Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory syscalls

Dave Martin
2019-05-29T15:18:50Z

On Wed, May 29, 2019 at 02:23:42PM +0100, Catalin Marinas wrote:
> On Wed, May 29, 2019 at 01:42:25PM +0100, Dave P Martin wrote:
> > On Tue, May 28, 2019 at 05:34:00PM +0100, Catalin Marinas wrote:
> > > On Tue, May 28, 2019 at 04:56:45PM +0100, Dave P Martin wrote:
> > > > On Tue, May 28, 2019 at 04:40:58PM +0100, Catalin Marinas wrote:
> > > >
> > > > [...]
> > > >
> > > > > My thoughts on allowing tags (quick look):
> > > > >
> > > > > brk - no
> > > >
> > > > [...]
> > > >
> > > > > mlock, mlock2, munlock - yes
> > > > > mmap - no (we may change this with MTE but not for TBI)
> > > >
> > > > [...]
> > > >
> > > > > mprotect - yes
> > > >
> > > > I haven't following this discussion closely... what's the rationale for
> > > > the inconsistencies here (feel free to refer me back to the discussion
> > > > if it's elsewhere).
> > >
> > > _My_ rationale (feel free to disagree) is that mmap() by default would
> > > not return a tagged address (ignoring MTE for now). If it gets passed a
> > > tagged address or a "tagged NULL" (for lack of a better name) we don't
> > > have clear semantics of whether the returned address should be tagged in
> > > this ABI relaxation. I'd rather reserve this specific behaviour if we
> > > overload the non-zero tag meaning of mmap() for MTE. Similar reasoning
> > > for mremap(), at least on the new_address argument (not entirely sure
> > > about old_address).
> > >
> > > munmap() should probably follow the mmap() rules.
> > >
> > > As for brk(), I don't see why the user would need to pass a tagged
> > > address, we can't associate any meaning to this tag.
> > >
> > > For the rest, since it's likely such addresses would have been tagged by
> > > malloc() in user space, we should allow tagged pointers.
> >
> > Those arguments seem reasonable. We should try to capture this
> > somewhere when documenting the ABI.
> >
> > To be clear, I'm not sure that we should guarantee anywhere that a
> > tagged pointer is rejected: rather the behaviour should probably be
> > left unspecified. Then we can tidy it up incrementally.
> >
> > (The behaviour is unspecified today, in any case.)
>
> What is specified (or rather de-facto ABI) today is that passing a user
> address above TASK_SIZE (e.g. non-zero top byte) would fail in most
> cases. If we relax this with the TBI we may end up with some de-facto

I may be being too picky, but "would fail in most cases" sounds like
"unspecified" ?

> ABI before we actually get MTE hardware. Tightening it afterwards may be
> slightly more problematic, although MTE needs to be an explicit opt-in.
>
> IOW, I wouldn't want to unnecessarily relax the ABI if we don't need to.

So long we don't block foreseeable future developments unnecessarily
either -- I agree there's a balance to be struck.

I guess this can be reviewed when we have nailed down the details a bit
further.

Cheers
---Dave