Standalone network block device client now working. :)

Denys Vlasenko vda.linux at
Wed Sep 22 00:19:17 UTC 2010

On Monday 13 September 2010 02:51, Rob Landley wrote:
> Finally got some time to debug this, and it now actually works.
>   nbd 9876 /dev/nbd0
>   mount /dev/nbd0 /blah
> It's not busyboxified at all (standalone, no external dependencies).  If a 
> server exports multiple NBDs it only supports selecting them by port number 
> (there's an extention to select them by name; I didn't need it so didn't 
> implement it).
> Its command line option parsing is somewhere between "pathetic" and 
> "nonexistent", so there's no if() around the call to daemon() and such, and 
> the 4k block size is hardwired (which means with an MTU of 1500 the packets 
> will be split, but just about all modern filesystems are using a 4k block size 
> anyway and gigabit ethernet with a 1500 byte mtu is silly).
> But it Worked For Me.
> Denys: you wanna busyboxify it, or wait for me to get around to it now that my 
> immediate need for something working is satisfied? :)

I am attaching the busyboxified version.

Outer for(;;) loop looks wrong. For one, on the second iteration
nbd file descriptor will be closed. I have doubts ioctls can work
on closed fds. Repeatedly daemonizing also looks strange.

I imagine the loop should just be removed, body should execute just once.

                // Open the device to force reread of the partition table.
                if (!fork()) {
                        char *s = strrchr(device, '/');
                        sprintf(data, "/sys/block/%.32s/pid", s ? s+1 : device);
                        // Is it up yet?
                        for (;;) {
                                temp = open(data, O_RDONLY);
                                if (temp == -1) sleep(1);
                                else {
                        close(open(device, O_RDONLY));

Why do you do it in a child? nbd-client's parent might actually
prefer to know that this step is complete before nbd-client
exits. As it stands, now reread happens "somewhere in the future,
maybe in 2050, who knows".

                // Process NBD requests until further notice.

                if (ioctl(nbd, NBD_DO_IT)>=0 || errno==EBADR) break;

Need more comments here.
Does this ioctl block for a long time, processing many requests?
On what condition does it return? Etc.


 * Copyright 2010 Rob Landley <rob at>
 * Licensed under GPLv2, see file LICENSE in this source tree.
#include "libbb.h"
#include <netinet/tcp.h>
#include <linux/fs.h>

//applet:IF_NBDCLIENT(APPLET_ODDNAME(nbd-client, nbdclient, _BB_DIR_USR_SBIN, _BB_SUID_DROP, nbdclient))

//kbuild:lib-$(CONFIG_NBDCLIENT) += nbd-client.o

//config:config NBDCLIENT
//config:	bool "nbd-client"
//config:	default y
//config:	help
//config:	  Network block device client

#define NBD_SET_SOCK          _IO(0xab, 0)
#define NBD_SET_BLKSIZE       _IO(0xab, 1)
#define NBD_SET_SIZE          _IO(0xab, 2)
#define NBD_DO_IT             _IO(0xab, 3)
#define NBD_CLEAR_SOCK        _IO(0xab, 4)
#define NBD_CLEAR_QUEUE       _IO(0xab, 5)
#define NBD_PRINT_DEBUG       _IO(0xab, 6)
#define NBD_SET_SIZE_BLOCKS   _IO(0xab, 7)
#define NBD_DISCONNECT        _IO(0xab, 8)
#define NBD_SET_TIMEOUT       _IO(0xab, 9)

//usage:#define nbdclient_trivial_usage
//usage:       "HOST PORT BLOCKDEV"
//usage:#define nbdclient_full_usage "\n\n"
//usage:       "Connect to HOST and provide a network block device on BLOCKDEV"

//TODO: more compat with nbd-client version 2.9.13. It says:
// Usage: nbd-client [bs=blocksize] [timeout=sec] host port nbd_device [-swap] [-persist] [-nofork]
// Or   : nbd-client -d nbd_device
// Or   : nbd-client -c nbd_device
// Default value for blocksize is 1024 (recommended for ethernet)
// Allowed values for blocksize are 512,1024,2048,4096
// Note, that kernel 2.4.2 and older ones do not work correctly with
// blocksizes other than 1024 without patches

int nbdclient_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int nbdclient_main(int argc, char **argv)
	int sock, nbd;
	unsigned long timeout = 0;
	char *host, *port, *device;
	struct nbd_header_t {
		uint64_t magic1; /* "NBDMAGIC" */
		uint64_t magic2; /* 0x420281861253 big endian */
		uint64_t devsize;
		uint32_t flags;
		char data[124];
	} nbd_header;
	struct bug_check {
		char c[offsetof(struct nbd_header_t, data) == 8+8+8+4 ? 1 : -1];

	// Parse command line stuff (just a stub now)
	if (argc != 4)

	host = argv[1];
	port = argv[2];
	device = argv[3];

	// Make sure the /dev/nbd exists.
	nbd = xopen(device, O_RDWR);

	// Repeat until spanked
	for (;;) {
		int ro;

		// Find and connect to server
		sock = create_and_connect_stream_or_die(host, xatou16(port));
		setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, &const_int_1, sizeof(const_int_1));

		// Log on to the server.
		xread(sock, &nbd_header, 8+8+8+4 + 124);
		if (memcmp(&nbd_header.magic1, "NBDMAGIC""\x00\x00\x42\x02\x81\x86\x12\x53", 16) != 0) {
			bb_error_msg_and_die("login failed");

		// Set 4k block size.  Everything uses that these days.
		ioctl(nbd, NBD_SET_BLKSIZE, 4096);
		ioctl(nbd, NBD_SET_SIZE_BLOCKS, SWAP_BE64(nbd_header.devsize)/4096);
		ioctl(nbd, NBD_CLEAR_SOCK);

		// If the sucker was exported read only, respect that locally.
		ro = (nbd_header.flags & SWAP_BE32(2)) / SWAP_BE32(2);
		if (ioctl(nbd, BLKROSET, &ro) < 0) {
			bb_error_msg_and_die("login failed");

		if (timeout)
			if (ioctl(nbd, NBD_SET_TIMEOUT, timeout))
		if (ioctl(nbd, NBD_SET_SOCK, sock))

		// if (swap) mlockall(MCL_CURRENT|MCL_FUTURE);

		// Open the device to force reread of the partition table.
		if (!fork()) {
			char *s = strrchr(device, '/');
			sprintf(, "/sys/block/%.32s/pid", s ? s+1 : device);
			// Is it up yet?
			for (;;) {
				int fd = open(, O_RDONLY);
				if (fd >= 0) {
			open(device, O_RDONLY);
			return 0;

		// Daemonize here.
		daemon(0, 0);

		// Process NBD requests until further notice.
		if (ioctl(nbd, NBD_DO_IT) >= 0 || errno == EBADR)

	// Flush queue and exit.
	ioctl(nbd, NBD_CLEAR_QUEUE);
	ioctl(nbd, NBD_CLEAR_SOCK);

	return 0;

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