executor: slow network device initialization

[Aleksandr 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 
 }