linux-kernel - Re: [PATCH 03/79] ovl: rearrange ovl_follow_link to it doesn't need to call ->put

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Date:	Tue, 5 May 2015 18:34:07 +1000
From:	NeilBrown <neilb@...e.de>
To:	"Hillf Danton" <hillf.zj@...baba-inc.com>
Cc:	"'Al Viro'" <viro@...iv.linux.org.uk>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/79] ovl: rearrange ovl_follow_link to it doesn't need
 to call ->put_link

On Tue, 05 May 2015 15:12:28 +0800 "Hillf Danton" <hillf.zj@...baba-inc.com>
wrote:

> > 
> > From: NeilBrown <neilb@...e.de>
> > 
> > ovl_follow_link current calls ->put_link on an error path.
> > However ->put_link is about to change in a way that it will be
> > impossible to call it from ovl_follow_link.
> > 
> > So rearrange the code to avoid the need for that error path.
> > Specifically: move the kmalloc() call before the ->follow_link()
> > call to the subordinate filesystem.
> > 
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> > ---
> >  fs/overlayfs/inode.c | 25 ++++++++++++-------------
> >  1 file changed, 12 insertions(+), 13 deletions(-)
> > 
> > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > index 04f1248..1b4b9c5e 100644
> > --- a/fs/overlayfs/inode.c
> > +++ b/fs/overlayfs/inode.c
> > @@ -145,6 +145,7 @@ static void *ovl_follow_link(struct dentry *dentry, struct nameidata *nd)
> >  	void *ret;
> >  	struct dentry *realdentry;
> >  	struct inode *realinode;
> > +	struct ovl_link_data *data = NULL;
> > 
> >  	realdentry = ovl_dentry_real(dentry);
> >  	realinode = realdentry->d_inode;
> > @@ -152,25 +153,23 @@ static void *ovl_follow_link(struct dentry *dentry, struct nameidata *nd)
> >  	if (WARN_ON(!realinode->i_op->follow_link))
> >  		return ERR_PTR(-EPERM);
> > 
> > -	ret = realinode->i_op->follow_link(realdentry, nd);
> > -	if (IS_ERR(ret))
> > -		return ret;
> > -
> >  	if (realinode->i_op->put_link) {
> > -		struct ovl_link_data *data;
> > -
> >  		data = kmalloc(sizeof(struct ovl_link_data), GFP_KERNEL);
> > -		if (!data) {
> > -			realinode->i_op->put_link(realdentry, nd, ret);
> > +		if (!data)
> >  			return ERR_PTR(-ENOMEM);
> > -		}
> >  		data->realdentry = realdentry;
> > -		data->cookie = ret;
> > +	}
> > 
> > -		return data;
> > -	} else {
> > -		return NULL;
> > +	ret = realinode->i_op->follow_link(realdentry, nd);
> > +	if (IS_ERR(ret)) {
> > +		kfree(data);
> > +		return ret;
> >  	}
> > +
> > +	if (data)
> 
> No need to check again(see the above kfree(data) please).

kfree(NULL) is perfectly valid.

NeilBrown


> 
> > +		data->cookie = ret;
> > +
> > +	return data;
> >  }
> > 
> >  static void ovl_put_link(struct dentry *dentry, struct nameidata *nd, void *c)
> > --
> > 2.1.4


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