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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 <> 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 :)


greg k-h

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