remoteloop: don't use O_DIRECT on s390x

Using O_DIRECT to open the image partition and then using that fd
for the backing of the loopback device will break the mounting of
the formatted partition, i.e mount will fail with:

  mount: /tmp/looptest-6qrtkp5e/mountpoint-root: wrong fs type,
  bad option, bad superblock on /dev/loop0, missing codepage or
  helper program, or other error.

Reproducible with the follow small-ish python script, executed via
'env PYTHONPATH=$(pwd) python3 looptest.py':

---- 8< ---- 8< ---- [ looptest.py ] ---- 8< ---- 8< ----

import contextlib
import json
import os
import subprocess
import stat
import tempfile

from osbuild import loop

@contextlib.contextmanager
def mount(source, dest):
    subprocess.run(["mount", source, dest], check=True)
    try:
        yield dest
    finally:
        subprocess.run(["umount", "-R", dest], check=True)

@contextlib.contextmanager
def os_open(path, flags):
    fd = os.open(path, flags)
    try:
        yield fd
    finally:
        os.close(fd)

def main():
    size = 512 * 1024 * 1024
    ptuuid = "0x14fc63d2"

    with contextlib.ExitStack() as cm:
        tmpdir = cm.enter_context(tempfile.TemporaryDirectory(prefix="looptest-"))
        print(f"Temporary directory at {tmpdir}")

        devdir = os.path.join(tmpdir, "dev")
        os.makedirs(devdir, exist_ok=True)
        dir_fd = cm.enter_context(os_open(devdir, os.O_DIRECTORY))

        image = os.path.join(tmpdir, "image")
        subprocess.run(["truncate", "--size", str(size), image], check=True)
        table = f"label: mbr\nlabel-id: {ptuuid}\nbootable, type=83"
        subprocess.run(["sfdisk", image], input=table, encoding='utf-8',
                       check=True)
        # read it back
        r = subprocess.run(["sfdisk", "--json", image],
                           stdout=subprocess.PIPE,
                           encoding='utf-8', check=True)
        table = json.loads(r.stdout)["partitiontable"]
        partitions = table["partitions"]
        start = partitions[0]["start"] * 512
        size = partitions[0]["size"] * 512

        # fails here with os.O_DIRECT
        image_fd = cm.enter_context(os_open(image, os.O_RDWR | os.O_DIRECT))

        control = loop.LoopControl()
        minor = control.get_unbound()
        lo = loop.Loop(minor)
        lo.set_fd(image_fd)
        lo.set_status(offset=start, sizelimit=size, autoclear=True)
        lo.mknod(dir_fd)
        loopdev = f"/dev/loop{minor}"

        # loopdev = os.path.join(devdir, lo.devname)
        # os.chmod(loopdev, os.stat(loopdev).st_mode | stat.S_IRGRP)

        subprocess.run(["ls", "-la", f"{devdir}"], check=True)
        subprocess.run(["mkfs.ext4", loopdev],
                       input="y", encoding='utf-8', check=True)

        subprocess.run(["blkid", loopdev], check=True)

        mountpoint = os.path.join(tmpdir, "mountpoint-root")
        os.makedirs(mountpoint, exist_ok=True)
        cm.enter_context(mount(loopdev, mountpoint))
        subprocess.run(["ls", "-la", tmpdir], check=True)
        subprocess.run(["ls", "-la", mountpoint], check=True)
        subprocess.run(["mount"], check=True)

if __name__ == '__main__':
    main()

osbuild/remoteloop.py

@@ -4,6 +4,7 @@ import contextlib
 import errno
 import json
 import os
+import platform
 import socket
 import threading
 from . import loop
@@ -117,7 +118,13 @@ class LoopClient:
     def device(self, filename, offset=None, sizelimit=None):
         req = {}
         fds = array.array("i")
-        fd = os.open(filename, os.O_RDWR | os.O_DIRECT)
+        oflags = os.O_RDWR
+
+        if platform.machine() != "s390x":
+            # O_DIRECT will break s390x currently
+            oflags |= os.O_DIRECT
+
+        fd = os.open(filename, oflags)
         dir_fd = os.open("/dev", os.O_DIRECTORY)
 
         fds.append(fd)