The previous code for handling payload length when converting
cmsg structures from host to target had a number of problems:
* we required the msg->msg_controllen to declare the buffer
to have enough space for final trailing padding (we were
checking against CMSG_SPACE), whereas the kernel does not
require this, and common userspace code assumes this. (In
particular, glibc's "try to talk to nscd" code that it will
run on startup will receive a cmsg with a 4 byte payload and
only allocate 4 bytes for it, which was causing us to do
the wrong thing on architectures that need 8-alignment.)
* we weren't correctly handling the fact that the SO_TIMESTAMP
payload may be larger for the target than the host
* we weren't marking the messages with MSG_CTRUNC when we did
need to truncate a message that wasn't truncated by the host,
but were instead logging a QEMU message; since truncation is
always the result of a guest giving us an insufficiently
sized buffer, we should report it to the guest as the kernel
does and don't log anything
Rewrite the parts of the function that deal with length to
fix these issues, and add a comment in target_to_host_cmsg
to explain why the overflow logging it does is a QEMU bug,
not a guest issue.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
c2aeb2586b