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@xxxxxxxxx>
> ---
> 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
>
