linux-kernel - Re: [patch 5/9] unprivileged mounts: allow unprivileged bind mounts

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Message-Id: <E1JCK11-0003ik-4f@pomaz-ex.szeredi.hu>
Date:	Tue, 08 Jan 2008 20:21:35 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	haveblue@...ibm.com
CC:	miklos@...redi.hu, akpm@...ux-foundation.org, hch@...radead.org,
	serue@...ibm.com, viro@....linux.org.uk, ebiederm@...ssion.com,
	kzak@...hat.com, linux-fsdevel@...r.kernel.org,
	containers@...ts.osdl.org, util-linux-ng@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 5/9] unprivileged mounts: allow unprivileged bind mounts

> > @@ -510,10 +533,16 @@ static struct vfsmount *clone_mnt(struct
> >                                         int flag)
> >  {
> >         struct super_block *sb = old->mnt_sb;
> > -       struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname);
> > +       struct vfsmount *mnt;
> > 
> > +       if (flag & CL_SETUSER) {
> > +               int err = reserve_user_mount();
> > +               if (err)
> > +                       return ERR_PTR(err);
> > +       }
> > +       mnt = alloc_vfsmnt(old->mnt_devname);
> >         if (!mnt)
> > -               return ERR_PTR(-ENOMEM);
> > +               goto alloc_failed;
> > 
> >         mnt->mnt_flags = old->mnt_flags;
> >         atomic_inc(&sb->s_active); 
> 
> I think there's a little race here.  We could have several users racing
> to get to this point when nr_user_mounts==max_user_mounts-1.  One user
> wins the race and gets their mount reserved.  The others get the error
> out of reserve_user_mount(), and return.
> 
> But, the winner goes on to error out on some condition further down in
> clone_mnt() and never actually instantiates the mount.
> 
> Do you think this is a problem?

For similar reasons as stated in the previous mail, I don't think this
matters.  If nr_user_mounts is getting remotely close to
max_user_mounts, then something is wrong (or the max needs to be
raised anyway).

Thanks for the review, Dave!

Miklos
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