segfault during packet capture with libvma

[jaycedowell]

We upgraded ADP to Ubuntu 18 (gcc 7.4.0), CUDA 10.2, and libvma 8.9.5 and started running into problems with packet capture. Specifically, the first packet captured with Bifrost compiled with VMA = 1 causes a segfault. The problem appears to be related to using an aligned data type here in udp_capture.cpp for accessing the packet's payload.

If I roll back gcc to the version that ships with Ubuntu 16 (5.5.0) I don't have any problems. If I instead keep gcc7 and I switch the definition of itype to a new unaligned type, like this one:

struct unaligned256_type {
	uint8_t data[32];
};

I also don't have any problems. The unaligned type also does not appear to affect the capture speed. Has anyone else run into this?

[benbarsdell]

I don't remember much about it but it's probably related to this comment: https://github.com/ledatelescope/bifrost/blob/master/src/udp_capture.cpp#L56

It sounds like maybe gcc5 wasn't vectorizing the loads but gcc7 is trying to.

[jaycedowell]

Yeah, I had wondered about that comment when I was trying to sort out what was happening. I guess that would imply that we should not be trying to access the packet's contents as aligned memory unless something changes in libvma.