linux-kernel - Re: [PATCH v4 22/29] x86/asm: Move 'status' from struct thread_info to struct thread

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Date:	Sun, 26 Jun 2016 17:40:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v4 22/29] x86/asm: Move 'status' from struct thread_info
 to struct thread_struct

On Sun, Jun 26, 2016 at 5:36 PM, Brian Gerst <brgerst@...il.com> wrote:
> On Sun, Jun 26, 2016 at 8:23 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Sun, Jun 26, 2016 at 4:55 PM, Brian Gerst <brgerst@...il.com> wrote:
>>> On Sun, Jun 26, 2016 at 5:55 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>>> --- a/arch/x86/kernel/asm-offsets.c
>>>> +++ b/arch/x86/kernel/asm-offsets.c
>>>> @@ -30,7 +30,6 @@
>>>>  void common(void) {
>>>>         BLANK();
>>>>         OFFSET(TI_flags, thread_info, flags);
>>>> -       OFFSET(TI_status, thread_info, status);
>>>
>>> TI_status can be deleted.  It's last users were removed in commit ee08c6bd.
>>
>> Indeed.
>>
>> Just to double-check: are you saying that this patch is okay?
>
> It looks OK to me, but I haven't tested it.  Another suggestion is to
> change the compat flag to a bitfield, since there is only one TS_*
> flag now and it's not referenced from asm.

That could also work.

As a silly alternative thought: we just might be able to get away with
shoving the "is ia32" flag into one of the high bits of
pt_regs->orig_ax.  It wouldn't break any 32-bit ptrace users because
they can't see the high bits.  It wouldn't break most 64-bit ptrace
users because they use the silly PTRACE_GETREGSET API that doesn't
show the high bits if the tracee is "32-bit".  It would change
behavior when a 64-bit tracer traces a 64-bit process that does int
$0x80, but at least strace already gets that case completely wrong.

Of course, this proposal has all kinds of problems.

--Andy