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 9c3db310ff
)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
stable-3.1
parent
1742e3c74e
commit
044b0bcedf
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@ -2461,6 +2461,8 @@ static int coroutine_fn raw_co_block_status(BlockDriverState *bs,
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off_t data = 0, hole = 0;
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int ret;
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assert(QEMU_IS_ALIGNED(offset | bytes, bs->bl.request_alignment));
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ret = fd_open(bs);
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if (ret < 0) {
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return ret;
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@ -2486,6 +2488,20 @@ static int coroutine_fn raw_co_block_status(BlockDriverState *bs,
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/* On a data extent, compute bytes to the end of the extent,
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* possibly including a partial sector at EOF. */
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*pnum = MIN(bytes, hole - offset);
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/*
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* We are not allowed to return partial sectors, though, so
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* round up if necessary.
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*/
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if (!QEMU_IS_ALIGNED(*pnum, bs->bl.request_alignment)) {
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int64_t file_length = raw_getlength(bs);
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if (file_length > 0) {
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/* Ignore errors, this is just a safeguard */
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assert(hole == file_length);
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}
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*pnum = ROUND_UP(*pnum, bs->bl.request_alignment);
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}
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ret = BDRV_BLOCK_DATA;
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} else {
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/* On a hole, compute bytes to the beginning of the next extent. */
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