Re: [RFC] Enable nxstack by default

Hi,

On Tue, Oct 18, 2011 at 12:53 PM, Oliver Pinter <oliver.pntr_at_gmail.com> wrote:
> 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:
>
yes, this is the conclusion I reached too. But this does not change
the fact that the VM should know about that, and this is missing from
Kostik's patch. I guess the first hunk should read:

_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(PAE) || defined(__amd64__) || defined(__powerpc64__) /*
both 64 and 32 bit */
+       1;
+#else
+       0;
+#endif
 SYSCTL_INT(__CONCAT(_kern_elf, __ELF_WORD_SIZE), OID_AUTO,
    nxstack, CTLFLAG_RW, &__elfN(nxstack), 0,
    __XSTRING(__CONCAT(ELF, __ELF_WORD_SIZE)) ": enable non-executable stack");

 - Arnaud