Re: Named address spaces on x86 GNU/Linux

[Richard Biener via Libc-alpha <libc-alpha@sourceware.org>]

On Thu, Jul 29, 2021 at 6:09 PM Joseph Myers <joseph@codesourcery.com> 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).

Thus flag_no_delete_null_pointer_checks checks should be replaced
with sth that takes the address-space as argument.  A good default
implementation would be to only have the default address space
covered by NULL pointer rules.

Richard.

> --
> Joseph S. Myers
> joseph@codesourcery.com