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

From: Guenter Roeck <[email-masked]>
Date: Wed, 21 Jan 2015 13:03:20 -0800

On 01/21/2015 12:06 PM, Al Viro wrote:
> On Wed, Jan 21, 2015 at 11:06:27AM -0800, Guenter Roeck wrote:
>> On 01/21/2015 10:29 AM, Al Viro wrote:
>>> On Wed, Jan 21, 2015 at 05:32:13AM -0800, Guenter Roeck wrote:
>>>> Another data point (though I have no idea if it is useful or what it means):
>>>>
>>>> In the working case, path_init sets nd->flags to 0x50 or 0x51.
>>>> In the non-working case (ie for all files with a '/' in the name),
>>>> it sets nd->flags to 0x10 or 0x11, even though it is always called
>>>> with the LOOKUP_RCU bit set in flags.
>>>
>>> Umm...  Are those path_init() succeeding or failing?  Note that path_init()
>>> includes "walk everything except for the last component", so your non-working
>>> case is "have it walk anything at all".  What's failing there?  path_init()
>>> or handling the remaining component?
>>>
>> path_init() returns -2. Guess that explains the unexpected flags ;-).
>> The failuere is from
>> 	link_path_walk()
>> 		walk_component()
>
> Which is to say, lookup gave it a negative dentry.  OK, let's just make
> vfs_mkdir() and walk_component() print what they are doing; on top of
> linux-next
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 323957f..8a4e22f 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1586,8 +1586,11 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
>   		inode = path->dentry->d_inode;
>   	}
>   	err = -ENOENT;
> -	if (!inode || d_is_negative(path->dentry))
> +	if (!inode || d_is_negative(path->dentry)) {
> +		printk(KERN_ERR "walk_component[%p,%pd4] -> negative\n",
> +			path->dentry, path->dentry);
>   		goto out_path_put;
> +	}
>
>   	if (should_follow_link(path->dentry, follow)) {
>   		if (nd->flags & LOOKUP_RCU) {
> @@ -3521,6 +3524,7 @@ int vfs_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
>   	error = dir->i_op->mkdir(dir, dentry, mode);
>   	if (!error)
>   		fsnotify_mkdir(dir, dentry);
> +	printk(KERN_ERR "mkdir[%p,%pd4] => %d\n", dentry, dentry, error);
>   	return error;
>   }
>   EXPORT_SYMBOL(vfs_mkdir);
>

ok case (putname commented out):

user_path_at_empty lookup usr flags 0x0
path_lookupat: calling path_init 'usr' flags=40
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=40[50] returned 0
walk_component: lookup_fast() returned 1
walk_component: lookup_slow() returned 0
walk_component: inode=  (null), negative=1
do_path_lookup(usr, 0x10)
path_lookupat: calling path_init 'usr' flags=50
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=50[50] returned 0
mkdir[c74012a0,/usr] => 0
user_path_at_empty lookup usr flags 0x1
path_lookupat: calling path_init 'usr' flags=41
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=41[51] returned 0
walk_component: inode=c74004a0, negative=0
user_path_at_empty lookup usr flags 0x1
path_lookupat: calling path_init 'usr' flags=41
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=41[51] returned 0

failing case:

path_lookupat: calling path_init 'usr' flags=40
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=40[50] returned 0
walk_component: lookup_fast() returned 1
walk_component: lookup_slow() returned 0
walk_component: inode=  (null), negative=1
do_path_lookup(usr, 0x10)
path_lookupat: calling path_init 'usr' flags=50
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=50[50] returned 0
mkdir[c74012a0,/kkk] => 0						<==== SIC!
user_path_at_empty lookup usr flags 0x1
path_lookupat: calling path_init 'usr' flags=41
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=41[51] returned 0
walk_component: lookup_fast() returned 1
walk_component: lookup_slow() returned 0
walk_component: inode=  (null), negative=1
user_path_at_empty lookup usr flags 0x1
path_lookupat: calling path_init 'usr' flags=41
path_init: link_path_walk() returned 0
path_lookupat: path_init 'usr' flags=41[51] returned 0
walk_component: lookup_fast() returned 1
walk_component: lookup_slow() returned 0
walk_component: inode=  (null), negative=1

Guenter