linux-kernel - Re: sysfs root link count broken in 2.6.22-git5

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Message-ID: <20070718033828.GA8582@suse.de>
Date:	Tue, 17 Jul 2007 20:38:28 -0700
From:	Greg KH <gregkh@...e.de>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	Tejun Heo <htejun@...il.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: sysfs root link count broken in 2.6.22-git5

On Tue, Jul 17, 2007 at 11:05:30PM +0200, Jean Delvare wrote:
> On Tue, 17 Jul 2007 11:36:52 -0700, Greg KH wrote:
> > On Tue, Jul 17, 2007 at 01:12:55PM +0200, Jean Delvare wrote:
> > > On Mon, 16 Jul 2007 20:48:44 -0700, Greg KH wrote:
> > > > On Sun, Jul 15, 2007 at 12:42:32PM +0200, Jean Delvare wrote:
> > > > > I'm running 2.6.22-git5 and noticed that the link count of the sysfs
> > > > > root is broken:
> > > > > 
> > > > > $ ls -ld /sys
> > > > > drwxr-xr-x 2 root root 0 Jul 15 12:27 /sys
> > > > > 
> > > > > sysfs is mounted, the link count should be 11, and is with kernel
> > > > > 2.6.22.1. find(1) complains about the bad link count.
> > > > 
> > > > I suggest updating your version of find(1), I get no such complaint
> > > > with:
> > > > 	$ find --version
> > > > 	GNU find version 4.3.8
> > > > 	Built using GNU gnulib version 2007-05-26
> > > > 	Features enabled: D_TYPE O_NOFOLLOW(enabled) LEAF_OPTIMISATION FTS() CBO(level=0)
> > > > 
> > > > What are you using?
> > > 
> > > $ find --version
> > > GNU find version 4.2.28
> > > Features enabled: D_TYPE O_NOFOLLOW(enabled) LEAF_OPTIMISATION
> > > 
> > > This is the standard version in openSuse 10.2. But how does it matter?
> > 
> > Well, some people feel that that message from find is not something that
> > should be bothering users all the time.  Hence it was fixed in newer
> > versions.
> 
> My understanding is that find uses the link count to speed up the
> search. So even if I admit that printing an error message when it
> detects that the count is wrong might confuse or annoy end-users, this
> is still a valuable for us developers that we got things wrong. I seem
> to remember that it helped us detect bugs in procfs and sysfs several
> times already.

I agree, I'm not trying to say it isn't a bug at all, sorry if it came
across that way.

> > > sysfs is broken, not find(1). Don't you see the sysfs root link count
> > > at 2 as I do? This needs to be fixed.
> > 
> > I'm not disagreeing with that, but other than find, what is the downside
> > of this not being correct?  And what should it be?
> 
> This breaks libsensors. libsensors uses libsysfs, and libsysfs is not
> very smart in that it will initialize successfully even if sysfs is not
> mounted.

libsysfs isn't smart at all, and isn't even supported anymore.  I'd
really suggest droping it entirely, it isn't worth it.

> So I added tests after the initialization, to make sure that
> sysfs is really there. These tests are:
> * The mount point exists.
> * The mount point is really mounted.

Do you know of a 2.6 based distro that does not mount sysfs at /sys?  We
took that check out a long time ago in udev and no one has complained :)

> The code looks like:
> 
>        if (sysfs_get_mnt_path(sensors_sysfs_mount, NAME_MAX)
>          || stat(sensors_sysfs_mount, &statbuf) < 0
>          || statbuf.st_nlink <= 2)      /* Empty directory */
>                 return 0;		/* Failure */
> 
> This works OK with 2.6.22.1, but the last test fails with the current
> git kernel even when sysfs is mounted.

Yeah, but is checking the number of hard links in the directory a safe
way to always verify that it isn't empty?  Isn't there some glibc
function that can detect the mount point of a filesystem or directory?
Something in glibc parses /proc/mounts for something, I can't remember
what it is right now though, sorry.

> You may object that this is not the right way to make sure that sysfs
> is mounted, but I don't want to rewrite half of sysfs_get_mnt_path() in
> libsensors when a simple stat should does the job.

Again, I recommend dropping libsysfs, it's gone from some distros
already :)

And yes, the bug should be fixed, I agree.  Thanks for letting us know.

thanks,

greg k-h
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