virtio-blk: report non-zero status when failing SG_IO requests

Linux really looks only at scsi->errors for SG_IO requests; it does
not look at the virtio request status at all.  Because of this, when
a SG_IO request is failed early with virtio_blk_req_complete(req,
VIRTIO_BLK_S_UNSUPP), without writing hdr.status, it will look like
a success to the guest.

This is their bug, but we can make it safe for older guests now by
forcing scsi->errors to have a non-zero value whenever a request
has to be failed.

But if we fix the bug in the guest driver, we will have another problem
because QEMU returns VIRTIO_BLK_S_IOERR if the status is non-zero, and
Linux translates that to -EIO.  Rather, the guest should succeed the
request and pass the non-zero status via the userspace-provided SG_IO
structure.  So, remove the case where virtio_blk_handle_scsi can
return VIRTIO_BLK_S_IOERR.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/virtio-blk.c

@@ -145,20 +145,12 @@ static VirtIOBlockReq *virtio_blk_get_request(VirtIOBlock *s)
     return req;
 }
 
-#ifdef __linux__
 static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
 {
-    struct sg_io_hdr hdr;
     int ret;
-    int status;
+    int status = VIRTIO_BLK_S_OK;
     int i;
 
-    if ((req->dev->vdev.guest_features & (1 << VIRTIO_BLK_F_SCSI)) == 0) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_UNSUPP);
-        g_free(req);
-        return;
-    }
-
     /*
      * We require at least one output segment each for the virtio_blk_outhdr
      * and the SCSI command block.
@@ -173,20 +165,26 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
     }
 
     /*
-     * No support for bidirection commands yet.
+     * The scsi inhdr is placed in the second-to-last input segment, just
+     * before the regular inhdr.
      */
-    if (req->elem.out_num > 2 && req->elem.in_num > 3) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_UNSUPP);
-        g_free(req);
-        return;
+    req->scsi = (void *)req->elem.in_sg[req->elem.in_num - 2].iov_base;
+
+    if ((req->dev->vdev.guest_features & (1 << VIRTIO_BLK_F_SCSI)) == 0) {
+        status = VIRTIO_BLK_S_UNSUPP;
+        goto fail;
     }
 
     /*
-     * The scsi inhdr is placed in the second-to-last input segment, just
-     * before the regular inhdr.
+     * No support for bidirection commands yet.
      */
-    req->scsi = (void *)req->elem.in_sg[req->elem.in_num - 2].iov_base;
+    if (req->elem.out_num > 2 && req->elem.in_num > 3) {
+        status = VIRTIO_BLK_S_UNSUPP;
+        goto fail;
+    }
 
+#ifdef __linux__
+    struct sg_io_hdr hdr;
     memset(&hdr, 0, sizeof(struct sg_io_hdr));
     hdr.interface_id = 'S';
     hdr.cmd_len = req->elem.out_sg[1].iov_len;
@@ -230,12 +228,7 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
     ret = bdrv_ioctl(req->dev->bs, SG_IO, &hdr);
     if (ret) {
         status = VIRTIO_BLK_S_UNSUPP;
-        hdr.status = ret;
-        hdr.resid = hdr.dxfer_len;
-    } else if (hdr.status) {
-        status = VIRTIO_BLK_S_IOERR;
-    } else {
-        status = VIRTIO_BLK_S_OK;
+        goto fail;
     }
 
     /*
@@ -258,14 +251,16 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
 
     virtio_blk_req_complete(req, status);
     g_free(req);
-}
 #else
-static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
-{
-    virtio_blk_req_complete(req, VIRTIO_BLK_S_UNSUPP);
+    abort();
+#endif
+
+fail:
+    /* Just put anything nonzero so that the ioctl fails in the guest.  */
+    stl_p(&req->scsi->errors, 255);
+    virtio_blk_req_complete(req, status);
     g_free(req);
 }
-#endif /* __linux__ */
 
 typedef struct MultiReqBuffer {
     BlockRequest        blkreq[32];