file struct's dentry being null?

file struct's dentry being null?

[Shaya Potter]

I decided to do an experiment, in fput, I did a 

struct file * next;
next = (struct file *) file->f_list.next;
if ( !next->dentry )
	printk("file struct: next's dentry is null, %d\n", next);

and I get a bunch of kernel output along these lines, with 5 different
next memory locations.  I only have 3 mount points, so it doesn't seem
to be related to that (though I would have thought it might have been
the list_head on the superblock).  anyways, just trying to figure out
what it's pointing to?

thanks,

shaya

Re: file struct's dentry being null?

[Nikita Danilov]

Shaya Potter writes:
 > I decided to do an experiment, in fput, I did a 
 > 
 > struct file * next;
 > next = (struct file *) file->f_list.next;

->f_list.next points to the ->f_list member of next struct file rather
than to the struct file itself. This is how it works for all struct
list_head embedded into objects.

 > if ( !next->dentry )
 > 	printk("file struct: next's dentry is null, %d\n", next);
 > 
 > and I get a bunch of kernel output along these lines, with 5 different
 > next memory locations.  I only have 3 mount points, so it doesn't seem
 > to be related to that (though I would have thought it might have been
 > the list_head on the superblock).  anyways, just trying to figure out
 > what it's pointing to?
 > 
 > thanks,
 > 
 > shaya
 > 

Nikita.

 > 

Re: file struct's dentry being null?

[Shaya Potter]

On Wed, 2003-07-02 at 11:58, Nikita Danilov wrote:
> Shaya Potter writes:
>  > I decided to do an experiment, in fput, I did a 
>  > 
>  > struct file * next;
>  > next = (struct file *) file->f_list.next;
> 
> ->f_list.next points to the ->f_list member of next struct file rather
> than to the struct file itself. This is how it works for all struct
> list_head embedded into objects.

yes, I understand that, the idea being that it's the first entry in the
struct that you want to link list, so can get the next struct by just
dereferencing the list_head (as &list_head == &struct)

I was just cheating, but that explains my point, I could see it being
null if I hit the real list_head (i.e. on the superblock), but I am
getting 5 distinct "hits", with only 3 mount points, so trying to figure
out why.

the issue I'm trying to figure out is (posted once b4) that I am getting
corrupt entries off of ext3's file list in it's superblock, so that when
fs_may_remount_ro() is called, it oops with a null pointer exception. 

Re: file struct's dentry being null?

[Eli Carter]

Shaya Potter wrote:
> On Wed, 2003-07-02 at 11:58, Nikita Danilov wrote:
> 
>>Shaya Potter writes:
>> > I decided to do an experiment, in fput, I did a 
>> > 
>> > struct file * next;
>> > next = (struct file *) file->f_list.next;
>>
>>->f_list.next points to the ->f_list member of next struct file rather
>>than to the struct file itself. This is how it works for all struct
>>list_head embedded into objects.
> 
> 
> yes, I understand that, the idea being that it's the first entry in the
> struct that you want to link list, so can get the next struct by just
> dereferencing the list_head (as &list_head == &struct)

No, his point was that &list_head != $struct, but rather &struct == 
&list_head - some_offset.

So the code you'd need would look like:
struct file *next;
next = list_entry( <address of the list_head>, file, f_list );

Eli
--------------------. "If it ain't broke now,
Eli Carter           \                  it will be soon." -- crypto-gram
eli.carter(a)inet.com `-------------------------------------------------

Re: file struct's dentry being null?

[Shaya Potter]

On Wed, 2003-07-02 at 12:18, Eli Carter wrote:
> Shaya Potter wrote:
> > On Wed, 2003-07-02 at 11:58, Nikita Danilov wrote:
> > 
> >>Shaya Potter writes:
> >> > I decided to do an experiment, in fput, I did a 
> >> > 
> >> > struct file * next;
> >> > next = (struct file *) file->f_list.next;
> >>
> >>->f_list.next points to the ->f_list member of next struct file rather
> >>than to the struct file itself. This is how it works for all struct
> >>list_head embedded into objects.
> > 
> > 
> > yes, I understand that, the idea being that it's the first entry in the
> > struct that you want to link list, so can get the next struct by just
> > dereferencing the list_head (as &list_head == &struct)
> 
> No, his point was that &list_head != $struct, but rather &struct == 
> &list_head - some_offset.
> 
> So the code you'd need would look like:
> struct file *next;
> next = list_entry( <address of the list_head>, file, f_list );

ok, right, looked at list_entry again, and I tried to cheat too much. 

that's for the pointer. 

Re: file struct's dentry being null?

[Shaya Potter]

On Wed, 2003-07-02 at 12:18, Eli Carter wrote:
> Shaya Potter wrote:
> > On Wed, 2003-07-02 at 11:58, Nikita Danilov wrote:
> > 
> >>Shaya Potter writes:
> >> > I decided to do an experiment, in fput, I did a 
> >> > 
> >> > struct file * next;
> >> > next = (struct file *) file->f_list.next;
> >>
> >>->f_list.next points to the ->f_list member of next struct file rather
> >>than to the struct file itself. This is how it works for all struct
> >>list_head embedded into objects.
> > 
> > 
> > yes, I understand that, the idea being that it's the first entry in the
> > struct that you want to link list, so can get the next struct by just
> > dereferencing the list_head (as &list_head == &struct)
> 
> No, his point was that &list_head != $struct, but rather &struct == 
> &list_head - some_offset.
> 
> So the code you'd need would look like:
> struct file *next;
> next = list_entry( <address of the list_head>, file, f_list );

ok, even with that fix, I'm still getting the same sort of output

I'm printing out 6 different distinct memory locations whose next entry
in the file list has a null dentry.

The system operates normally like this, so just trying to figure out
what its dereferencing.

this is the block of code I stick in fput()

{
	struct file * next;
	next = list_entry(file->f_list.next, struct file, f_list);
	if ( !next->f_dentry )
	{
		printk("deleting dentry whose next is null!, next = %d\n",next);
	}

}