On Fri, Aug 23, 2019 at 05:19:13PM +0100, Catalin Marinas wrote:

On Thu, Aug 22, 2019 at 05:37:23PM +0100, Dave P Martin wrote:

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On Thu, Aug 22, 2019 at 04:55:32PM +0100, Catalin Marinas wrote:

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On Wed, Aug 21, 2019 at 07:46:51PM +0100, Dave P Martin wrote:

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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.

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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.

Cool...

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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 ;).

[reply deleted]

Cheers
---Dave