Named address spaces on x86 GNU/Linux

[Richard Biener]

On Sat, Jul 31, 2021 at 9:34 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Thu, Jul 29, 2021 at 04:08:36PM +0000, Joseph Myers wrote:
> > On Thu, 29 Jul 2021, Florian Weimer via Gcc wrote:
> > > On GNU/Linux, SEGFS is used to implement the thread pointer, to avoid
> > > dedicating a general-purpose register to it.  At address zero with the
> > > SEGFS prefix, the offset itself is stored so that userspace can read it
> > > without having to call into the kernel.  So the SEGFS null pointer is a
> > > valid address, and so are some bytes after it (depending on TCB layout,
> > > some of which is specified by the ABI or is part of the de-facto ABI
> > > used by GCC).
> >
> > That suggests that we need a target hook to describe null pointer
> > properties for a given address space.  In an address space where null
> > pointers are valid to dereference, there should be no diagnostics for
> > arithmetic on / dereferencing them - and more generally,
> > -fno-delete-null-pointer-checks should be in effect for pointers to such
> > an address space (so I don't think this is just a warning issue, you can
> > probably get wrong code from null pointer check deletion in such an
> > address space).
>
> There already is TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID?  So this just
> isn't used everywhere it should?

Yeah, looks like so.

Richard.

>
> Segher