[PATCH] usb: gadget: f_fs: Remove WARN_ON in functionfs_bind

[Akash M <akash.m5@samsung.com>]

This commit addresses an issue related to below kernel panic where
panic_on_warn is enabled. It is caused by the unnecessary use of WARN_ON
in functionsfs_bind, which easily leads to the following scenarios.

1.adb_write in adbd               2. UDC write via configfs
  =================	             =====================

->usb_ffs_open_thread()           ->UDC write
 ->open_functionfs()               ->configfs_write_iter()
  ->adb_open()                      ->gadget_dev_desc_UDC_store()
   ->adb_write()                     ->usb_gadget_register_driver_owner
                                      ->driver_register()
->StartMonitor()                       ->bus_add_driver()
 ->adb_read()                           ->gadget_bind_driver()
<times-out without BIND event>           ->configfs_composite_bind()
                                          ->usb_add_function()
->open_functionfs()                        ->ffs_func_bind()
 ->adb_open()                               ->functionfs_bind()
                                       <ffs->state !=FFS_ACTIVE>

The adb_open, adb_read, and adb_write operations are invoked from the
daemon, but trying to bind the function is a process that is invoked by
UDC write through configfs, which opens up the possibility of a race
condition between the two paths. In this race scenario, the kernel panic
occurs due to the WARN_ON from functionfs_bind when panic_on_warn is
enabled. This commit fixes the kernel panic by removing the unnecessary
WARN_ON.

Kernel panic - not syncing: kernel: panic_on_warn set ...
[   14.542395] Call trace:
[   14.542464]  ffs_func_bind+0x1c8/0x14a8
[   14.542468]  usb_add_function+0xcc/0x1f0
[   14.542473]  configfs_composite_bind+0x468/0x588
[   14.542478]  gadget_bind_driver+0x108/0x27c
[   14.542483]  really_probe+0x190/0x374
[   14.542488]  __driver_probe_device+0xa0/0x12c
[   14.542492]  driver_probe_device+0x3c/0x220
[   14.542498]  __driver_attach+0x11c/0x1fc
[   14.542502]  bus_for_each_dev+0x104/0x160
[   14.542506]  driver_attach+0x24/0x34
[   14.542510]  bus_add_driver+0x154/0x270
[   14.542514]  driver_register+0x68/0x104
[   14.542518]  usb_gadget_register_driver_owner+0x48/0xf4
[   14.542523]  gadget_dev_desc_UDC_store+0xf8/0x144
[   14.542526]  configfs_write_iter+0xf0/0x138

Fixes: ddf8abd25994 ("USB: f_fs: the FunctionFS driver")
Cc: stable@vger.kernel.org
Signed-off-by: Akash M <akash.m5@samsung.com>

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 2920f8000bbd..92c883440e02 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -2285,7 +2285,7 @@ static int functionfs_bind(struct ffs_data *ffs, struct usb_composite_dev *cdev)
 	struct usb_gadget_strings **lang;
 	int first_id;
 
-	if (WARN_ON(ffs->state != FFS_ACTIVE
+	if ((ffs->state != FFS_ACTIVE
 		 || test_and_set_bit(FFS_FL_BOUND, &ffs->flags)))
 		return -EBADFD;
 
-- 
2.17.1