[Bug 1905979] Re: Check if F_OFD_SETLK is supported may give wrong result

[Olaf Seibert]

The kernel version is Linux hostname 4.15.0-124-generic #127-Ubuntu SMP
Fri Nov 6 10:54:43 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

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https://bugs.launchpad.net/bugs/1905979

Title:
  Check if F_OFD_SETLK is supported may give wrong result

Status in QEMU:
  Incomplete

Bug description:
  In util/osdep.c there is a function qemu_probe_lock_ops() to check if
  file locks F_OFD_SETLK and F_OFD_GETLK (of the style "Open file
  description locks (non-POSIX)") are supported.

  This test is done by trying a lock operation on the file /dev/null.

  This test can get a wrong result.

  The result is (probably) if the operating system *in general* supports
  these locks. However, it does not guarantee that the file system where
  the lock is really wanted (for instance, in caller
  raw_check_lock_bytes() in block/file-posix.c) does support these
  locks.

  (In theory it could even be that /dev/null, being a device special
  file, does not support the lock type while a plain file would.)

  This is in particular relevant for disk images which are stored on a
  shared file system (my particular use case is the Quobyte file system,
  which appears not to support these locks).

  The code as mentioned above is present in the master branch (I checked
  commit ea8208249d1082eae0444934efb3b59cd3183f05) but also for example
  on stable-2.11 commit 0982a56a551556c704dc15752dabf57b4be1c640)

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