Investigate valgrind report wrt uninitalized mem in sendmsg

[omoerbeek]

• Program: dnsdist, but potentially more
• Issue type: Bug report

Short description

As reported by valgrind

==30898== Syscall param sendmsg(msg.msg_control) points to uninitialised byte(s)
==30898==    at 0x5DECEB0: sendmsg (sendmsg.c:28)
==30898==    by 0xC2C010: sendfromto(int, char const*, unsigned long, int, ComboAddress const&, ComboAddress const&) (iputils.cc:241)
==30898==    by 0x74CBC0: sendUDPResponse(int, char const*, unsigned short, int, ComboAddress const&, ComboAddress const&) (dnsdist.cc:479)

But do consider:

// if you end up here because valgrind told you were are doing something wrong
// with msgh->msg_controllen, please refer to https://github.com/PowerDNS/pdns/pull/3962
// first.
void addCMsgSrcAddr(struct msghdr* msgh, void* cmsgbuf, const ComboAddress* source, int itfIndex)

But note that this valgrind report is about msg_control and not msg_controllen

[omoerbeek]

Apart form the alignment thing below the code looks ok.
So the reason why valgrind complains could be msg_controllen is set to CMSG_SPACE(size), but only size is filled in addCMsgSrcAddr. So alignment bytes are left uninitialized. It's debatable if valgrind should complain about that.

That said, I do noticed that the buffer is not aligned explicitly. At least OpenBSD recommends that. A PR will be upcoming.