Subscriber OOM crash | suggestion to implement broker incoming message size limit about mosquitto

[karlp]

I suspect (prettttty sure) you're seeing VM overcommit effects. This can be especially obvious on smaller platforms like openwrt. With big enough messages, the malloc will fail, mosquitto can report as you desire. With "just right(wrong)" big messages, you'll be out of memory, but inside the VM overcommit ratio. You can probably play with those tunings to confirm this. See https://www.kernel.org/doc/Documentation/vm/overcommit-accounting

[domasgim]

Most likely not mosquitto library related issue but a platform dependent one, for some reason mosquitto__malloc does not return error on specific payload sizes on my particular device

mosquitto/lib/packet_mosq.c

Line 512 in 15292b2

mosq->in_packet.payload = mosquitto__malloc(mosq->in_packet.remaining_length*sizeof(uint8_t));

Afterwards it fills up the buffer until OOM is invoked when calling the following

mosquitto/lib/packet_mosq.c

Line 520 in 15292b2

read_length = net__read(mosq, &(mosq->in_packet.payload[mosq->in_packet.pos]), mosq->in_packet.to_process);

Still, it would be great to have some configurable value to limit the maximum incoming message from the broker even for non MQTT5 clients