Regression wrt mounting /proc in user namespace in 3.13

[Serge E. Hallyn]

Quoting Serge E. Hallyn (serge at hallyn.com):
> Quoting Gao feng (gaofeng at cn.fujitsu.com):
> > On 11/18/2013 11:19 AM, Serge E. Hallyn wrote:
> > > Quoting Serge E. Hallyn (serge at hallyn.com):
> > >> Low on power and no charger, but a quick test printing out if a mount is
> > >> !S_ISDIR or has nlink !=2 in fs_fully_visible() gives me:
> > >>
> > >> [   92.939650] nlink is 1 for ino 8733 (0:3)
> > >>
> > >> (that's major 0 minor 3)
> > > 
> > > Ok, so that is for binfmt_misc on /proc/sys/fs/binfmt_misc.  The
> > > underlying directory is empty, and nlink is showing up as 1.
> > >  
> > > Can we just get the nlink check changed to check for < 3 instead
> > > of ==2 ?
> > > 
> > 
> > I already reported this problem to Eric,hi is working on fix this problem.
> > 
> > nlink is not the right thing to check if a directory is null. since
> > in all of filesystems, parent dir's nlink is increase only when we
> > create sub-dir.
> 
> This whole thing feels very brittle.  May I also point out that simply
> setting perms appears to work just fine instead of overmounting.  If I
> chmod 700 /proc/swaps, unshare my pid and mount namespaces and remount
> /proc, then /proc/swaps is 700 in the new mount.  Since our concern is
> with a new user namespace, which will be limited to world perms, this
> should suffice and allow us to skip all this nonsense.
> 
> Eric?
> 
> -serge

So yeah, I think this patch should be reverted, rather than "fixed".

-serge