linux-kernel - Re: linux-next: Tree for Jan 20 -- Kernel panic

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Date:	Wed, 21 Jan 2015 15:42:16 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Sabrina Dubroca <sd@...asysnail.net>,
	Guenter Roeck <linux@...ck-us.net>,
	Paul Moore <pmoore@...hat.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-audit@...hat.com,
	Richard Guy Briggs <rgb@...hat.com>
Subject: Re: linux-next: Tree for Jan 20 -- Kernel panic - Unable to mount
 root fs

On Wed, Jan 21, 2015 at 12:05:39PM +0100, Sabrina Dubroca wrote:
> 2015-01-21, 04:36:38 +0000, Al Viro wrote:
> > On Tue, Jan 20, 2015 at 08:01:26PM -0800, Guenter Roeck wrote:
> > > With this patch:
> > > 
> > > sys_mkdir .:40775 returned -17
> > > sys_mkdir usr:40775 returned 0
> > > sys_mkdir usr/lib:40775 returned 0
> > > sys_mkdir usr/share:40755 returned 0
> > > sys_mkdir usr/share/udhcpc:40755 returned 0
> > > sys_mkdir usr/bin:40775 returned 0
> > > sys_mkdir usr/sbin:40775 returned 0
> > > sys_mkdir mnt:40775 returned 0
> > > sys_mkdir proc:40775 returned 0
> > > sys_mkdir root:40775 returned 0
> > > sys_mkdir lib:40775 returned 0
> > > sys_mkdir lib/modules:40775 returned 0
> > > ...
> > > 
> > > and the problem is fixed.
> 
> This patch also works for me.
> 
> 
> > ... except that it simply confirms that something's fishy with getname_kernel()
> > of ->name of struct filename returned by getname().  IOW, I still do not
> > understand the mechanism of breakage there.
> 
> I'm not so sure about that.  I tried to copy name to a new string in
> do_path_lookup and that didn't help.
> 
> Now, I've removed the
> 
>         putname(filename);
> 
> line from do_path_lookup and I don't get the panic.

That would indicate that somehow the refcount got unbalanced. Looking
more closely it seems like the various audit_*() function do take a
reference, but maybe that's not enough.

But debugging this further I see no indication that the memory is ever
freed, or otherwise corrupted.

I did collect a bit more data, perhaps that's useful. I started seeing
this issue as well on devices that boot over NFS. After reading this
thread I also realized that another warning that I was seeing might be
related:

	[   28.261930] Warning: unable to open an initial console.

I've added a couple of printks and see that the reason for this is that
/dev/console doesn't get created. /dev however does get created.

	[   11.786627] sys_mkdir dev:40755 returned 0
	...
	[   11.978748] sys_mknod dev/console:20600 returned -2

The chain that fails turns out to be this:

	sys_mknod()
	  sys_mknodat()
	    user_path_create()
	      kern_path_create()
	        do_path_lookup()
	          filename_lookup()
	            path_lookupat()
	              path_init()
	                link_path_walk()
	                  walk_component()

walk_components() ends up calling lookup_slow() and the result is that
inode == NULL and d_is_negative(path->dentry) returns true, therefore
causing -ENOENT to be returned.

I tried to figure out why inode would be NULL at that point or why
d_is_negative() returned true, but I ended up getting completely lost,
so I thought it best to report my findings before I confuse everything.

Is there anything else I can investigate to track this down?

Thierry

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