Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

[Andy Lutomirski]

On Oct 7, 2014 1:03 PM, "David Daney" <ddaney.cavm@xxxxxxxxx> wrote:
>
> On 10/07/2014 12:28 PM, Andy Lutomirski wrote:
>>
>> On Tue, Oct 7, 2014 at 12:21 PM, Rich Felker <dalias@xxxxxxxx> wrote:
>>>
>>> On Tue, Oct 07, 2014 at 12:16:59PM -0700, Leonid Yegoshin wrote:
>>>>
>>>> On 10/07/2014 12:09 PM, Rich Felker wrote:
>>>>>
>>>>> I agree completely here. We should not break things (or, as it
>>>>> seems, leave them broken) for common usage cases that affect
>>>>> everyone just to coddle proprietary vendor-specific instructions.
>>>>> The latter just should not be used in delay slots unless the chip
>>>>> vendor also promises to provide fpu branch in hardware. Rich
>>>>
>>>> And what do you propose - remove a current in-stack emulation and
>>>> you still think it doesn't break a status-quo?
>>>
>>>
>>> The in-stack trampoline support could be left but used only for
>>> emulating instructions the kernel doesn't know. This would make all
>>> normal binaries immediately usable with non-executable stack, and
>>> would avoid the only potential source of regressions. Ultimately I
>>> think the "xol" stuff should be removed, but that could be a long term
>>> goal.
>>
>>
>> Does anything break if the xol stuff is disabled for PT_GNU_STACK tasks?
>>
>
> The instructions must be executed, if you turn on a non-executable stack, you cannot execute them on the stack, so they must be handled in another way, which is the subject of this thread.
>
> Options:
>
> 1a) XOL kernel manages the memory
> 1b) XOL userspace manages the menory
> 2) Emulate the instructions.
> 3) I don't think there is a 3rd. option.

4) SIGILL

5) single-step or use an HW breakpoint if available


But, yes, 3 seems reasonable.

--Andy
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