[Bug 1829459] Re: qemu seems to lack support for pid namespace.

[crocket <crockabiscuit@gmail.com>]

In a native chroot, `sudo unshare --pid -- echo hello world` works
without a problem.

In a qemu-aarch64 chroot, `sudo unshare --keep-caps --pid -- echo hello
world` fails with the same error described in this issue.

`qemu: qemu_thread_create: Invalid argument`

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https://bugs.launchpad.net/bugs/1829459

Title:
  qemu seems to lack support for pid namespace.

Status in QEMU:
  New

Bug description:
  # Version

  qemu-4.0.0
  glibc-2.28

  # commands used to launch qemu-aarch64 in user mode.

  : ${QEMU_BINFMT_FLAGS:=OC}

  printf '%s\n' ':qemu-
  aarch64:M::\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xb7\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin
  /qemu-aarch64:'"${QEMU_BINFMT_FLAGS}"
  >/proc/sys/fs/binfmt_misc/register

  > sudo cp /usr/bin/qemu-aarch64 $RPI/usr/bin
  > sudo chroot $RPI /bin/ksh -l

  # host

  Gentoo Linux amd64

  # Guest

  Gentoo Linux aarch64

  # The problem that I have

  "emerge" program fails due to the error, "qemu: qemu_thread_create: Invalid argument".
  "emerge" is Gentoo's package manager that compiles and installs packages.

  # Workaround

  Disable pid-sandbox in emerge.

  # How to reproduce the issue

  Execute

  unshare --pid -- echo hello world

  or

  python -c "import portage.process; portage.process.spawn(['echo',
  'hello', 'world'], unshare_pid=True)"

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