linux-kernel - Re: odd behavior from /sys/block (sysfs)

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Message-ID: <4605.1290892451@marajade.sandelman.ca>
Date:	Sat, 27 Nov 2010 16:14:11 -0500
From:	Michael Richardson <mcr@...delman.ca>
To:	Greg KH <gregkh@...e.de>
cc:	linux-kernel@...r.kernel.org, bart@...ie.net
Subject: Re: odd behavior from /sys/block (sysfs) 

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>>>>> "Greg" == Greg KH <gregkh@...e.de> writes:
    >> {please CC me}
    >> 
    >> I was capturing data from my laptop's /sys file system as test input
    >> for some code that needs to grovel through /sys a bit.  I found it weird
    >> that tar got different answers than ls!  See below (at end) for original
    >> observation.
    >> 
    >> It seems that this is because lstat64() on sysfs returns st_size=0 for
    >> the link, and tar does not know how to deal with this, while ls does.
    >> I don't know if it is tar that is wrong, or sysfs.
    >> lstat64(3) suggests that it is sysfs that is at fault, that it should
    >> set st_size.  The behaviour of ls, suggests that perhaps other systems
    >> have worked around st_size=0 for symlinks.  (I'm on 2.6.32-bpo.5
    >> from debian) 

    Greg> So, what do you think should be changed here?

Iif st_size=0 is not a valid return from readlink(2), then I think sysfs
should be fixed.    I will cook a patch.

While tar might not useful (I was successful at using cp -r, btw),
having working file operations makes sense. 

    Greg> I wouldn't ever recommend using tar on sysfs as it doesn't make any
    Greg> sense (sysfs is a virtual file system, like /proc/ and I think
    Greg> that tar doesn't like /proc either, right?)

Are there things on /sys for which a read is not idempotent?

On /proc, there are files which never terminate, because the process is
introspecting itself.  Taking a snapshot of /sys is kinda a useful thing
if you collecting diagnostics, I think. 

- -- 
]       He who is tired of Weird Al is tired of life!           |  firewalls  [
]   Michael Richardson, Sandelman Software Works, Ottawa, ON    |net architect[
] mcr@...delman.ottawa.on.ca http://www.sandelman.ottawa.on.ca/ |device driver[
   Kyoto Plus: watch the video <http://www.youtube.com/watch?v=kzx1ycLXQSE>
	               then sign the petition. 


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