IPC + PID Namespace from host does not work in combination

[mario19911]

Description

I am trying to run two containers, both with --ipc=host and --pid=host. With this combination I am not able to see the shared memories from container A in container B. When I remove --pid=host it is working.

Steps to reproduce the issue:

1. Create a container which creates a shared memory (shm_server from https://www.cs.cf.ac.uk/Dave/C/node27.html)

2. Startup container A with podman run -it --ipc=host --pid=host --rm /bin/bash

3. Startup shm_server in container and check it with ipcs -m

4. Startup the second container with podman run -it --ipc=host --pid=host --rm /bin/bash and check the same with ipcs -m (cannot see the shared memory created in container A)

5. Startup the second container again with podman run -it --ipc=host --rm /bin/bash and check ipcs -m --> does work

Describe the results you received:
When both namespaces are set to "host" I cannot see the generated shared memories.

Describe the results you expected:
To see the shared memory of container A in container B.

Additional information you deem important (e.g. issue happens only occasionally):

Output of podman version:

Version:      2.0.3
API Version:  1
Go Version:   go1.14.2
Built:        Thu Jan  1 01:00:00 1970
OS/Arch:      linux/amd64

Output of podman info --debug:

host:
  arch: amd64
  buildahVersion: 1.15.0
  cgroupVersion: v1
  conmon:
    package: 'conmon: /usr/libexec/podman/conmon'
    path: /usr/libexec/podman/conmon
    version: 'conmon version 2.0.18, commit: '
  cpus: 1
  distribution:
    distribution: ubuntu
    version: "20.04"
  eventLogger: file
  hostname: mario-VirtualBox
  idMappings:
    gidmap: null
    uidmap: null
  kernel: 5.4.0-42-generic
  linkmode: dynamic
  memFree: 138342400
  memTotal: 2084356096
  ociRuntime:
    name: runc
    package: 'runc: /usr/sbin/runc'
    path: /usr/sbin/runc
    version: 'runc version spec: 1.0.1-dev'
  os: linux
  remoteSocket:
    path: /run/podman/podman.sock
  rootless: false
  slirp4netns:
    executable: ""
    package: ""
    version: ""
  swapFree: 383512576
  swapTotal: 470282240
  uptime: 16h 29m 40.51s (Approximately 0.67 days)
registries:
  search:
  - docker.io
  - quay.io
store:
  configFile: /etc/containers/storage.conf
  containerStore:
    number: 8
    paused: 0
    running: 1
    stopped: 7
  graphDriverName: overlay
  graphOptions: {}
  graphRoot: /var/lib/containers/storage
  graphStatus:
    Backing Filesystem: extfs
    Native Overlay Diff: "true"
    Supports d_type: "true"
    Using metacopy: "false"
  imageStore:
    number: 548
  runRoot: /var/run/containers/storage
  volumePath: /var/lib/containers/storage/volumes
version:
  APIVersion: 1
  Built: 0
  BuiltTime: Thu Jan  1 01:00:00 1970
  GitCommit: ""
  GoVersion: go1.14.2
  OsArch: linux/amd64
  Version: 2.0.3

**Additional environment details (AWS, VirtualBox, physical, etc.):Ubuntu 20.04 in VirtualBox

[mheon]

This is fixed now, via #7109