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SubjectRe: [patch 5/9] unprivileged mounts: allow unprivileged bind mounts
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On Tue, 2008-01-08 at 12:35 +0100, Miklos Szeredi wrote:
> @@ -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?

I think just about the one solution is to block new mounters on a
sleepable lock until the race winner actually finishes their mount
operation.

-- Dave



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