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Date:   Wed, 29 May 2019 14:23:42 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Dave Martin <Dave.Martin@arm.com>
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        Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
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Subject: Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory
 syscalls
Message-ID: <20190529132341.27t3knoxpb7t7y3g@mbp>
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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
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.

-- 
Catalin