From: Dave Martin <Dave.Martin@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arch@vger.kernel.org, linux-doc@vger.kernel.org,
Szabolcs Nagy <szabolcs.nagy@arm.com>,
Andrey Konovalov <andreyknvl@google.com>,
Kevin Brodsky <kevin.brodsky@arm.com>,
Will Deacon <will.deacon@arm.com>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Will Deacon <will@kernel.org>,
Dave Hansen <dave.hansen@intel.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v9 3/3] arm64: Relax Documentation/arm64/tagged-pointers.rst
Date: Thu, 22 Aug 2019 17:37:23 +0100 [thread overview]
Message-ID: <20190822163723.GF27757@arm.com> (raw)
In-Reply-To: <20190822155531.GB55798@arrakis.emea.arm.com>
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.
> That said, on_sig_stack() probably needs some untagging as it does user
> pointer arithmetics with potentially different tags.
Good point.
> > > 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 ;)
There are other cases like (non-)propagation of the tag to si_addr
when a fault is reported via a signal, but I think we already have
appropriate wording to cover that.
Cheers
---Dave
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next prev parent reply other threads:[~2019-08-22 16:37 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-21 16:47 [PATCH v9 0/3] arm64 tagged address ABI Catalin Marinas
2019-08-21 16:47 ` [PATCH v9 1/3] mm: untag user pointers in mmap/munmap/mremap/brk Catalin Marinas
2019-08-21 16:47 ` [PATCH v9 2/3] arm64: Define Documentation/arm64/tagged-address-abi.rst Catalin Marinas
2019-08-21 16:57 ` Andrey Konovalov
2019-08-22 9:38 ` Kevin Brodsky
2019-08-21 17:35 ` Will Deacon
2019-08-22 14:17 ` [PATCH] arm64: Add tagged-address-abi.rst to index.rst Vincenzo Frascino
2019-08-21 16:47 ` [PATCH v9 3/3] arm64: Relax Documentation/arm64/tagged-pointers.rst Catalin Marinas
2019-08-21 17:33 ` Will Deacon
2019-08-21 18:46 ` Dave Martin
2019-08-22 15:55 ` Catalin Marinas
2019-08-22 16:37 ` Dave Martin [this message]
2019-08-23 16:19 ` Catalin Marinas
2019-08-23 16:32 ` Dave Martin
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