Re: [RFC] Add option to mount only a pids subset

On Sun, Mar 12, 2017 at 08:19:33PM -0700, Andy Lutomirski wrote:

On Sat, Mar 11, 2017 at 6:13 PM, Al Viro [off-list ref] wrote:

quoted

PS: AFAICS, simple mount --bind of your pid-only mount will suddenly
expose the full thing.  And as for the lifetimes making no sense...
note that you are simply not freeing these structures of yours.
Try to handle that and you'll get a serious PITA all over the
place.

What are you trying to achieve, anyway?  Why not add a second vfsmount
pointer per pid_namespace and make it initialized on demand, at the
first attempt of no-pid mount?  Just have a separate no-pid instance
created for those namespaces where it had been asked for, with
separate superblock and dentry tree not containing anything other
that pid-only parts + self + thread-self...

Can't we just make procfs work like most other filesystems and have
each mount have its own superblock?  If we need to do something funky
to stat() output to keep existing userspace working, I think that's
okay.

First of all, most of the filesystems do *NOT* guarantee anything of
that sort.  And what's the point of having more instances than
necessary, anyway?

As far as I can tell, proc_mnt is very nearly useless -- it seems to
be used for proc_flush_task (which claims to be purely an optimization
and could be preserved in the common case where there's only one
relevant mount) and for sysctl_binary.  For the latter, we could
create proc_mnt but make actual user-initiated mounts be new
superblocks anyway.

Again, what for?  It won't salvage that kludge...  It's not as if it
had been hard to have separate pid-only instance created when asked
for (and reused every time when we are asked for pid-only).  What's
the point of ever having more than two instances per pidns?  IDGI...

Folks, there is no one-to-one correspondence between mountpoints and
superblocks.  Not since 2000 or so.  Just don't try to shove your
per-superblock stuff into vfsmount; it simply won't work.  If you
want a separate instance for that thing, then just go ahead and
have ->mount() decide which one to use (and whether to create a new
one).  All there is to it...