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Date:	Mon, 20 Oct 2014 18:42:19 +0400
From:	Andrey Ryabinin <>
To:	Theodore Ts'o <>
Cc:	Daniel Borkmann <>,
	Andrew Morton <>,
	Thomas Gleixner <>,
	Ingo Molnar <>,
	"H. Peter Anvin" <>,
	Peter Zijlstra <>,
	Michal Marek <>,
	Sasha Levin <>,,,,
	Andreas Dilger <>,
	Dmitry Vyukov <>,
	Konstantin Khlebnikov <>
Subject: Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes

On 10/20/2014 06:16 PM, Theodore Ts'o wrote:
> On Mon, Oct 20, 2014 at 04:09:30PM +0200, Daniel Borkmann wrote:
>>> It's triggering when input_rotate == 0, so UBSan complains about right shift in rol32()
>>> static inline __u32 rol32(__u32 word, unsigned int shift)
>>> {
>>> 	return (word << shift) | (word >> (32 - shift));
>>> }
>> So that would be the case when the entropy store's input_rotate calls
>> _mix_pool_bytes() for the very first time ... I don't think it's an
>> issue though.
> I'm sure it's not an issue, but it's still true that 
> 	return (word << 0) | (word >> 32);
> is technically not undefined, and while it would be unfortunate (and
> highly unlikely) if gcc were to say, start nethack, it's technically
> allowed by the C spec.  :-)

There are plenty of such technically not defined but really
harmless things in kernel.
I've reported only few UBs which seemed to me like a real bugs.
So if it's not an issue we could ignore it, OTOH it would be nice
to fix it to shut up ubsan.

>      	     	   	    	 	     - Ted

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