linux-kernel - Re: lustre: why does cfs_get_random

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Date:	Sat, 5 Oct 2013 10:21:21 -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 Sat, Oct 05, 2013 at 06:10:54AM +0000, Dilger, Andreas wrote:
> >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 think that depends on the network driver.  The Cray systems have some
> very strange networking hardware that is beyond our control - definitely
> not ethernet or Infiniband.

add_device_randomness() is called from __dev_open() and
dev_set_mac_address() in net/core/dev.c.  This is above the ethernet
and infiniband level.  So as long as it looks like a Linux network
device, and they are setting the hardware media access address in the
standard place (dev->dev_addr), it should work fine.

If they don't then they should fix their drivers to call
add_device_randomness(); the answer shouldn't be to make every single
users of the Linux random number generation infrastructure work around
the problem at the subsystem or file system level!

						- Ted
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