tree-wide: take BSD lock on loopback devices we dissect/mount/operate on

So here's something we should always keep in mind:

systemd-udevd actually does *two* things with BSD file locks on block
devices:

1. While it probes a device it takes a LOCK_SH lock. Thus everyone else
   taking a LOCK_EX lock will temporarily block udev from probing
   devices, which is good when making changes to it.

2. Whenever a device is closed after write (detected via inotify), udevd
   will issue BLKRRPART (requesting the kernel to reread the partition
   table). It does this while holding a LOCK_EX lock on the block
   device. Thus anyone else taking LOCK_SH or LOCK_EX will temporarily
   block udevd from issuing that ioctl. And that's quite relevant, since
   the kernel will temporarily flush out all partitions while re-reading
   the partition table and then create them anew. Thus it is smart to
   take LOCK_SH when dissecting a block device to ensure that no
   BLKRRPART is issued in the background, until we mounted the devices.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/portable/portable.c b/src/portable/portable.c
index 0c10b161f0..7bba7b47e4 100644
--- a/src/portable/portable.c
+++ b/src/portable/portable.c
@@ -359,6 +359,10 @@ static int portable_extract_by_path(
                 /* We now have a loopback block device, let's fork off a child in its own mount namespace, mount it
                  * there, and extract the metadata we need. The metadata is sent from the child back to us. */
 
+                r = loop_device_flock(d, LOCK_SH);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to acquire lock on loopback block device: %m");
+
                 BLOCK_SIGNALS(SIGCHLD);
 
                 r = mkdtemp_malloc("/tmp/inspect-XXXXXX", &tmpdir);