virtio-9p: message header is 7-byte long

The 9p spec at http://man.cat-v.org/plan_9/5/intro reads:

 "Each 9P message begins with a four-byte size field specify-
  ing the length in bytes of the complete message including
  the four bytes of the size field itself.  The next byte is
  the message type, one of the constants in the enumeration in
  the include file <fcall.h>.  The next two bytes are an iden-
  tifying tag, described below."

ie, each message starts with a 7-byte long header.

The core 9P code already assumes this pretty much everywhere. This patch
does the following:
- makes the assumption explicit in the common 9p.h header, since it isn't
  related to the transport
- open codes the header size in handle_9p_output() and hardens the sanity
  check on the space needed for the reply message

Signed-off-by: Greg Kurz <[email protected]>
Acked-by: Stefano Stabellini <[email protected]>

diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index c886ba78d2ee8bb006d7b5461679fc992bd2a6f0..aac1b0b2ce3d498a2092f7a9f395f89e929d29fc 100644 (file)
--- a/hw/9pfs/9p.h
+++ b/hw/9pfs/9p.h
@@ -124,6 +124,11 @@ typedef struct {
     uint8_t id;
     uint16_t tag_le;
 } QEMU_PACKED P9MsgHeader;
+/* According to the specification, 9p messages start with a 7-byte header.
+ * Since most of the code uses this header size in literal form, we must be
+ * sure this is indeed the case.
+ */
+QEMU_BUILD_BUG_ON(sizeof(P9MsgHeader) != 7);
 
 struct V9fsPDU
 {

diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
index 3380bfc0c5519d9b74e0a67f67e271e96036e5ae..1a68c1622d3a6faf9aec2d80b20abfd98960f6c4 100644 (file)
--- a/hw/9pfs/virtio-9p-device.c
+++ b/hw/9pfs/virtio-9p-device.c
@@ -53,17 +53,15 @@ static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
             goto out_free_pdu;
         }
 
-        if (elem->in_num == 0) {
+        if (iov_size(elem->in_sg, elem->in_num) < 7) {
             virtio_error(vdev,
                          "The guest sent a VirtFS request without space for "
                          "the reply");
             goto out_free_req;
         }
-        QEMU_BUILD_BUG_ON(sizeof(out) != 7);
 
-        len = iov_to_buf(elem->out_sg, elem->out_num, 0,
-                         &out, sizeof(out));
-        if (len != sizeof(out)) {
+        len = iov_to_buf(elem->out_sg, elem->out_num, 0, &out, 7);
+        if (len != 7) {
             virtio_error(vdev, "The guest sent a malformed VirtFS request: "
                          "header size is %zd, should be 7", len);
             goto out_free_req;