[dm-devel] Re: [Bugme-new] [Bug 12680] New: Not having a VIA PadLock hardware incurs a long delay in probing on modules insertion attempt.

[Andrew Morton]

(cc dm-devel)

On Wed, 11 Feb 2009 17:27:42 +0100 Valentin QUEQUET <v.quequet-techniques at orange.fr> wrote:

> 
> I've finally found why my computer seems to hang (pause) quite lengthy 
> when I boot Pristine Linux 2.6.29-rcX... instead of Pristine Linux 
> 2.6.28.4 (for example).
> 
> The reason is that the cryptographic keys generation for the Device 
> Mapper takes longer with 2.6.29 than with 2.6.28 under certain 
> circumstances.

So it's device-mapper userspace?

Is this new behaviour in recent kernel versions?  Some kernel change
caused /dev/random accesses to wait for longer before sufficient
entropy has been gathered?


> To notice a non-negligible delay in the key generation phase, the system 
> must fit the following both 2 conditions:
> 
>    1) The system PRNG entropy pool must lack of entropy normally brought 
> in the form of environmental noise.
> 
>    2) The system must initiate its Device-Mapper-Encrypted (dm-crypt) 
> partitions with boot-time dynamically generated
>         cryptographic keys using "/dev/random" as key file. (the 3rd 
> field of "/etc/crypttab" ; see "man crypttab")
> 
> 
> Such a long delay in the key generation phase can be avoided if the 
> system fits either of the following 2 conditions:
> 
>    1) The excitated user stresses its keyboard and mouse (generates much 
> environmental noise) to provide the PRNG entropy pool with much entropy. 
> (Or some other peripheral generates noise : network interface, ...)
> 
>    2) The system initiates dm-crypt partitions using "/dev/urandom" as 
> key file.
> 
> 
> But in the scenario where both
>    1) environmental noise is reduced to the minimum (no user 
> 'excitation' and mouse and NIC unplugged)
> and
>    2) where dm-crypt partitions are initialized with "/dev/random" as 
> key file,
> there is a huge difference whether I boot Linux 2.6.28.y or Linux 
> 2.6.29-rcX... .
> 
> 
> In order to provide you with meaningful information but not too much, I 
> join a few "bootchart"-generated logs (bootchart*.tgz) plus their 
> ".svgz" corresponding diagrams (Pruned and Not-Pruned) for the following 
> test cases:
> 
> Having always environmental noise reduced at its minimum possible level.
> Using alternately 2.6.28 and 2.6.29 Linux versions.
> Using alternately "/dev/random" and "/dev/urandom" as dm-crypt key file.
> 
> There are then 4 test cases for which I join files, and for each test 
> case, I provide:
>    - The "bootchart*.tgz" bootchart report.
>    - The Not-Pruned ".svgz" corresponding SVG diagram.
>    - The Pruned ".svgz" corresponding SVG diagram.
> 
> Thus leading to the following 12 files:
> 
> -r--r--r-- 1 testr testr 174682 Feb 11 17:10 
> DevRandom_bootchart-2.6.28.4.BootChart_Report.tgz
> -r--r--r-- 1 testr testr 102648 Feb 11 17:10 
> DevRandom_bootchart-2.6.28.4.Not-Pruned_SVG_Diagram.svgz
> -r--r--r-- 1 testr testr  26010 Feb 11 17:10 
> DevRandom_bootchart-2.6.28.4.Pruned_SVG_Diagram.svgz
> -r--r--r-- 1 testr testr 327701 Feb 11 17:10 
> DevRandom_bootchart-2.6.29-rc4-git1.BootChart_Report.tgz
> -r--r--r-- 1 testr testr 175522 Feb 11 17:10 
> DevRandom_bootchart-2.6.29-rc4-git1.Not-Pruned_SVG_Diagram.svgz
> -r--r--r-- 1 testr testr  39844 Feb 11 17:10 
> DevRandom_bootchart-2.6.29-rc4-git1.Pruned_SVG_Diagram.svgz
> -r--r--r-- 1 testr testr 138401 Feb 11 17:10 
> DevUrandom_bootchart-2.6.28.4.BootChart_Report.tgz
> -r--r--r-- 1 testr testr  80691 Feb 11 17:10 
> DevUrandom_bootchart-2.6.28.4.Not-Pruned_SVG_Diagram.svgz
> -r--r--r-- 1 testr testr  21136 Feb 11 17:10 
> DevUrandom_bootchart-2.6.28.4.Pruned_SVG_Diagram.svgz
> -r--r--r-- 1 testr testr 152979 Feb 11 17:10 
> DevUrandom_bootchart-2.6.29-rc4-git1.BootChart_Report.tgz
> -r--r--r-- 1 testr testr  78323 Feb 11 17:10 
> DevUrandom_bootchart-2.6.29-rc4-git1.Not-Pruned_SVG_Diagram.svgz
> -r--r--r-- 1 testr testr  20745 Feb 11 17:10 
> DevUrandom_bootchart-2.6.29-rc4-git1.Pruned_SVG_Diagram.svgz
> 
> But for the sake of convenience, I tar them all as 
> "Dev-Random_regression_on_post-2.6.28_kernels.tar"
> 
> In hope my report will prove useful.
> 
> Sincerely,
> Valentin QUEQUET
> 
> n.b. : Don't hesitate to ask me for more files or explanations.
>