[Qemu-devel] [PATCH] QCOW2: fix bug - report read success on failure

[Chunqiang Tang <ctang@us.ibm.com>]

This patch fixes bugs in QCOW2's error handling paths of read operations.
When an I/O operation fails, the QCOW2 driver mistakenly reports it as success
to the uper layer.

This bug was found by Fast Virtual Disk (FVD)'s fully automated testing tool,
when it injected failures. Specifically, the following test triggered the bug.

/bin/rm -rf /var/ramdisk/truth.raw /var/ramdisk/test.qcow2 /var/ramdisk/zero-500M.raw
dd if=/dev/zero of=/var/ramdisk/truth.raw count=0 bs=1 seek=1112250368
dd if=/dev/zero of=/var/ramdisk/zero-500M.raw count=0 bs=1 seek=575525376
./qemu-img create -f qcow2 -ocluster_size=65536,backing_fmt=blksim -b /var/ramdisk/zero-500M.raw /var/ramdisk/test.qcow2 1112250368
./qemu-io --auto --seed=184915369 --truth=/var/ramdisk/truth.raw --format=qcow2 --test=blksim:/var/ramdisk/test.qcow2 --verify_write=true --compare_before=false --compare_after=true --round=100000 --parallel=100 --io_size=1048576 --fail_prob=0.1 --cancel_prob=0 --instant_qemubh=true

Signed-off-by: Chunqiang Tang <ctang@us.ibm.com>
---
 block/qcow2.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 8c906d1..6f6d56f 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -479,8 +479,10 @@ static void qcow2_aio_read_cb(void *opaque, int ret)
                 BLKDBG_EVENT(bs->file, BLKDBG_READ_BACKING_AIO);
                 acb->hd_aiocb = bdrv_aio_readv(bs->backing_hd, acb->sector_num,
                                     &acb->hd_qiov, n1, qcow2_aio_read_cb, acb);
-                if (acb->hd_aiocb == NULL)
+                if (acb->hd_aiocb == NULL) {
+                    ret = -EIO;
                     goto done;
+                }
             } else {
                 ret = qcow2_schedule_bh(qcow2_aio_read_bh, acb);
                 if (ret < 0)
@@ -495,8 +497,10 @@ static void qcow2_aio_read_cb(void *opaque, int ret)
         }
     } else if (acb->cluster_offset & QCOW_OFLAG_COMPRESSED) {
         /* add AIO support for compressed blocks ? */
-        if (qcow2_decompress_cluster(bs, acb->cluster_offset) < 0)
+        if (qcow2_decompress_cluster(bs, acb->cluster_offset) < 0) {
+            ret = -EIO;
             goto done;
+        }
 
         qemu_iovec_from_buffer(&acb->hd_qiov,
             s->cluster_cache + index_in_cluster * 512,
-- 
1.7.0.4