linux-kernel - Re: Modprobe as script breaks initramfs kernel?

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Date:	Thu, 22 Feb 2007 23:52:16 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
CC:	dsaxena@...xity.net, linux-kernel@...r.kernel.org
Subject: Re: Modprobe as script breaks initramfs kernel?

Jan Engelhardt wrote:
> On Feb 22 2007 14:30, Michael Tokarev wrote:
>> Deepak Saxena wrote:
>>> We attempted an experiment in cleaning up some modprobe messages during 
>>> initramfs bootup when the modules directory is missing by moving modprobe 
>>> to modprobe-bin and replacing modprobe with the following simple shell script:
>>>
>>> #!/bin/sh
>>> # Clean up bootup when modules are not present
>>>
>>> if [ -e "/lib/modules/'uname -r'/modules.dep" ] ; then
>>> 	/sbin/modprobe-bin $*
> 
> The nitpick guide says: `uname -r` and "$@" instead of $*  ;-)

Here, it's possible to workaround the pipefs issue... with another kernel
bugfix.  uname -r is available in /proc/sys/kernel/version, but the problem
is that reading almost any file in /proc with shell, which does read by a
single byte, returns only first byte of of the file being read.  The bug
has been addressed by Andrew recently, but I don't remember if the fix is
in 2.6.20 or 2.6.19.x yet.  I discovered it exactly because of this very
issue - hit pipefs bug and tried to work around it by replacing the above
construct with

   read version < /proc/sys/kernel/version
   ... /lib/modules/$version/modules.dep ...

wich results in $version being set to.. "2" (from "2.6...." :)

But sure, the usage is wrong, albiet ok for some minimal debugging.

>>> BUG: unable to handle kernel NULL pointer dereference at virtual address 00000014
>>> Trace; c0107737 <sys_pipe+17/60>

>> This is the same issue I reported much earlier with /sbin/hotplug being a script
>> in initrfamfs.  The problem is because pipefs isn't initialized yet at the time
>> the script gets called, and causes a NULL-pointer deref.  Obviously you're using
>> pipe above.
> 
> Btw, has this pipefs issue been adressed (by moving pipefs before initramfs
> stage), or something?

Not as I know of.  It has been discussed, but seems to be of low priority.

At least, reading /proc file is fixed... somewhere... :)  So it's now possible
to work around it, at least in some places (because sometimes pipe is really
useful anyway and difficult to substitute with something ;)

/mjt
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