scsi: do not return short responses for emulated commands

The inquiry command, for the case of VPD=1, was returning short
responses; the number of returned bytes was just the number of bytes
in the request, without padding to the specified allocation length
with zero bytes.  This is usually harmless, but it is a violation
of the SCSI specification.

To fix this, always pad with zero bytes to r->cmd.xfer in
scsi_disk_emulate_command, and return at most r->buflen bytes
(the size of the buffer for command data) rather than at most
buflen bytes (the number of bytes that was filled in).

Before this patch, "strace sg_inq -p0x83 /dev/sda" would report a
non-zero resid value.  After this patch, it reports resid=0.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi-disk.c

@@ -652,7 +652,6 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
     if (buflen > SCSI_MAX_INQUIRY_LEN) {
         buflen = SCSI_MAX_INQUIRY_LEN;
     }
-    memset(outbuf, 0, buflen);
 
     outbuf[0] = s->qdev.type & 0x1f;
     outbuf[1] = (s->features & (1 << SCSI_DISK_F_REMOVABLE)) ? 0x80 : 0;
@@ -1596,24 +1595,26 @@ static int32_t scsi_disk_emulate_command(SCSIRequest *req, uint8_t *buf)
         break;
     }
 
+    /*
+     * FIXME: we shouldn't return anything bigger than 4k, but the code
+     * requires the buffer to be as big as req->cmd.xfer in several
+     * places.  So, do not allow CDBs with a very large ALLOCATION
+     * LENGTH.  The real fix would be to modify scsi_read_data and
+     * dma_buf_read, so that they return data beyond the buflen
+     * as all zeros.
+     */
+    if (req->cmd.xfer > 65536) {
+        goto illegal_request;
+    }
+    r->buflen = MAX(4096, req->cmd.xfer);
+
     if (!r->iov.iov_base) {
-        /*
-         * FIXME: we shouldn't return anything bigger than 4k, but the code
-         * requires the buffer to be as big as req->cmd.xfer in several
-         * places.  So, do not allow CDBs with a very large ALLOCATION
-         * LENGTH.  The real fix would be to modify scsi_read_data and
-         * dma_buf_read, so that they return data beyond the buflen
-         * as all zeros.
-         */
-        if (req->cmd.xfer > 65536) {
-            goto illegal_request;
-        }
-        r->buflen = MAX(4096, req->cmd.xfer);
         r->iov.iov_base = qemu_blockalign(s->qdev.conf.bs, r->buflen);
     }
 
     buflen = req->cmd.xfer;
     outbuf = r->iov.iov_base;
+    memset(outbuf, 0, r->buflen);
     switch (req->cmd.buf[0]) {
     case TEST_UNIT_READY:
         assert(!s->tray_open && bdrv_is_inserted(s->qdev.conf.bs));
@@ -1694,12 +1695,14 @@ static int32_t scsi_disk_emulate_command(SCSIRequest *req, uint8_t *buf)
         outbuf[5] = 0;
         outbuf[6] = s->qdev.blocksize >> 8;
         outbuf[7] = 0;
-        buflen = 8;
         break;
     case REQUEST_SENSE:
         /* Just return "NO SENSE".  */
         buflen = scsi_build_sense(NULL, 0, outbuf, r->buflen,
                                   (req->cmd.buf[1] & 1) == 0);
+        if (buflen < 0) {
+            goto illegal_request;
+        }
         break;
     case MECHANISM_STATUS:
         buflen = scsi_emulate_mechanism_status(s, outbuf);
@@ -1770,7 +1773,6 @@ static int32_t scsi_disk_emulate_command(SCSIRequest *req, uint8_t *buf)
             }
 
             /* Protection, exponent and lowest lba field left blank. */
-            buflen = req->cmd.xfer;
             break;
         }
         DPRINTF("Unsupported Service Action In\n");
@@ -1827,7 +1829,7 @@ static int32_t scsi_disk_emulate_command(SCSIRequest *req, uint8_t *buf)
         return 0;
     }
     assert(!r->req.aiocb);
-    r->iov.iov_len = MIN(buflen, req->cmd.xfer);
+    r->iov.iov_len = MIN(r->buflen, req->cmd.xfer);
     if (r->iov.iov_len == 0) {
         scsi_req_complete(&r->req, GOOD);
     }