hw/net/e1000e_core: Adjust e1000e_write_payload_frag_to_rx_buffers() assert

An assertion in e1000e_write_payload_frag_to_rx_buffers() attempts to
guard against the calling code accidentally trying to write too much
data to a single RX descriptor, such that the E1000EBAState::cur_idx
indexes off the end of the EB1000BAState::written[] array.

Unfortunately it is overzealous: it asserts that cur_idx is in
range after it has been incremented. This will fire incorrectly
for the case where the guest configures four buffers and exactly
enough bytes are written to fill all four of them.

The only places where we use cur_idx and index in to the written[]
array are the functions e1000e_write_hdr_frag_to_rx_buffers() and
e1000e_write_payload_frag_to_rx_buffers(), so we can rewrite this to
assert before doing the array dereference, rather than asserting
after updating cur_idx.

Cc: qemu-stable@nongnu.org
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/e1000e_core.c

@@ -1392,10 +1392,13 @@ e1000e_write_payload_frag_to_rx_buffers(E1000ECore *core,
                                         dma_addr_t data_len)
 {
     while (data_len > 0) {
-        uint32_t cur_buf_len = core->rxbuf_sizes[bastate->cur_idx];
-        uint32_t cur_buf_bytes_left = cur_buf_len -
-                                      bastate->written[bastate->cur_idx];
-        uint32_t bytes_to_write = MIN(data_len, cur_buf_bytes_left);
+        uint32_t cur_buf_len, cur_buf_bytes_left, bytes_to_write;
+
+        assert(bastate->cur_idx < MAX_PS_BUFFERS);
+
+        cur_buf_len = core->rxbuf_sizes[bastate->cur_idx];
+        cur_buf_bytes_left = cur_buf_len - bastate->written[bastate->cur_idx];
+        bytes_to_write = MIN(data_len, cur_buf_bytes_left);
 
         trace_e1000e_rx_desc_buff_write(bastate->cur_idx,
                                         ba[bastate->cur_idx],
@@ -1414,8 +1417,6 @@ e1000e_write_payload_frag_to_rx_buffers(E1000ECore *core,
         if (bastate->written[bastate->cur_idx] == cur_buf_len) {
             bastate->cur_idx++;
         }
-
-        assert(bastate->cur_idx < MAX_PS_BUFFERS);
     }
 }