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SubjectRe: [PATCH v3 2/4] x86, mpx: hook #BR exception handler to allocate bound tables
On 01/28/2014 01:21 PM, Andy Lutomirski wrote:
> On Mon, Jan 27, 2014 at 7:35 PM, Ren Qiaowei <qiaowei.ren@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 don't mean the same instruction that just trapped.

> I think that failure to fix up the exception should either let the
> normal bounds error through or should raise SIGBUS.
>
Maybe we need HPA help answer this question. Peter, what do you think
about it? If allocation of the bound table fail, what should we do?

>>
>>>> + 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.)
>
do_trap() -> do_trap_no_signal() call similar code to check if the fault
occurred in userspace or kernel space. You can see previous discussion
for the first version of this patchset.

Thanks,
Qiaowei


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