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

[Maxim Devaev <mdevaev@gmail.com>]

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.

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.

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);
+
+	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