On Mon, Jan 27, 2014 at 7:35 PM, Ren Qiaowei <qiaowei....@intel.com> wrote:
> On 01/28/2014 04:36 AM, Andy Lutomirski wrote:
>>>
>>> + bd_entry = status & MPX_BNDSTA_ADDR_MASK;
>>> + if ((bd_entry >= bd_base) && (bd_entry < bd_base + bd_size))
>>> + allocate_bt(bd_entry);
>>
>>
>> What happens if this fails? Retrying forever isn't very nice.
>>
> If allocation of the bound table fail, the related entry in the bound
> directory is still invalid. The following access to this entry still produce
> #BR fault.
>
By the "following access" I think you mean the same instruction that
just trapped -- it will trap again because the exception hasn't been
fixed up. Then mmap will fail again, and you'll retry again, leading
to an infinite loop.
I think that failure to fix up the exception should either let the
normal bounds error through or should raise SIGBUS.
>
>>> + if (!user_mode(regs)) {
>>> + if (!fixup_exception(regs)) {
>>> + tsk->thread.error_code = error_code;
>>> + tsk->thread.trap_nr = X86_TRAP_BR;
>>> + die("bounds", regs, error_code);
>>> + }
>>
>>
>> Why the fixup? Unless I'm missing something, the kernel has no business
>> getting #BR on access to a user address.
>>
>> Or are you adding code to allow the kernel to use MPX itself? If so,
>> shouldn't this use an MPX-specific fixup to allow normal C code to use
>> this stuff?
>>
> It checks whether #BR come from user-space. You can see do_trap_no_signal().
Wasn't #BR using do_trap before? do_trap doesn't call
fixup_exception. I don't see why it should do it now. (I also don't
think it should come from kernel space until someone adds kernel-mode
MPX support.)
>
>
>>> + goto exit;
>>> + }
>>> +
>>> + if (!boot_cpu_has(X86_FEATURE_MPX)) {
>>> + do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code,
>>> NULL);
>>> + goto exit;
>>
>>
>> This, as well as the status == 0 case, should probably document that the
>> exception is from BOUND, not MPX.
>>
> Ok. I will add one comment for this.
>
>
>
>>> + break;
>>> +
>>> + case 1: /* Bound violation. */
>>> + case 0: /* No MPX exception. */
>>> + do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code,
>>> NULL);
>>> + break;
>>
>>
>> What does "No Intel MPX exception" mean? Surely that has business
>> sending #BR.
>>
> Oh. It comes from spec, and just mean it is not from MPX. :) I will change
> it to be accurate.
>
>
>>> +
>>> + default:
>>> + break;
>>
>>
>> What does status 3 mean? The docs say "reserved". Presumably this
>> should log and kill the process.
>
> I guess it should be a good suggestion.
>
> Thanks,
> Qiaowei
>
--
Andy Lutomirski
AMA Capital Management, LLC
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