block: Fix NULL deference for unaligned write if qiov is NULL

For zero write, callers pass in NULL qiov (qemu-io "write -z" or
scsi-disk "write same").

Commit fc3959e466 fixed bdrv_co_write_zeroes which is the common case
for this bug, but it still exists in bdrv_aio_write_zeroes. A simpler
fix would be in bdrv_co_do_pwritev which is the NULL dereference point
and covers both cases.

So don't access it in bdrv_co_do_pwritev in this case, use three aligned
writes.

[Initialize ret to 0 in bdrv_co_do_zero_pwritev() to avoid uninitialized
variable warning with gcc 4.9.2.
--Stefan]

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1431522721-3266-3-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 9eeb6dd1b2)
Conflicts:
	block/io.c

* moved hunks into corresponding location in block.c due to lack of
  61007b316 in v2.3.0
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

block.c

@@ -3363,6 +3363,94 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
     return ret;
 }
 
+static int coroutine_fn bdrv_co_do_zero_pwritev(BlockDriverState *bs,
+                                                int64_t offset,
+                                                unsigned int bytes,
+                                                BdrvRequestFlags flags,
+                                                BdrvTrackedRequest *req)
+{
+    uint8_t *buf = NULL;
+    QEMUIOVector local_qiov;
+    struct iovec iov;
+    uint64_t align = MAX(BDRV_SECTOR_SIZE, bs->request_alignment);
+    unsigned int head_padding_bytes, tail_padding_bytes;
+    int ret = 0;
+
+    head_padding_bytes = offset & (align - 1);
+    tail_padding_bytes = align - ((offset + bytes) & (align - 1));
+
+
+    assert(flags & BDRV_REQ_ZERO_WRITE);
+    if (head_padding_bytes || tail_padding_bytes) {
+        buf = qemu_blockalign(bs, align);
+        iov = (struct iovec) {
+            .iov_base   = buf,
+            .iov_len    = align,
+        };
+        qemu_iovec_init_external(&local_qiov, &iov, 1);
+    }
+    if (head_padding_bytes) {
+        uint64_t zero_bytes = MIN(bytes, align - head_padding_bytes);
+
+        /* RMW the unaligned part before head. */
+        mark_request_serialising(req, align);
+        wait_serialising_requests(req);
+        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_HEAD);
+        ret = bdrv_aligned_preadv(bs, req, offset & ~(align - 1), align,
+                                  align, &local_qiov, 0);
+        if (ret < 0) {
+            goto fail;
+        }
+        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_AFTER_HEAD);
+
+        memset(buf + head_padding_bytes, 0, zero_bytes);
+        ret = bdrv_aligned_pwritev(bs, req, offset & ~(align - 1), align,
+                                   &local_qiov,
+                                   flags & ~BDRV_REQ_ZERO_WRITE);
+        if (ret < 0) {
+            goto fail;
+        }
+        offset += zero_bytes;
+        bytes -= zero_bytes;
+    }
+
+    assert(!bytes || (offset & (align - 1)) == 0);
+    if (bytes >= align) {
+        /* Write the aligned part in the middle. */
+        uint64_t aligned_bytes = bytes & ~(align - 1);
+        ret = bdrv_aligned_pwritev(bs, req, offset, aligned_bytes,
+                                   NULL, flags);
+        if (ret < 0) {
+            goto fail;
+        }
+        bytes -= aligned_bytes;
+        offset += aligned_bytes;
+    }
+
+    assert(!bytes || (offset & (align - 1)) == 0);
+    if (bytes) {
+        assert(align == tail_padding_bytes + bytes);
+        /* RMW the unaligned part after tail. */
+        mark_request_serialising(req, align);
+        wait_serialising_requests(req);
+        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_TAIL);
+        ret = bdrv_aligned_preadv(bs, req, offset, align,
+                                  align, &local_qiov, 0);
+        if (ret < 0) {
+            goto fail;
+        }
+        BLKDBG_EVENT(bs, BLKDBG_PWRITEV_RMW_AFTER_TAIL);
+
+        memset(buf, 0, bytes);
+        ret = bdrv_aligned_pwritev(bs, req, offset, align,
+                                   &local_qiov, flags & ~BDRV_REQ_ZERO_WRITE);
+    }
+fail:
+    qemu_vfree(buf);
+    return ret;
+
+}
+
 /*
  * Handle a write request in coroutine context
  */
@@ -3403,6 +3491,11 @@ static int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs,
      */
     tracked_request_begin(&req, bs, offset, bytes, true);
 
+    if (!qiov) {
+        ret = bdrv_co_do_zero_pwritev(bs, offset, bytes, flags, &req);
+        goto out;
+    }
+
     if (offset & (align - 1)) {
         QEMUIOVector head_qiov;
         struct iovec head_iov;
@@ -3476,14 +3569,14 @@ static int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs,
                                flags);
 
 fail:
-    tracked_request_end(&req);
 
     if (use_local_qiov) {
         qemu_iovec_destroy(&local_qiov);
     }
     qemu_vfree(head_buf);
     qemu_vfree(tail_buf);
-
+out:
+    tracked_request_end(&req);
     return ret;
 }