Re: [PATCH 1/2] x86, mpx: explicitly disable 32-bit MPX support on 64-bit kernels

Andy Lutomirski
2014-12-23T18:49:14Z

On Tue, Dec 23, 2014 at 1:19 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Andy Lutomirski <luto@amacapital.net> wrote:
>
>> On Mon, Dec 22, 2014 at 12:21 PM, Dave Hansen <dave@sr71.net> wrote:
>> > On 12/22/2014 12:17 PM, Andy Lutomirski wrote:
>> >>> > /*
>> >>> > + * 32-bit binaries on 64-bit kernels are currently
>> >>> > + * unsupported.
>> >>> > + */
>> >>> > + if (IS_ENABLED(CONFIG_X86_64) && test_thread_flag(TIF_IA32))
>> >>> > + return MPX_INVALID_BOUNDS_DIR;
>> >> Should this check mm->ia32_compat instead?
>> >
>> > set_personality_64bit/ia32() seem to make that and TIF_IA32 awfully
>> > equivalent. Is there a specific reason for wanting it done this way?
>>
>> My general desire to remove various bogus TIF_IA32 references.
>> [...]
>
> So we generally want to use mm->context.ia32_compat instead of
> TIF_IA32, because in the end TIF_IA32 will go away altogether?
>
> Or do you just want to audit all TIF_IA32 places (because most of
> them are wrong), and using mm->context.ia32_compat where it's
> justified and eliminating TIF_IA32 use is a nice way to document
> that ongoing audit without breaking stuff and such?

TBH, I haven't gotten that far. But of the uses I've looked at, most
seem to want TS_COMPAT instead (is_ia32_task, which is IMO terribly
named), some are so buggier than just using a strange flag, and I
suspect that the rest would make more sense using
mm->context.ia32_compat.

I was planning on sending some patches to incrementally improve the
situation and then seeing where we end up.

>
>> [...] But this is only temporary, so I don't really care.
>
> New code that touches this area should better use new principles,
> so I have no problem with requiring this, as long as it's well
> explained and logical and desirable to everyone.
>

Given that this patch is a fix for 3.19, the code looks correct, and
the check will go away in 3.20, I see no reason to worry about this
particular patch.

--Andy

> Thanks,
>
> Ingo

--
Andy Lutomirski
AMA Capital Management, LLC