Re: [RFC PATCH v4 03/12] vfs - move mnt_namespace definition to linux/mount.h

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Mar 17, 2015 at 10:45:09AM +0800, Ian Kent wrote:
> From: Ian Kent <ikent@redhat.com>
> 
> The mnt_namespace definition will be needed by the usermode helper
> contained execution implementation, move it to include/linux/mount.h.

I really don't like that.  AFAICS, the root of the evil is that fscking
nsproxy keeps a pointer to mnt_namespace while all it really needs to
know about is the address of ns_common field embedded into it.  Let's see...

fs/namespace.c:697:     struct mnt_namespace *ns = current->nsproxy->mnt_ns;
fs/namespace.c:770:     return mnt->mnt_ns == current->nsproxy->mnt_ns;
fs/namespace.c:1502:    return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
fs/namespace.c:1587:    return current->nsproxy->mnt_ns->seq >= mnt_ns->seq;
fs/namespace.c:2293:    struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
fs/namespace.c:2961:    touch_mnt_namespace(current->nsproxy->mnt_ns);
fs/namespace.c:3003:    init_task.nsproxy->mnt_ns = ns;
fs/namespace.c:3101:    ns_root.mnt = &current->nsproxy->mnt_ns->root->mnt;
fs/namespace.c:3119:    struct mnt_namespace *ns = current->nsproxy->mnt_ns;
fs/namespace.c:3159:            ns = &nsproxy->mnt_ns->ns;
fs/namespace.c:3187:    put_mnt_ns(nsproxy->mnt_ns);
fs/namespace.c:3188:    nsproxy->mnt_ns = mnt_ns;
fs/pnode.c:282: user_ns = current->nsproxy->mnt_ns->user_ns;
fs/proc_namespace.c:246:        if (!nsp || !nsp->mnt_ns) {
fs/proc_namespace.c:251:        ns = nsp->mnt_ns;
include/linux/nsproxy.h:33:     struct mnt_namespace *mnt_ns;
kernel/nsproxy.c:37:    .mnt_ns                 = NULL,
kernel/nsproxy.c:70:    new_nsp->mnt_ns = copy_mnt_ns(flags, tsk->nsproxy->mnt_ns, user_ns, new_fs);
kernel/nsproxy.c:71:    if (IS_ERR(new_nsp->mnt_ns)) {
kernel/nsproxy.c:72:            err = PTR_ERR(new_nsp->mnt_ns);
kernel/nsproxy.c:113:   if (new_nsp->mnt_ns)
kernel/nsproxy.c:114:           put_mnt_ns(new_nsp->mnt_ns);
kernel/nsproxy.c:160:   if (ns->mnt_ns)
kernel/nsproxy.c:161:           put_mnt_ns(ns->mnt_ns);

OK, so we need

a) add in fs/mount.h
static inline struct mnt_namespace *current_mnt_ns(void)
{
	return current->nsproxy->mnt_ns;
}

and replace a lot of open-coded instances.

b) lift to_mnt_ns() into fs/mount.h (as static inline)

c) switch put_mnt_ns() to struct ns_common *, replacing put_mnt_ns(ns)
with put_mnt_ns(&ns->ns) and using to_mnt_ns() in the body to recover
the original.

d) make copy_mnt_ns() take and return struct ns_common * (same treatment as for
put_mnt_ns() in (c); replace
        new_nsp->mnt_ns = copy_mnt_ns(flags, tsk->nsproxy->mnt_ns, user_ns, new_fs);
        if (IS_ERR(new_nsp->mnt_ns)) {
                err = PTR_ERR(new_nsp->mnt_ns);
                goto out_ns;
        }
with
	ns = copy_mnt_ns(flags, &tsk->nsproxy->mnt_ns->ns, user_ns, new_fs);
	if (IS_ERR(ns)) {
		err = PTR_ERR(ns);
		goto out_ns;
	}
	new_nsp->mnt_ns = to_mnt_ns(ns);

e) replace struct mnt_namespace *mnt_ns with struct ns_common *__mnt_ns in
nsproxy.h, deal with users of mnt_ns by
* everywhere in the tree replace put_mnt_ns(&...->mnt_ns->ns) with
put_mnt_ns(...->__mnt_ns) (i.e. modify the lines modified by (c) again).
* in fs/namespace.c replace
	init_task.nsproxy->mnt_ns = ns;
with
	init_task.nsproxy->__mnt_ns = &ns->ns;
replace
	ns = &nsproxy->mnt_ns->ns;
with
	ns = nsproxy->__mnt_ns;
replace
	nsproxy->mnt_ns = mnt_ns;
with
	nsproxy>__mnt_ns = &mnt_ns->ns;
* in fs/proc_namespace.c replace
	ns = nsp->mnt_ns;
with
	ns = to_mnt_ns(nsp->__mnt_ns);
and
	if (!nsp || !nsp->mnt_ns) {
with
	if (!nsp || !nsp->__mnt_ns) {
* in kernel/nsproxy.c: replace
	ns = copy_mnt_ns(flags, &tsk->nsproxy->mnt_ns->ns, user_ns, new_fs);
added in (d) with
	ns = copy_mnt_ns(flags, tsk->nsproxy->__mnt_ns, user_ns, new_fs);
and
	new_nsp->mnt_ns = to_mnt_ns(ns);
with
	new_nsp->__mnt_ns = ns;
The rest of mnt_ns in that file get replaced by __mnt_ns.
* in current_mnt_ns() replace ...->mnt_ns with to_mnt_ns(...->__mnt_ns)

Do you want me to push such a series in vfs.git?  After such 5 commits we have
no pointers to struct mnt_namespace in nsproxy.h *and* no need for your patches
to dig into ->mnt_ns->ns - we already have that as ->__mnt_ns.