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SubjectRe: [PATCH 1/2] fs: Extend mount_ns with support for a fast namespace to vfsmount function
On Fri, Mar 23, 2018 at 04:41:40PM -0500, Eric W. Biederman wrote:

> struct dentry *mount_ns(struct file_system_type *fs_type,
> int flags, void *data, void *ns, struct user_namespace *user_ns,
> + struct vfsmount *(*ns_to_mnt)(void *ns),
> int (*fill_super)(struct super_block *, void *, int))
> {
> struct super_block *sb;
> -
> + int (*test_super)(struct super_block *, void *) = ns_test_super;
> /* Don't allow mounting unless the caller has CAP_SYS_ADMIN
> * over the namespace.
> */
> if (!(flags & SB_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN))
> return ERR_PTR(-EPERM);
>
> - sb = sget_userns(fs_type, ns_test_super, ns_set_super, flags,
> - user_ns, ns);
> + if (ns_to_mnt) {
> + test_super = NULL;
> + if (!(flags & SB_KERNMOUNT)) {
> + struct vfsmount *m = ns_to_mnt(ns);
> + if (IS_ERR(m))
> + return ERR_CAST(m);
> + atomic_inc(&m->mnt_sb->s_active);
> + down_write(&m->mnt_sb->s_umount);
> + return dget(m->mnt_root);

This is completely wrong. Look:
* SB_KERNMOUNT and !SB_KERNMOUNT cases are almost entirely isolated;
completely so once that ns_to_mnt becomes unconditionally non-NULL.
* in !SB_KERNMOUNT passing ns_to_mnt() is pointless - you might as
well pass existing vfsmount (or ERR_PTR()) and use _that_. fill_super()
is not used at all in that case.
* is SB_KERNMOUNT ns_to_mnt serves only as a flag, eventually
constant true.

So let's split it in two helpers and give them sane arguments.

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