Re: [PATCH] vfs namespace: Don't assume mount namespace has valid root

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

On Wed, Mar 14, 2012 at 04:58:30PM -0400, Sasha Levin wrote:
> This patch fixes the assumption that a mnt namespace will always have a valid
> root object.

It's not an assumption, it's an invariant that should hold unless you have
run into a bug somewhere.

Instances of struct mnt_namespace should *all* come from alloc_mnt_ns().
There are only two callers - dup_mnt_namespace() and create_mnt_ns().
The latter will assign non-NULL vfsmount to ->root or die NULL pointer
dereference in
                mnt->mnt_ns = new_ns;
The former will either assign non-NULL to ->root or kfree() mnt_namespace
before anyone can see it.

And nothing should modify ->root after that assignment for as long as
the instance of struct mnt_namespace is allocated.

Mind explaining how have you managed to get mnt_namespace with NULL ->root
passed to dup_mnt_ns()?