Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt

[Dave Martin]

On Thu, Jun 13, 2019 at 01:28:21PM +0100, Catalin Marinas wrote:
> On Thu, Jun 13, 2019 at 12:37:32PM +0100, Dave P Martin wrote:
> > On Thu, Jun 13, 2019 at 11:15:34AM +0100, Vincenzo Frascino wrote:
> > > On 12/06/2019 16:35, Catalin Marinas wrote:
> > > > On Wed, Jun 12, 2019 at 03:21:10PM +0100, Vincenzo Frascino wrote:
> > > >> +  - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged
> > > >> +                             Address ABI.
> [...]
> > Is there a canonical way to detect whether this whole API/ABI is
> > available?  (i.e., try to call this prctl / check for an HWCAP bit,
> > etc.)
> 
> The canonical way is a prctl() call. HWCAP doesn't make sense since it's
> not a hardware feature. If you really want a different way of detecting
> this (which I don't think it's worth), we can reinstate the AT_FLAGS
> bit.

Sure, I think this probably makes sense -- I'm still getting my around
which parts of the design are directly related to MTE and which aren't.

I was a bit concerned about the interaction between
PR_SET_TAGGED_ADDR_CTRL and the sysctl: the caller might conclude that
this API is unavailable when actually tagged addresses are stuck on.

I'm not sure whether this matters, but it's a bit weird.

One option would be to change the semantics, so that the sysctl just
forbids turning tagging from off to on.  Alternatively, we could return
a different error code to distinguish this case.

Or we just leave it as proposed.

Cheers
---Dave