Re: USB Media card - works at boot-up, removal works, re-insert doesn't

[Luke Kenneth Casson Leighton]

*cackle* i had a word with phil (the nutter) hands and out of a random
conversation he said "why don't you use a userspace filesystem?" and i
went "duh, of course, yadayada".

... so i downloaded fuse, with the intention of writing a proxy
filesystem only to find that fuse/example/fusexmp is EXACTLY what
is needed.

haha!

due to the way that fusexmp works (it's a stateless proxy redirector)
the issue of umount on the proxied filesystems attempting to kick the
feet from out under a fusexmp-mounted filesystem just... doesn't happen!

okay, the plan:

* using fuse, remount partitions that the user is to be allowed to see:

	/media on /mnt/media
	/home/username on /Documents

* modify KVM (or more likely, HAL) to pseudo-"chroot" anything that goes
  into /media/[somedir] to make it look like it's on /mnt/media/[somedir]

the thing about the fusexmp example is that it doesn't _actually_ touch
the filesystem until access is requested.

e.g. the xmp_read function does an open(path, O_RDONLY), a pread(),
followed by a close().  likewise for write, getdir [consisting of an
opendir(), readdir() sequence and a closedir()]

i sincerely couldn't _give_ a stuff how inefficient that is: it cleanly
and clearly solves the problem [of usb media getting unplugged without
warning].


i note with interest that fuse has kernel-level "cacheing"
(fusermount -c).  obviously this would need to be disabled [the -c
option not used on a mount].



the only issue (for me) is.... fuse doesn't support xattrs, and i aim to
use this on an selinux system.

*sigh*.  a-hacking-i-will-go...

On Tue, Sep 21, 2004 at 10:48:42PM +0100, Luke Kenneth Casson Leighton wrote:
> On Tue, Sep 21, 2004 at 11:08:24PM +0200, David Zeuthen wrote:
> > On Tue, 2004-09-21 at 21:29 +0100, Luke Kenneth Casson Leighton wrote:
> > > hi david,
> > > 
> > > okay i tracked down a bit further, i think i outlined a bit towards the
> > > end of the original message:
> > > 
> > 
> > This is quite interesting,
> > 
> > > * umount -l /media/usbdisk7 is being done but konqueror still has
> > >   directory handles open on it.
> > > 
> > > * therefore ioctl("/dev/sdc", BLKRRPART) returns an error "Device Busy".
> > > 
> > > * therefore no notifications go to the child volumes.
> > > 
> > > i "solved" the problem by doing this drastic and vicious assault on
> > > KDE programs:
> > > 
> > > 	system("lsof %s | cut -d' ' -f2 | sort | uniq | xargs kill -TERM",
> > > 	       mount_point);
> > > 
> > > when that is done from HAL just before the unmount -l, the problem goes
> > > away.
> > > 
> > 
> > Heh :-)
> > 
> > > *beam* :)
> > > 
> > > ... of course, i wouldn't dream of suggesting to anyone that this
> > > technique actually be used in a production environment, but it _does_
> > > work.
> > > 
> > > *lol*.
> > > 
> > > but seriously, the problems are caused, i believe, by KDE's kio_kfile
> > > plugin, which does "directory notify" requests.
> > > 
> > > i'm endeavouring to track down whether disabling "dir notify" solves the
> > > problem of konqueror keeping directory handles open.
> > > 
> > 
> > To me, it seems like a severe kernel bug if a userspace process,
> > *especially* if it's unprivileged, can keep the kernel from emitting
> > hotplug remove events when a device is physically detached. It would be
> > interesting to create a minimal program to reproduce this.
> 
> that's quite straightforward: i guess that an appx 30 character perl program
> or a 3 line python program 'd do the job.
> 
> or just using opendir() in c, here y'go...
> 
> 
> 	#include <stdio.h>
> 	#include <dirent.h>
> 
> 	int main(int argc, char *argv[])
> 	{
> 		opendir(argv[1]);
> 		sleep(3600);
> 	}
> 
> compile and test:
> 
> 	lkcl@highfield:~/src$ gcc opendir.c
> 	lkcl@highfield:~/src$ ./a.out /tmp &
> 	[2] 23520
> 	lkcl@highfield:~/src$ 
> 	lkcl@highfield:~/src$ 
> 	lkcl@highfield:~/src$ lsof /tmp
> 	COMMAND   PID USER   FD   TYPE DEVICE SIZE  NODE NAME
> 	a.out   23520 lkcl    3r   DIR    3,2 4096 96983 /tmp
> 	lkcl@highfield:~/src$ konqueror file:/tmp &
> 	[3] 23527
> 	lkcl@highfield:~/src$ konqueror: WARNING: KGenericFactory: instance requested but no instance name passed to the constructor!
> 	lsof /tmp
> 	COMMAND     PID USER   FD   TYPE DEVICE SIZE  NODE NAME
> 	a.out     23520 lkcl    3r   DIR    3,2 4096 96983 /tmp
> 	konqueror 23527 lkcl  128r   DIR    3,2 4096 96983 /tmp
> 
> 
> [of course, changing it to "umount -lf" _also_ solves the
>  problem by making konqueror break: result, after the first remove,
>  you have to manually close konqueror, insert the media, remove
>  the media card (again), reinsert it (again), re-run konqueror]
> 
> l.
> 
> -- 
> --
> Truth, honesty and respect are rare commodities that all spring from
> the same well: Love.  If you love yourself and everyone and everything
> around you, funnily and coincidentally enough, life gets a lot better.
> --
> <a href="http://lkcl.net";>      lkcl.net      </a> <br />
> <a href="mailto:lkcl@xxxxxxxx";> lkcl@xxxxxxxx </a> <br />
> 

-- 
--
Truth, honesty and respect are rare commodities that all spring from
the same well: Love.  If you love yourself and everyone and everything
around you, funnily and coincidentally enough, life gets a lot better.
--
<a href="http://lkcl.net";>      lkcl.net      </a> <br />
<a href="mailto:lkcl@xxxxxxxx";> lkcl@xxxxxxxx </a> <br />

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