[BusyBox] Busybox-1.0-pre7: "Could not find a loop device"

[Rob Landley]

On Thursday 05 February 2004 11:08, graeme.n.brown at bt.com wrote:
> I 'm putting together a small embedded Linux distro based on Buybox and
> uClibc. Trying Busybox-1.0-pre7 I am having difficulties with /bin/mount of
> loop devices. I get an error message "Could not find a spare loop device"
> when I try to load a CRAMFS image from my startup script [which happens to
> be /etc/rc.d/rc.S] with lines such as

The problem is that a real mtab says ",loop" in the fourth field, and when 
/etc/mtab is a symlink to /proc/mounts the fourth field does NOT say this, so 
umount doesn't know to do a losetup -d on the loop device.  So it leaks loop 
devices, and eventually run out.  (Note: the original mount command does this 
too, it just might take a while to notice.)

However, /proc/mounts DOES indicate what block device the mount is attached to 
(/dev/loop5 or such, it's the first field), and presumably if we do losetup 
-d on each one (or the relevant syscall) after unmount it, it'd silently fail 
without doing any harm when it's not a loop device.  (We could also check the 
major/minor numbers to see if it's a loop device before doing the syscall, 
but that's evil for a number of reasons and almost certainly unnecessary 
work.)

I.E. mount needs a patch, which I've pondered doing for a while but haven't 
gotten around to...

Ooh, hang on.  Look at umount.c, line 179:

#if defined CONFIG_FEATURE_MOUNT_LOOP
    if (freeLoop && blockDevice != NULL && !strncmp("/dev/loop", blockDevice, 
9))
        /* this was a loop device, delete it */
        del_loop(blockDevice);
#endif

Figure out how to switch on CONFIGURE_FEATURE_MOUNT_LOOP and you should be 
good.  (Don't ask me what the business with freeLoop is, I don't know...)

Rob