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

Catalin Marinas
2019-08-23T16:19:18Z

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:
> > > On Wed, Aug 21, 2019 at 06:33:53PM +0100, Will Deacon wrote:
> > > > On Wed, Aug 21, 2019 at 05:47:30PM +0100, Catalin Marinas wrote:
> > > > > @@ -59,6 +63,11 @@ be preserved.
> > > > > The architecture prevents the use of a tagged PC, so the upper byte will
> > > > > be set to a sign-extension of bit 55 on exception return.
> > > > >
> > > > > +This behaviour is maintained when the AArch64 Tagged Address ABI is
> > > > > +enabled. In addition, with the exceptions above, the kernel will
> > > > > +preserve any non-zero tags passed by the user via syscalls and stored in
> > > > > +kernel data structures (e.g. ``set_robust_list()``, ``sigaltstack()``).
> > >
> > > 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.

> > > > Hmm. I can see the need to provide this guarantee for things like
> > > > set_robust_list(), but the problem is that the statement above is too broad
> > > > and isn't strictly true: for example, mmap() doesn't propagate the tag of
> > > > its address parameter into the VMA.
> > > >
> > > > So I think we need to nail this down a bit more, but I'm having a really
> > > > hard time coming up with some wording :(
> > >
> > > Time for some creative vagueness?
> > >
> > > We can write a statement of our overall intent, along with examples of
> > > a few cases where the tag should and should not be expected to emerge
> > > intact.
> > >
> > > 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 ;).

--
Catalin