linux-kernel - Re: lustre: why does cfs_get_random

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Date:	Thu, 3 Oct 2013 10:26:21 -0700
From:	Greg KH <greg@...ah.com>
To:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <andreas.dilger@...el.com>,
	Peng Tao <tao.peng@....com>, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org, Nikita Danilov <nikita@...sterfs.com>
Subject: Re: lustre: why does cfs_get_random_bytes() exist?

On Thu, Oct 03, 2013 at 12:39:08PM -0400, Theodore Ts'o wrote:
> I've been auditing uses of get_random_bytes() since there are places
> where get_random_bytes() is getting used where something weaker, such
> as prandom_u32() is quite sufficient.  Basically, if kernel code just
> needs a random number which does not have any cryptographic
> requirements (such as in ext[234]. which gets the new block group used
> for inode allocations using get_random_bytes), then prandom_u32()
> should be used instead of get_random_bytes() to save CPU overhead and
> to reduce the drain on the /dev/urandom's entropy pool.
> 
> Typically, the reason for this is either for historical reasons, since
> prandom_u32() hadn't existed when the code was written, or because
> historical code was cut and pasted into newer code.
> 
> When I came across staging/lustre/lustre/libcfs/prng.c, I saw
> something which is **really** weird.  It defines a cfs_rand() which is
> functionally identical to prandom_u32().  More puzzlingly, it also
> defines cfs_get_random_bytes() which calls get_random_bytes() and then
> xor's the result with cfs_rand().  That last step has no cryptographic
> effect, so I'm really wondering who thought this as a good idea and/or
> necessary.
> 
> What I think should happen is that staging/lustre/lustre/libcfs/prng.c
> should be removed, and calls to cfs_rand() should get replaced
> prandom_u32(), and cfs_get_random_bytes() should get replaced with
> get_random_bytes().
> 
> Does this sound reasonable?

Sounds reasonable to me, care to send a patch to do so?

thanks,

greg k-h
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