From 9c3db310ff0b7473272ae8dce5e04e2f8a825390 Mon Sep 17 00:00:00 2001 From: Max Reitz Date: Wed, 15 May 2019 06:15:40 +0200 Subject: [PATCH] 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 Reviewed-by: Eric Blake Signed-off-by: Kevin Wolf --- block/file-posix.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/block/file-posix.c b/block/file-posix.c index e09e15bbf8..d018429672 100644 --- a/block/file-posix.c +++ b/block/file-posix.c @@ -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. */