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

[Alan Stern <stern@rowland.harvard.edu>]

On Wed, Apr 06, 2022 at 12:24:45PM +0300, Maxim Devaev wrote:
> Using the SIGUSR1 signal sent to the "file-storage" thread
> from the userspace, it is possible to break IO operations
> that block the gadget. Thus, it is possible to implement
> "force eject" without stopping the gadget and umounting
> it from the host side.

It's not clear to me how breaking I/O operations allows you to do a 
"force eject".  It seems that what you would need is something like 
fsg_store_file() that omits the curlun->prevent_medium_removal check.
Interrupting a lengthy I/O operation doesn't really have anything to do 
with this.

> There are two problems here:
> 
>   - In order to send a signal, we need to find the thread
>     in procfs, but if several mass storage gadgets are created
>     in the system, each process has the same name and it is
>     impossible to distinguish one gadget from another.
> 
>   - Root privileges are required to send the signal.
> 
> The proposed "break_io" interface solves both problems.
> It allows us to get rid of the procfs search and delegate
> sending the signal to a regular user.

Or to keep this ability restricted to the superuser, if that is desired.

> Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
> ---
>  drivers/usb/gadget/function/f_mass_storage.c | 22 ++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
> index 6ad669dde41c..e9b7c59e1dc4 100644
> --- a/drivers/usb/gadget/function/f_mass_storage.c
> +++ b/drivers/usb/gadget/function/f_mass_storage.c
> @@ -3239,6 +3239,27 @@ static ssize_t fsg_opts_stall_store(struct config_item *item, const char *page,
>  
>  CONFIGFS_ATTR(fsg_opts_, stall);
>  
> +static ssize_t fsg_opts_break_io_store(struct config_item *item,
> +				       const char *page, size_t len)
> +{
> +	struct fsg_opts *opts = to_fsg_opts(item);
> +	unsigned long flags;
> +
> +	mutex_lock(&opts->lock);
> +	spin_lock_irqsave(&opts->common->lock, flags);
> +
> +	if (opts->common->thread_task)
> +		send_sig_info(SIGUSR1, SEND_SIG_PRIV,
> +			      opts->common->thread_task);

You should not call send_sig_info() directly; instead call 
raise_exception().  It already does the work you need (including some 
things you left out).

Alan Stern

> +
> +	spin_unlock_irqrestore(&opts->common->lock, flags);
> +	mutex_unlock(&opts->lock);
> +
> +	return len;
> +}
> +
> +CONFIGFS_ATTR_WO(fsg_opts_, break_io);
> +
>  #ifdef CONFIG_USB_GADGET_DEBUG_FILES
>  static ssize_t fsg_opts_num_buffers_show(struct config_item *item, char *page)
>  {
> @@ -3283,6 +3304,7 @@ CONFIGFS_ATTR(fsg_opts_, num_buffers);
>  
>  static struct configfs_attribute *fsg_attrs[] = {
>  	&fsg_opts_attr_stall,
> +	&fsg_opts_attr_break_io,
>  #ifdef CONFIG_USB_GADGET_DEBUG_FILES
>  	&fsg_opts_attr_num_buffers,
>  #endif
> -- 
> 2.35.1
>