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

[Will Deacon]

On Wed, Oct 14, 2020 at 04:43:23PM -0700, Peter Collingbourne wrote:
> On Fri, Sep 18, 2020 at 1:30 AM Will Deacon <will at kernel.org> wrote:
> > 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.
> 
> Our main requirement on the Android side is to provide an API for
> changing the tag checking mode in all threads in a process while
> multiple threads are running. I think we've been able to accomplish
> this [1] by using a libc private real-time signal which is sent to all
> threads. The implementation has been tested on FVP via the included
> unit tests. The code has also been tested on real hardware in a
> multi-threaded app process (of course we don't have MTE-enabled
> hardware, so the implementation was tested on hardware by hacking it
> to disable the tagged address ABI instead of changing the tag checking
> mode, and then verifying via ptrace(PTRACE_GETREGSET) that the tagged
> address ABI was disabled in all threads).
> 
> That being said, as with any code at the nexus of concurrency and
> POSIX signals, the implementation is quite tricky so I would say it
> falls more into the category of "no obvious problems" than "obviously
> no problems". It also relies on changes to the implementations of
> pthread APIs so it wouldn't catch threads created directly via clone()
> rather than via pthread_create(). I think we would be able to ignore
> such threads on Android without causing compatibility issues because
> we can require the process to not create threads via clone() before
> calling the function. I imagine this may not necessarily work for
> other libcs like glibc, though, but as I understand it glibc has no
> plan to offer such an API.
> 
> I feel confident enough in the kernel API though that I think that
> it's reasonable as a starting point at least, and that if a problem
> with the API is discovered I would expect it to be fixable by adding
> new APIs, so:
> 
> Acked-by: Peter Collingbourne <pcc at google.com>

Thanks, Peter. This series has already landed upstream, so I'm unable to
add your Ack now, but the text above is very helpful.

Cheers,

Will

> [1] https://android-review.googlesource.com/c/platform/bionic/+/1427377