From: Joseph Myers <joseph@codesourcery.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: gcc@gcc.gnu.org, libc-alpha@sourceware.org
Subject: Re: Named address spaces on x86 GNU/Linux
Date: Thu, 29 Jul 2021 16:08:36 +0000 [thread overview]
Message-ID: <alpine.DEB.2.22.394.2107291603410.528367@digraph.polyomino.org.uk> (raw)
In-Reply-To: <87czr12u3t.fsf@oldenburg.str.redhat.com>
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).
--
Joseph S. Myers
joseph@codesourcery.com
next prev parent reply other threads:[~2021-07-29 16:08 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-29 12:16 Named address spaces on x86 GNU/Linux Florian Weimer via Libc-alpha
2021-07-29 16:08 ` Joseph Myers [this message]
2021-07-30 7:00 ` Richard Biener via Libc-alpha
2021-07-31 19:32 ` Segher Boessenkool
2021-08-02 10:06 ` Richard Biener via Libc-alpha
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