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

Catalin Marinas
2019-08-22T15:55:32Z

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.

That said, on_sig_stack() probably needs some untagging as it does user
pointer arithmetics with potentially different tags.

> > 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.

--
Catalin