Re: [RFC] Enable nxstack by default

On 10/18/11, Arnaud Lacombe <lacombar_at_gmail.com> wrote:
> Hi,
>
> On Tue, Oct 18, 2011 at 11:44 AM, Garrett Cooper <yanegomi_at_gmail.com> wrote:
>> On Tue, 18 Oct 2011, Arnaud Lacombe wrote:
>>
>>> Hi,
>>>
>>> On Tue, Oct 18, 2011 at 5:07 AM, Kostik Belousov <kostikbel_at_gmail.com>
>>> wrote:
>>>>
>>>> On Mon, Oct 17, 2011 at 09:30:56PM +0200, Oliver Pinter wrote:
>>>>>
>>>>> Hi all!
>>>>>
>>>>> I think, it's the time to enable the nxstack feature. Any comments,
>>>>> pros, cons?
>>>>
>>>> I dragged the change long enough for it to miss the 9.0.
>>>> After the 9.0 is released, I will flip the switch with the following
>>>> change.
>>>>
>>>> diff --git a/sys/kern/imgact_elf.c b/sys/kern/imgact_elf.c
>>>> index 8455f48..926fe64 100644
>>>> --- a/sys/kern/imgact_elf.c
>>>> +++ b/sys/kern/imgact_elf.c
>>>> _at__at_ -118,7 +118,12 _at__at_ static int elf_legacy_coredump = 0;
>>>>  SYSCTL_INT(_debug, OID_AUTO, __elfN(legacy_coredump), CTLFLAG_RW,
>>>>     &elf_legacy_coredump, 0, "");
>>>>
>>>> -static int __elfN(nxstack) = 0;
>>>> +int __elfN(nxstack) =
>>>> +#if defined(__amd64__) || defined(__powerpc64__) /* both 64 and 32 bit
>>>> */
>>>>
>>> Why leaving 32bits x86 CPU supporting the NX feature behind ?
>>
>> Most likely because it was assumed that i386 doesn't fully support it.
>> According to ye great Wikipedia, NX support didn't roll into i386 until
>> Prescott, which was pretty late in the non-64-bit capable family of CPUs,
>> as
>> its successor -- Conroe -- was 64-bit. Intel detuned some of the early
>> Dual
>> Core Pentiums, e.g. the Yonahs to not talk 64-bit. Not sure about AMD.
>>
>> There are probably more details in binutils, gcc, etc, that I'm missing
>> and
>> Kostik can expound on.
>>
> NX support is advertised in the cpuid flags, just add the logic to
> handle this interface. Kostik's patch is just incomplete, but he's got
> a commit bit so he can commit it as-is, as he will.
>
> If nonexec_stack becomes the default, it should be on every CPU
> supporting the feature, not just the low-hanging one.
>
>  - Arnaud
>

the NX detection code already implemented in i386, but this feature
required PAE:

_at_initializecpu(void):

»       »       }
#ifdef PAE
»       »       if ((amd_feature & AMDID_NX) != 0) {
»       »       »       uint64_t msr;

»       »       »       msr = rdmsr(MSR_EFER) | EFER_NXE;
»       »       »       wrmsr(MSR_EFER, msr);
»       »       »       pg_nx = PG_NX;
»       »       }
#endif
»       »       break