* Brian Gerst <brge...@gmail.com> wrote:
> > Ok, so your goal is to allow the x32 ABI, but not 32-bit user-space?
>
> It just seems odd that x32 (which is really a 64-bit ABI with 32-bit
> pointers)
> depended on enabling 32-bit support. Other than both using the core compat
> code, they are not really related.
Yeah.
> > I suppose that makes some sense, it might be a valid 'attack surface
> > reduction' technique, while still allowing the x32 ABI.
> >
> > But I'm not sure we should bother and complicate things: 32-bit compat
> > isn't
> > going away anytime soon, and most of CONFIG_COMPAT is needed for x32.
>
> Many of the compat syscalls are unused by x32. It only needs to handle
> syscalls
> with pointers embedded in data structures differently than native 64-bit.
Yeah, but in fact those are the 'most interesting' (read: most complex) aspects
of
the generic compat machinery. So most of the 'core compat' functionality is
used -
even though we don't use many of the (trivial) argument-converted syscall
variants.
> 64-bit integer arguments (ie., loff_t) do not need special handling, since
> they
> can be passed in a single register instead of a pair of 32-bit registers.
> This
> won't solve that particular issue yet, but it's something to be aware of for
> future cleanups.
Yes, and I think 'X32' is a misnomer in that sense: in reality it's a 90%
64-bit
ABI that just happens to have a handful of additional system calls that can
deal
with data pointers truncated to 32 bits.
So 'C64' would have been a better name: a compacted 64-bit ABI - but that
particular name has its own problems ;-) Maybe S64 (small 64-bit memory model)?
'S64' would also have been an easier sell to distros: they generally resist
adding
anything that smells old, 32-bit ... but kernel hackers and marketing were
always
somewhat disjunct sets ;-)
I'm wondering whether we'll ever see a 48-bit user-space pointer model ;-) They
are misaligned by 32 bits, but x86 CPUs generally handle 32-bit misalignment
just
fine. The killer would be to zero-extend from 48 bits to 64 bits I suspect -
there's no natural instruction for that.
> > So maybe we could introduce CONFIG_X86_32_ABI=y or so, which would cover
> > just
> > the 32-bit entry code and the signal frame compatibility layer?
>
> Yes.
Ok, then it sounds good to me!
Thanks,
Ingo
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