[Qemu-devel] [PATCH v6 2/3] block: Fix NULL deference for unaligned write if qiov is NULL

[Fam Zheng <famz@redhat.com>]

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.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/io.c | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)

diff --git a/block/io.c b/block/io.c
index 4e5a92e..d766220 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1174,6 +1174,97 @@ 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;
+
+    head_padding_bytes = offset & (align - 1);
+    tail_padding_bytes = align - ((offset + bytes) & (align - 1));
+    tracked_request_begin(&req, bs, offset, bytes, true);
+
+    mark_request_serialising(&req, align);
+    wait_serialising_requests(&req);
+
+    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. */
+        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((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((offset & (align - 1)) == 0);
+    if (bytes) {
+        assert(align == tail_padding_bytes + bytes);
+        /* RMW the unaligned part after tail. */
+        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);
+        printf("tail part %ld %d\n", offset, bytes);
+        ret = bdrv_aligned_pwritev(bs, &req, offset, align,
+                                   &local_qiov, flags & ~BDRV_REQ_ZERO_WRITE);
+    }
+fail:
+    tracked_request_end(&req);
+    if (buf) {
+        qemu_vfree(buf);
+    }
+    return ret;
+
+}
+
 /*
  * Handle a write request in coroutine context
  */
@@ -1207,6 +1298,10 @@ static int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs,
         bdrv_io_limits_intercept(bs, bytes, true);
     }
 
+    if (!qiov) {
+        return bdrv_co_do_zero_pwritev(bs, offset, bytes, flags);
+    }
+
     /*
      * Align write if necessary by performing a read-modify-write cycle.
      * Pad qiov with the read parts and be sure to have a tracked request not
-- 
2.1.0