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

On Thu, Mar 09, 2017 at 12:26:49PM +0100, Djalal Harouni wrote:

I'm bit lost in the two discussion, however the main concern I was
discussing with Andy was if you have per superblock proc mounts then
each mount will end up with its own device ID st_dev, right now they
share the same ID if they are in the same pid namespace, but if we
change that then we may break the following:
http://man7.org/linux/man-pages/man7/namespaces.7.html

In fact, nothing has changed. I added a parameter that affects
the mountpoint, not the entire pid namespace.

The procfs will still be global. The device ID will be the same as before.

Both new NS_GET_PARENT and NS_GET_USERNS ioctl() that return an fd,
suggests to follow up with fstat() to identify the namespaces..
"By applying fstat(2) to the returned file descriptor, one obtains a
stat structure whose st_dev (resident device) and st_ino (inode
number) fields together identify the owning/parent namespace."

Other /proc/self/ns/* comparison and stat() logic...

Andy suggested that we may have the same st_dev for mounts in the same
pid namespace... I'm not sure which side effect this may bring!

Basically we have here a issue because other proc options (hidepid for
example) affect the entire pid namespace, but, I guess, have to affect
the mountpoint.

# grep ^proc /proc/mounts 
proc /proc proc rw,relatime 0 0

# mount -t proc proc /tmp/proc
# mount -o remount,hidepid=2 -t proc proc /tmp/proc

# grep ^proc /proc/mounts 
proc /proc proc rw,relatime,hidepid=2 0 0
proc /tmp/proc proc rw,relatime,hidepid=2 0 0

-- 
Rgrds, legion