very good discussion.... On Mon, Oct 20, 2008 at 6:46 PM, Luciano Rocha <luciano@xxxxxxxxxxx> wrote: > On Mon, Oct 20, 2008 at 03:28:07PM +0530, rishi agrawal wrote: >> Hello Everyone, >> >> I would like to know about the use of "losetup". >> >> By use I want mean that in what all situations will I use losetup. Its >> okay that it attaches a loop device to any regular file but how do we >> use this feature. > > Anytime you use the option loop to mount, you use (indirectly) losetup: > > mount -o loop,ro distro.iso /mnt > YES, and sometimes, u don't need to mount with loop option to read these iso files, (or files created via dd + mkfs): http://user-mode-linux.sourceforge.net/new/index.html (ie, the UML executable will take as input these files and mount it as "ubd" - UML block device, naming is my guess). Implementation? Where is it located? it is in drivers/block (which is why it worked only for regular file and block devices, but network loopback does exists as well): loop.c: static int __init loop_init(void) { int i, nr; if (register_blkdev(LOOP_MAJOR, "loop")) return -EIO; ===>lsmod |grep loop "loop" must exists first before u can do any mounting. For UML, the udb is implemented inside arch/um/drivers/ubd_*.c. Probing further, how is the information flowed from drivers/block/loop.c all the way to fs/isofs, where ISO9660 file format recognition is implemented? We start here: file root_fs_val ===> root_fs_val: Linux rev 1.0 ext2 filesystem data So the file format is ext2 fs formatted. Next we use "strace" (output is highly truncated for sake of clarity): strace mount -o loop -t ext2 root_fs_val /mnt/test3====> execve("/bin/mount", ["mount", "-o", "loop", "-t", "ext2", "root_fs_val", "/mnt/test3"], [/* 66 vars */]) = 0 stat64("/dev/loop0", {st_mode=S_IFBLK|0640, st_rdev=makedev(7, 0), ...}) = 0 open("/dev/loop0", O_RDONLY|O_LARGEFILE) = 3 open("root_fs_val", O_RDWR|O_LARGEFILE) = 3 open("/dev/loop0", O_RDWR|O_LARGEFILE) = 4 stat64("/sbin/mount.ext2", 0xbf8e9eb0) = -1 ENOENT (No such file or directory) mount("/dev/loop0", "/mnt/test3", "ext2", MS_MGC_VAL, "") = 0 read(4, "/dev/sdb1 / ext3 rw,acl,user_xat"..., 482) = 482 write(4, "/hdc2/download1/qimage/root_fs_v"..., 74) = 74 The last two lines are reading/writing the ACL attributes. There goes losetup under the hood :-). -- Regards, Peter Teoh -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
