executor: slow network device initialization

[a-nogikh]

When executor restarts are frequent, we're observing increased executor startup times.

Some observations:

• We spend ~4 seconds here. This time is stable and does not depend on the number of procs.

syzkaller/executor/common_linux.h

Line 1203 in 1e153dc

write_file("/sys/bus/netdevsim/del_device", "%u", addr);

I wonder if it can be replaced with a DEVLINK_CMD_RELOAD command.

In general, I've found little to nothing documentation about netdevsim, which does not make things easier.

• We spend from 1 to 10 seconds here. It's very quick when procs=1 and gets 1-2 seconds slower with every extra proc.

syzkaller/executor/common_linux.h

Line 1490 in 1e153dc

static void initialize_netdevices(void)

Most time is spent here:

syzkaller/executor/common_linux.h

Lines 1679 to 1691 in 1e153dc

	for (i = 0; i < sizeof(devices) / (sizeof(devices[0])); i++) {
	// Assign some unique address to devices. Some devices won't up without this.
	// Shift addresses by 10 because 0 subnet address can mean special things.
	char addr[32];
	sprintf(addr, DEV_IPV4, i + 10);
	netlink_add_addr4(&nlmsg, sock, devices[i].name, addr);
	if (!devices[i].noipv6) {
	sprintf(addr, DEV_IPV6, i + 10);
	netlink_add_addr6(&nlmsg, sock, devices[i].name, addr);
	}
	uint64 macaddr = DEV_MAC + ((i + 10ull) << 40);
	netlink_device_change(&nlmsg, sock, devices[i].name, true, 0, &macaddr, devices[i].macsize, NULL);
	}

Note To get these observations under -debug, we need to remove setting procs=1 in the debug mode:

syzkaller/syz-manager/manager.go

Lines 803 to 806 in 1e153dc

	if *flagDebug {
	fuzzerV = 100
	procs = 1
	}