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


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