy with this per-thread control ABI but the discussion
> > went on whether to expand it in the future (with a new bit) to
> > synchronise the tag checking mode across all threads of a process. This
> > adds some complications for the kernel as it needs an IPI to the other
> > CPUs to set SCTLR_EL1 and it's also racy with multiple threads
> > requesting different modes.
> > 
> > Now, in the glibc land, if the tag check mode is controlled via
> > environment variables, the dynamic loader can set this at process start
> > while still in single-threaded mode and not touch it at run-time. The
> > MTE checking can still be enabled at run-time, per mapped memory range
> > via the PROT_MTE flag. This approach doesn't require any additional
> > changes to the current patches. But it's for Szabolcs to confirm once
> > he's back.
> > 
> > > Wasn't there a man page kicking around too? Would be good to see that
> > > go upstream (to the manpages project, of course).
> > 
> > Dave started writing one for the tagged address ABI, not sure where that
> > is. For the MTE additions, we are waiting for the ABI to be upstreamed.
> 
> The tagged address ABI control stuff is upstream in the man-pages-5.08
> release.
> 
> I don't think anyone drafted anything for MTE yet.  Do we consider the
> MTE ABI to be sufficiently stable now for it to be worth starting
> drafting something?

I think so, yes. I'm hoping to queue it for 5.10, once I have an Ack from
the Android tools side on the per-thread ABI.

Will