Commit 9c3db310 authored May 15, 2019 by Max Reitz Committed by Kevin Wolf May 20, 2019

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>

parent 012056f4

block/file-posix.c

+16 −0

Original line number	Diff line number	Diff line
		@@ -2488,6 +2488,8 @@ static int coroutine_fn raw_co_block_status(BlockDriverState *bs,
		off_t data = 0, hole = 0;
		int ret;

		assert(QEMU_IS_ALIGNED(offset \| bytes, bs->bl.request_alignment));

		ret = fd_open(bs);
		if (ret < 0) {
		return ret;
		@@ -2513,6 +2515,20 @@ static int coroutine_fn raw_co_block_status(BlockDriverState *bs,
		/* On a data extent, compute bytes to the end of the extent,
		* possibly including a partial sector at EOF. */
		*pnum = MIN(bytes, hole - offset);

		/*
		* We are not allowed to return partial sectors, though, so
		* round up if necessary.
		*/
		if (!QEMU_IS_ALIGNED(*pnum, bs->bl.request_alignment)) {
		int64_t file_length = raw_getlength(bs);
		if (file_length > 0) {
		/* Ignore errors, this is just a safeguard */
		assert(hole == file_length);
		}
		pnum = ROUND_UP(pnum, bs->bl.request_alignment);
		}

		ret = BDRV_BLOCK_DATA;
		} else {
		/* On a hole, compute bytes to the beginning of the next extent. */