Re: [PATCH 3.19 3/3] x86, mpx: Change the MPX enable/disable API to arch_prctl

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Jan 5, 2015 at 1:18 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote:
> On 01/05/2015 12:42 PM, Andi Kleen wrote:
>> Dave Hansen <dave.hansen@linux.intel.com> writes:
>>> On 12/29/2014 04:52 PM, Andy Lutomirski wrote:
>>>> This has the benefit the it avoids cluttering prctl with more
>>>> arch-specific functionality.  The down side is that arch_prctl will
>>>> need to be wired up as a 32-bit syscall to add 32-bit support for
>>>> MPX.
>>>
>>> There is existing userspace out there which depends on the existing
>>> prctl() setup.  There isn't a _lot_ and it might still be able to be
>>> changed easily, but this isn't a given.
>>>
>>> I'll check in with the folks doing the gcc (runtime) part of this next
>>> week and see what they think.
>>
>> It'll be quite messy for 32bit because they would need to use syscall(),
>> as glibc won't have a arch_prctl stub.

We should avoid arch_prctl because glibc won't add a syscall stub that
libgcc or whatever would want?  My mind boggles.

>
> Yeah, I'd _really_ prefer not to change it.  The code is in a gcc
> branch, but is getting pulled in to the 5.0 release.  We've got
> *absolutely* no shortage of prctl numbers.

We do, however, have a severe shortage of sanity in the prctl implementation.

Anyway, if it's actually a problem to change it, I have no real
problem keeping it, but I think we *really* need to validate the rest
of the arguments at the very least.

--Andy