Re: [PATCH] usb: gadget: f_mass_storage: break IO operations via configfs

[Alan Stern]

On Sun, Apr 10, 2022 at 01:42:28AM +0300, Maxim Devaev wrote:
> В Sat, 9 Apr 2022 16:22:29 -0400
> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > > I'm using Raspberry Pi with DWC2. So:
> > > - Connect RPi-based gadget to the Linux host.
> > > - Set image in the "file" attribute.  
> > 
> > Exactly what is the full pathname you're using for the "file" attribute?
> 
> /sys/kernel/config/usb_gadget/kvmd/functions/mass_storage.usb0/lun.0/file

Yeah, that doesn't seem right at all.

You're doing this under KVM, right?  Is the gadget driver running in the 
host OS or the guest OS?  And the sysfs file accesses -- are they in the 
host's filesystem or in the guest's?

What happens if you don't use KVM and just load the gadget driver on the 
physical machine?

> > > - Mount gadget's drive on the Linux host.
> > > - Umount it.
> > > - Try to eject using emptying the "file" attribute.
> > > - Get EBUSY error.  
> > 
> > This must mean that some program on the host is keeping the device file 
> > open, even though it isn't mounted.  (I tried running a similar test on 
> > my system and it worked perfectly, with no other programs accessing the 
> > device).  You might be able to identify which program is accessing the 
> > device by running lsof on the host and searching the output for the 
> > device name.
> 
> I've been thinking about it too. I tried lsof but it didn't display anything
> related with sr0 device. But after execution the "eject" command on the host,
> I was able to emptify the "file" attribute of the gadget.

The eject command should set the "file" attribute to an empty string all 
by itself, with no need for you to do anything else.

> > I also tried sending a USR1 signal to the driver's kernel thread while 
> > an image was mounted and being accessed.  It did clear the prevent_allow 
> > flag, so I could eject the image.  But it also caused a 30-second delay 
> > on the host, as predicted.  Now, maybe you don't care about such delays 
> > when you're going to eject the media anyway, but it still seems like a 
> > bad thing to do.
> 
> It looks like the prevent_medium_removal flag switching really works better in this case.

I don't understand that comment.  In what case?  Works better than what?

> > > > > I have reflected on the rest of your arguments and changed my mind.
> > > > > I think that "forced_eject" for a specific lun without interrupting operations would
> > > > > really be the best solution. I wrote a simple patch and tested it, everything seems
> > > > > to work. What do you think about something like this?
> > > > > 
> > > > > 
> > > > > static ssize_t fsg_lun_opts_forced_eject_store(struct config_item *item,
> > > > >                                                const char *page, size_t len)
> > > > > {
> > > > >         struct fsg_lun_opts *opts = to_fsg_lun_opts(item);
> > > > >         struct fsg_opts *fsg_opts = to_fsg_opts(opts->group.cg_item.ci_parent);
> > > > >         int ret;
> > > > > 
> > > > >         opts->lun->prevent_medium_removal = 0;
> > > > >         ret = fsg_store_file(opts->lun, &fsg_opts->common->filesem, "", 0);
> > > > >         return ret < 0 ? ret : len;
> > > > > }
> > > > > 
> > > > > CONFIGFS_ATTR_WO(fsg_lun_opts_, forced_eject);    
> > > > 
> > > > The basic idea is right.  But this should not be a CONFIGFS option; it 
> > > > should be an ordinary LUN attribute.  For an example, see the definition of 
> > > > file_store() in f_mass_storage.c; your routine should look very similar.  
> > > 
> > > Okay, but where this attribute is located in sysfs? How can I use it?  
> > 
> > Well, it's going to be in different places depending on what UDC driver 
> > your gadget uses.  On my system I'm using the dummy_udc driver, so the 
> > sysfs "file" attribute is located at:
> > 
> > 	/sys/devices/platform/dummy_ucd.0/gadget/lun0/file
> > 
> > If instead you're looking at
> > 
> > 	/sys/module/g_mass_storage/parameters/file
> > 
> > or in some configfs directory, that's the wrong place.  You can eject 
> > the media simply by doing (as root):
> > 
> > 	echo >/sys/devices/.../gadget/lun0/file
> > 
> > (fill in the "..." appropriately for your system).
> > 
> > > Sorry for the stupid question.  
> > 
> > Not at all.
> 
> Thanks! Unfortunately I'm using dwc2 driver and it doesn't have any gadget parameters
> outside of the configfs:
> 
> [root@pikvm ~]# find /sys -iname lun0
> [root@pikvm ~]# find /sys -iname lun.0
> /sys/kernel/config/usb_gadget/kvmd/functions/mass_storage.usb0/lun.0
> [root@pikvm ~]#
> 
> So in my local case configfs is only way to place forced_eject :(

That can't possibly be right.  Again, we may be miscommunicating because 
of the way you're using KVM.

What happens if you set up the gadget using g-mass-storage instead of 
configfs?  For example:

	modprobe g-mass-storage cdrom=y removable=y ro=y file=...

> Could we add both device attrs and configfs file?

No.  Configfs files are for setting up the gadget in the first place, or 
changing its configuration while it isn't attached to a host.  Device 
attribute files are for modifying the gadget while it is running.

Alan Stern