Re: linux-next: Tree for Jan 20 -- Kernel panic - Unable to mount root fs

Thierry Reding
2015-01-21T14:42:21Z

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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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