block/file-posix: Unaligned O_DIRECT block-status

block/file-posix: Unaligned O_DIRECT block-status

Currently, qemu crashes whenever someone queries the block status of an
unaligned image tail of an O_DIRECT image:
$ echo > foo
$ qemu-img map --image-opts driver=file,filename=foo,cache.direct=on
Offset          Length          Mapped to       File
qemu-img: block/io.c:2093: bdrv_co_block_status: Assertion `*pnum &&
QEMU_IS_ALIGNED(*pnum, align) && align > offset - aligned_offset'
failed.

This is because bdrv_co_block_status() checks that the result returned
by the driver's implementation is aligned to the request_alignment, but
file-posix can fail to do so, which is actually mentioned in a comment
there: "[...] possibly including a partial sector at EOF".

Fix this by rounding up those partial sectors.

There are two possible alternative fixes:
(1) We could refuse to open unaligned image files with O_DIRECT
    altogether.  That sounds reasonable until you realize that qcow2
    does necessarily not fill up its metadata clusters, and that nobody
    runs qemu-img create with O_DIRECT.  Therefore, unpreallocated qcow2
    files usually have an unaligned image tail.

(2) bdrv_co_block_status() could ignore unaligned tails.  It actually
    throws away everything past the EOF already, so that sounds
    reasonable.
    Unfortunately, the block layer knows file lengths only with a
    granularity of BDRV_SECTOR_SIZE, so bdrv_co_block_status() usually
    would have to guess whether its file length information is inexact
    or whether the driver is broken.

Fixing what raw_co_block_status() returns is the safest thing to do.

There seems to be no other block driver that sets request_alignment and
does not make sure that it always returns aligned values.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 9c3db310ff0b7473272ae8dce5e04e2f8a825390)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>