linux-kernel - Re: lustre: why does cfs_get_random

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Message-ID: <20131003234545.GA19796@thunk.org>
Date:	Thu, 3 Oct 2013 19:45:45 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"Dilger, Andreas" <andreas.dilger@...el.com>
Cc:	Greg KH <greg@...ah.com>, Peng Tao <tao.peng@....com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: lustre: why does cfs_get_random_bytes() exist?

On Thu, Oct 03, 2013 at 11:06:58PM +0000, Dilger, Andreas wrote:
> 
> The Lustre cfs_get_random_bytes() incorporates (via cfs_rand()) a seed
> which
> also hashes in the addresses from any network interfaces that are
> configured.
> Conversely, cfs_rand() also is seeded at startup from get_random_bytes() in
> case a hardware RNG is available.  This ensures even with identical initial
> conditions cfs_get_random_bytes() gets a different random stream on each
> node.

With modern kernels, the /dev/random driver has the
add_device_randomness() interface which is used to mix in
personalization information, which includes the network MAC address.
So that particular concern should be covered without the hack of
mixing in cfs_rand().

> I'm not against cleaning this up, if there is some mechanism for the
> startup code to add in the node interface addresses into the entropy
> pool, and this is also used to perturb the prandom_u32() sequence
> after that point.

That's handled too, via the late initcall prandom_reseed().

Cheers,

					- Ted
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