Re: odd behavior from /sys/block (sysfs)

[Greg KH <gregkh@suse.de>]

On Sat, Nov 27, 2010 at 04:14:11PM -0500, Michael Richardson wrote:
> 
> >>>>> "Greg" == Greg KH <gregkh@suse.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. 

I agree, a patch would be most welcome.

>     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?

There might be some binary files in /sys where this does not happen.

Also note that other filesystems are mounted under /sys, like debugfs
which is in /sys/kernel/debug/ and all bets are off as to what are in
those files and if they ever terminate :)

thanks,

greg k-h