linux-kernel - Re: [PATCH v3 2/4] x86, mpx: hook #BR exception handler to allocate bound tables

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Date:	Tue, 28 Jan 2014 13:39:44 +0800
From:	Ren Qiaowei <qiaowei.ren@...el.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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@...el.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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