Re: [PATCH 5/5] NFS: Correct the NFS mount path when following a referral

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Tue, 2009-06-23 at 15:42 -0500, Serge E. Hallyn wrote:
> Quoting Trond Myklebust (Trond.Myklebust@netapp.com):
> > Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
> > ---
> > 
> >  fs/nfs/super.c |   24 ++++++++++++++++++++++++
> >  1 files changed, 24 insertions(+), 0 deletions(-)
> > 
> > 
> > diff --git a/fs/nfs/super.c b/fs/nfs/super.c
> > index 8da7e59..daecbad 100644
> > --- a/fs/nfs/super.c
> > +++ b/fs/nfs/super.c
> > @@ -2548,6 +2548,27 @@ static struct vfsmount *nfs_do_root_mount(struct file_system_type *fs_type,
> >  	return root_mnt;
> >  }
> > 
> > +static void nfs_fix_devname(const struct path *path, struct vfsmount *mnt)
> > +{
> > +	char *page = (char *) __get_free_page(GFP_KERNEL);
> > +	char *devname, *tmp;
> > +
> > +	if (page == NULL)
> > +		return;
> > +	devname = nfs_path(path->mnt->mnt_devname,
> > +			path->mnt->mnt_root, path->dentry,
> > +			page, PAGE_SIZE);
> > +	if (devname == NULL)
> > +		goto out_freepage;
> > +	tmp = kstrdup(devname, GFP_KERNEL);
> > +	if (tmp == NULL)
> > +		goto out_freepage;
> > +	kfree(mnt->mnt_devname);
> > +	mnt->mnt_devname = tmp;
> 
> (looking through patch 3 a bit) is this expected to be safe because all
> callers will send in a mnt which was privately mounted as nfs root_mnt through
> vfs_kern_mount?  So that at this point noone else can have a ref to
> mnt?
> 
> If that isn't the intent, then this seems problematic...  (If it is, it
> seems worth commenting both so that every reader doesn't feel compelled
> to verify, and so that no new callers will naively violate that
> expectation)

The call to nfs_fix_devname() is only applied to the 'mnt_target'
vfsmount, which is the one that was passed down directly from
do_kern_mount() to the ->get_sb() method. It is entirely unreferenced by
any other process since we haven't yet called 'do_add_mount()' to
publish it.

Cheers
  Trond

> thanks,
> -serge
> 
> > +out_freepage:
> > +	free_page((unsigned long)page);
> > +}
> > +
> >  static int nfs_follow_remote_path(struct vfsmount *root_mnt,
> >  		const char *export_path, struct vfsmount *mnt_target)
> >  {
> > @@ -2574,6 +2595,9 @@ static int nfs_follow_remote_path(struct vfsmount *root_mnt,
> >  	mnt_target->mnt_sb = s;
> >  	mnt_target->mnt_root = dget(nd.path.dentry);
> > 
> > +	/* Correct the device pathname */
> > +	nfs_fix_devname(&nd.path, mnt_target);
> > +
> >  	path_put(&nd.path);
> >  	down_write(&s->s_umount);
> >  	return 0;
> > 
> > --
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-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com