When binding the ConfigFS gadget to a UDC, the functions in each
configuration are added in list order. However, if usb_add_function()
fails, the failed function is put back on its configuration's
func_list and purge_configs_funcs() is called to further clean up.

purge_configs_funcs() iterates over the configurations and functions
in forward order, calling unbind() on each of the previously added
functions. But after doing so, each function gets moved to the
tail of the configuration's func_list. This results in reshuffling
the original order of the functions within a configuration such
that the failed function now appears first even though it may have
originally appeared in the middle or even end of the list. At this
point if the ConfigFS gadget is attempted to re-bind to the UDC,
the functions will be added in a different order than intended,
with the only recourse being to remove and relink the functions all
over again.

An example of this as follows:

ln -s functions/mass_storage.0 configs/c.1
ln -s functions/ncm.0 configs/c.1
ln -s functions/ffs.adb configs/c.1	# oops, forgot to start adbd
echo "<udc device>" > UDC		# fails
start adbd
echo "<udc device>" > UDC		# now succeeds, but...
					# bind order is
					# "ADB", mass_storage, ncm

[30133.118289] configfs-gadget gadget: adding 'Mass Storage Function'/ffffff810af87200 to config 'c'/ffffff817d6a2520
[30133.119875] configfs-gadget gadget: adding 'cdc_network'/ffffff80f48d1a00 to config 'c'/ffffff817d6a2520
[30133.119974] using random self ethernet address
[30133.120002] using random host ethernet address
[30133.139604] usb0: HOST MAC 3e:27:46:ba:3e:26
[30133.140015] usb0: MAC 6e:28:7e:42:66:6a
[30133.140062] configfs-gadget gadget: adding 'Function FS Gadget'/ffffff80f3868438 to config 'c'/ffffff817d6a2520
[30133.140081] configfs-gadget gadget: adding 'Function FS Gadget'/ffffff80f3868438 --> -19
[30133.140098] configfs-gadget gadget: unbind function 'Mass Storage Function'/ffffff810af87200
[30133.140119] configfs-gadget gadget: unbind function 'cdc_network'/ffffff80f48d1a00
[30133.173201] configfs-gadget a600000.dwc3: failed to start g1: -19
[30136.661933] init: starting service 'adbd'...
[30136.700126] read descriptors
[30136.700413] read strings
[30138.574484] configfs-gadget gadget: adding 'Function FS Gadget'/ffffff80f3868438 to config 'c'/ffffff817d6a2520
[30138.575497] configfs-gadget gadget: adding 'Mass Storage Function'/ffffff810af87200 to config 'c'/ffffff817d6a2520
[30138.575554] configfs-gadget gadget: adding 'cdc_network'/ffffff80f48d1a00 to config 'c'/ffffff817d6a2520
[30138.575631] using random self ethernet address
[30138.575660] using random host ethernet address
[30138.595338] usb0: HOST MAC 2e:cf:43:cd:ca:c8
[30138.597160] usb0: MAC 6a:f0:9f:ee:82:a0
[30138.791490] configfs-gadget gadget: super-speed config #1: c

Fix this by reversing the iteration order of the functions in
purge_config_funcs() when unbinding them, and adding them back to
the config's func_list at the head instead of the tail. This
ensures that we unbind and unwind back to the original list order.

Fixes: 88af8bbe ("usb: gadget: the start of the configfs interface")
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Link: https://lore.kernel.org/r/20201229224443.31623-1-jackp@codeaurora.org
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>