hw/net/msf2-emac: Don't modify descriptor in-place in emac_store_desc()

hw/net/msf2-emac: Don't modify descriptor in-place in emac_store_desc()

The msf2-emac ethernet controller has functions emac_load_desc() and
emac_store_desc() which read and write the in-memory descriptor
blocks and handle conversion between guest and host endianness.

As currently written, emac_store_desc() does the endianness
conversion in-place; this means that it effectively consumes the
input EmacDesc struct, because on a big-endian host the fields will
be overwritten with the little-endian versions of their values.
Unfortunately, in all the callsites the code continues to access
fields in the EmacDesc struct after it has called emac_store_desc()
-- specifically, it looks at the d.next field.

The effect of this is that on a big-endian host networking doesn't
work because the address of the next descriptor is corrupted.

We could fix this by making the callsite avoid using the struct; but
it's more robust to have emac_store_desc() leave its input alone.

(emac_load_desc() also does an in-place conversion, but here this is
fine, because the function is supposed to be initializing the
struct.)

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20230424151919.1333299-1-peter.maydell@linaro.org