During a rebase operation data is copied from the backing chain into the target image using a loop, and each iteration looks for a contiguous region of allocated data of at most IO_BUF_SIZE (2 MB).
Once that region is found, and in order to avoid partial writes, its boundaries are extended so they are aligned to the (sub)clusters of the target image (see commit 12df580b). This operation can however result in a region that exceeds the maximum allowed IO_BUF_SIZE, crashing qemu-img. This can be easily reproduced when the source image has a smaller cluster size than the target image: base <- int <- active $ qemu-img create -f qcow2 base.qcow2 4M $ qemu-img create -f qcow2 -F qcow2 -b base.qcow2 -o cluster_size=1M int.qcow2 $ qemu-img create -f qcow2 -F qcow2 -b int.qcow2 -o cluster_size=2M active.qcow2 $ qemu-io -c "write -P 0xff 1M 2M" int.qcow2 $ qemu-img rebase -F qcow2 -b base.qcow2 active.qcow2 qemu-img: qemu-img.c:4102: img_rebase: Assertion `written + pnum <= IO_BUF_SIZE' failed. Aborted Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3174 Fixes: 12df580b3b7f ("qemu-img: rebase: avoid unnecessary COW operations") Signed-off-by: Alberto Garcia <[email protected]> --- qemu-img.c | 2 +- tests/qemu-iotests/024 | 46 ++++++++++++++++++++++++++++++++++++++ tests/qemu-iotests/024.out | 27 ++++++++++++++++++++++ 3 files changed, 74 insertions(+), 1 deletion(-) diff --git a/qemu-img.c b/qemu-img.c index a7791896c1..454da88c73 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -4081,7 +4081,7 @@ static int img_rebase(const img_cmd_t *ccmd, int argc, char **argv) n += offset - QEMU_ALIGN_DOWN(offset, write_align); offset = QEMU_ALIGN_DOWN(offset, write_align); n += QEMU_ALIGN_UP(offset + n, write_align) - (offset + n); - n = MIN(n, size - offset); + n = MIN(n, MIN(size - offset, IO_BUF_SIZE)); assert(!bdrv_is_allocated(unfiltered_bs, offset, n, &n_alloc) && n_alloc == n); diff --git a/tests/qemu-iotests/024 b/tests/qemu-iotests/024 index b29c76e161..b59d825b42 100755 --- a/tests/qemu-iotests/024 +++ b/tests/qemu-iotests/024 @@ -315,6 +315,52 @@ echo $QEMU_IMG map "$OVERLAY" | _filter_qemu_img_map +# Check that the region to copy to the overlay during a rebase +# operation does not exceed the I/O buffer size. +# +# backing_new <-- backing_old <-- overlay +# +# Backing (new): -- -- -- -- <-- Empty image, size 4MB +# Backing (old):|--|ff|ff|--| <-- 4 clusters, 1MB each +# Overlay: |-- --|-- --| <-- 2 clusters, 2MB each +# +# The data at [1MB, 3MB) must be copied from the old backing image to +# the overlay. However the rebase code will extend that region to the +# overlay's (sub)cluster boundaries to avoid CoW (see commit 12df580b). +# This test checks that IO_BUF_SIZE (2 MB) is taken into account. + +echo +echo "=== Test that the region to copy does not exceed 2MB (IO_BUF_SIZE) ===" +echo + +echo "Creating backing chain" +echo + +TEST_IMG=$BASE_NEW _make_test_img 4M +TEST_IMG=$BASE_OLD CLUSTER_SIZE=1M _make_test_img -b "$BASE_NEW" -F $IMGFMT +TEST_IMG=$OVERLAY CLUSTER_SIZE=2M _make_test_img -b "$BASE_OLD" -F $IMGFMT + +echo +echo "Writing data to region [1MB, 3MB)" +echo + +$QEMU_IO "$BASE_OLD" -c "write -P 0xff 1M 2M" | _filter_qemu_io + +echo +echo "Rebasing" +echo + +$QEMU_IMG rebase -b "$BASE_NEW" -F $IMGFMT "$OVERLAY" + +echo "Verifying the data" +echo + +$QEMU_IO "$OVERLAY" -c "read -P 0x00 0 1M" | _filter_qemu_io +$QEMU_IO "$OVERLAY" -c "read -P 0xff 1M 2M" | _filter_qemu_io +$QEMU_IO "$OVERLAY" -c "read -P 0x00 2M 1M" | _filter_qemu_io + +$QEMU_IMG map "$OVERLAY" | _filter_qemu_img_map + echo # success, all done diff --git a/tests/qemu-iotests/024.out b/tests/qemu-iotests/024.out index 3d1e31927a..cc18ee0290 100644 --- a/tests/qemu-iotests/024.out +++ b/tests/qemu-iotests/024.out @@ -243,4 +243,31 @@ Offset Length File 0 0x20000 TEST_DIR/subdir/t.IMGFMT 0x40000 0x20000 TEST_DIR/subdir/t.IMGFMT +=== Test that the region to copy does not exceed 2MB (IO_BUF_SIZE) === + +Creating backing chain + +Formatting 'TEST_DIR/subdir/t.IMGFMT.base_new', fmt=IMGFMT size=4194304 +Formatting 'TEST_DIR/subdir/t.IMGFMT.base_old', fmt=IMGFMT size=4194304 backing_file=TEST_DIR/subdir/t.IMGFMT.base_new backing_fmt=IMGFMT +Formatting 'TEST_DIR/subdir/t.IMGFMT', fmt=IMGFMT size=4194304 backing_file=TEST_DIR/subdir/t.IMGFMT.base_old backing_fmt=IMGFMT + +Writing data to region [1MB, 3MB) + +wrote 2097152/2097152 bytes at offset 1048576 +2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) + +Rebasing + +Verifying the data + +read 1048576/1048576 bytes at offset 0 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +read 2097152/2097152 bytes at offset 1048576 +2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +Pattern verification failed at offset 2097152, 1048576 bytes +read 1048576/1048576 bytes at offset 2097152 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +Offset Length File +0 0x400000 TEST_DIR/subdir/t.IMGFMT + *** done -- 2.47.3
