Re: [PATCH] debugfs: fix potential infinite loop in debugfs_remove_recursive

[yukuai (C)]

Thanks for your explanation

On 2019/11/14 4:17, Steven Rostedt wrote:
> On Thu, 31 Oct 2019 21:34:44 +0800
> yu kuai <yukuai3@xxxxxxxxxx> wrote:
>
> > debugfs_remove_recursive uses list_empty to judge weather a dentry has
> > any subdentry or not. This can lead to infinite loop when any subdir is in
> > use.
> >
> > The problem was discoverd by the following steps in the console.
> > 1. use debugfs_create_dir to create a dir and multiple subdirs(insmod);
> > 2. cd to the subdir with depth not less than 2;
> > 3. call debugfs_remove_recursive(rmmod).
> >
> > After removing the subdir, the infinite loop is triggered bucause
>
>    s/bucause/because/
>
> > debugfs_remove_recursive uses list_empty to judge if the current dir
> > doesn't have any subdentry, if so, remove the current dir and which
> > will never happen.
> >
> > Fix the problem by using simple_empty instead of list_empty.
> >
> > Fixes: 776164c1faac ('debugfs: debugfs_remove_recursive() must not rely on list_empty(d_subdirs)')
> > Reported-by: chenxiang66@xxxxxxxxxxxxx
> > Signed-off-by: yu kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   fs/debugfs/inode.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> > index 7b975db..42b28acc 100644
> > --- a/fs/debugfs/inode.c
> > +++ b/fs/debugfs/inode.c
> > @@ -773,8 +773,10 @@ void debugfs_remove_recursive(struct dentry *dentry)
> >   		if (!simple_positive(child))
> >   			continue;
> >   
> > -		/* perhaps simple_empty(child) makes more sense */
> > -		if (!list_empty(&child->d_subdirs)) {
> > +		/* use simple_empty to prevent infinite loop when any
> > +		 * subdentry of child is in use
> > +		 */
>
> Nit, multi-line comments should be of the form:
>
> 	/*
> 	 * comment line 1
> 	 * comment line 2
> 	 */
>
> Not
>
> 	/* comment line 1
> 	 * comment line 2
> 	 */
>
> It's known that the networking folks like that method, but it's not
> acceptable anywhere outside of networking.
Do you agree with that list_empty(&chile->d_subdirs) here is not 
appropriate? Since it can't skip the subdirs that is not 
simple_positive(simple_positive() will return false), which is the 
reason of infinite loop.
> > +		if (!simple_empty(child)) {
>
> Have you tried this with lockdep enabled? I'm thinking that you might
> get a splat with holding parent->d_lock and simple_empty(child) taking
> the child->d_lock.
The locks are taken and released in the right order:
take parent->d_lock
	take child->d_lock
		list_for_each_entry(c, &child->d_sundirs, d_child)
			take c->d_lock
			release c->d_lock
	release child->d_lock
release parent->d_lock
I don't see anything wrong, am I missing something?

Thanks
Yu Kuai
> -- Steve
>
>
> >   			spin_unlock(&parent->d_lock);
> >   			inode_unlock(d_inode(parent));
> >   			parent = child;
>
>
> .