linux-kernel - Re: [PATCH v3 4/4] x86, mpx: extend siginfo structure to include bound violation information

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Date:	Mon, 27 Jan 2014 09:34:34 +0800
From:	Ren Qiaowei <qiaowei.ren@...el.com>
To:	David Rientjes <rientjes@...gle.com>
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
Subject: Re: [PATCH v3 4/4] x86, mpx: extend siginfo structure to include
 bound violation information

On 01/27/2014 05:38 AM, David Rientjes wrote:
> On Sun, 26 Jan 2014, Ren Qiaowei wrote:
>
>>> arch/x86/kernel/mpx.c: In function ‘do_mpx_bounds’:
>>> arch/x86/kernel/mpx.c:407:3: warning: cast to pointer from integer of
>>> different size [-Wint-to-pointer-cast]
>>> arch/x86/kernel/mpx.c:409:3: warning: cast to pointer from integer of
>>> different size [-Wint-to-pointer-cast]
>>>
>>> and the documentation says you explicitly want to support this config.
>>>
>>> These types of warnings are usually indicative of real problems when
>>> you're storing upper and lower bits in 32-bit fields after casting them
>>> from 64-bit values.
>>>
>>> I'm also not sure if the added fields to the generic struct siginfo can be
>>> justified for this.
>>>
>> According to MPX spec, for 32-bit case, the upper 32-bits of 64-bits bound
>> register are ignored, and so casting to pointer from 64-bit values should be
>> not produce any problems.
>>
>
> Ok, so this is intended per the spec which nobody reading the code is
> going to know and people who report the compile warnings are going to
> continue to question it.
>
> How are you planning on suppressing the warnings?  It will probably
> require either
>
>   - separate 64-bit and 32-bit helper functions to do_mpx_bounds() to
>     do appropriate casts before casting to a pointer, or
>
>   - a macro defined as a no-op for 64-bit and as a cast to 32-bit value
>     for 32-bit configs that will be used in do_mpx_bounds() and casted
>     to the pointer.
>
I agree with you and we should suppress all the warnings as possible. If 
I use (unsgined long) to cast like the following code, what do you think 
about it? sizeof(long) will be 4 for 32-bit.

     info->si_lower = (void __user *)(unsigned long)
         (xsave_buf->bndregs.bndregs[2*bndregno]);
     info->si_upper = (void __user *)(unsigned long)
	(~xsave_buf->bndregs.bndregs[2*bndregno+1]);

Thanks,
Qiaowei
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