Block layer patches

- NBD server: Fix crashes related to switching between AioContexts
 - file-posix: Workaround for discard/write_zeroes on buggy filesystems
 - Follow-up fixes for the reopen vs. permission changes
 - quorum: Fix error handling for flush
 - block-copy: Refactor copy_range handling
 - docs: Describe how to use 'null-co' block driver
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Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches

- NBD server: Fix crashes related to switching between AioContexts
- file-posix: Workaround for discard/write_zeroes on buggy filesystems
- Follow-up fixes for the reopen vs. permission changes
- quorum: Fix error handling for flush
- block-copy: Refactor copy_range handling
- docs: Describe how to use 'null-co' block driver

# gpg: Signature made Wed 02 Jun 2021 14:44:15 BST
# gpg:                using RSA key DC3DEB159A9AF95D3D7456FE7F09B272C88F2FD6
# gpg:                issuer "kwolf@redhat.com"
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>" [full]
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  docs/secure-coding-practices: Describe how to use 'null-co' block driver
  block-copy: refactor copy_range handling
  block-copy: fix block_copy_task_entry() progress update
  nbd/server: Use drained block ops to quiesce the server
  block-backend: add drained_poll
  block: improve permission conflict error message
  block: simplify bdrv_child_user_desc()
  block/vvfat: inherit child_vvfat_qcow from child_of_bds
  block: improve bdrv_child_get_parent_desc()
  block-backend: improve blk_root_get_parent_desc()
  block: document child argument of bdrv_attach_child_common()
  block/file-posix: Try other fallbacks after invalid FALLOC_FL_ZERO_RANGE
  block/file-posix: Fix problem with fallocate(PUNCH_HOLE) on GPFS
  block: drop BlockBackendRootState::read_only
  block: drop BlockDriverState::read_only
  block: consistently use bdrv_is_read_only()
  block/vvfat: fix vvfat_child_perm crash
  block/vvfat: child_vvfat_qcow: add .get_parent_aio_context, fix crash
  qemu-io-cmds: assert that we don't have .perm requested in no-blk case
  block/quorum: Provide .bdrv_co_flush instead of .bdrv_co_flush_to_disk

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/devel/secure-coding-practices.rst

@@ -104,3 +104,12 @@ structures and only process the local copy.  This prevents
 time-of-check-to-time-of-use (TOCTOU) race conditions that could cause QEMU to
 crash when a vCPU thread modifies guest RAM while device emulation is
 processing it.
+
+Use of null-co block drivers
+----------------------------
+
+The ``null-co`` block driver is designed for performance: its read accesses are
+not initialized by default. In case this driver has to be used for security
+research, it must be used with the ``read-zeroes=on`` option which fills read
+buffers with zeroes. Security issues reported with the default
+(``read-zeroes=off``) will be discarded.