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 11:35:15 +0800
From:	Ren Qiaowei <qiaowei.ren@...el.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
CC:	x86@...nel.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 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.

>> +	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().

>> +		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

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