[PATCH] usb: gadget: composite: fix possible kernel oops in composite_setup()

[Sergey Shtylyov <s.shtylyov@omp.ru>]

list_first_entry() should never return NULL -- which makes Svace complain
about the next *if* statement's condition being always false. What's worse,
list_first_entry() won't work correctly when the list is empty -- in that
case, passing config->descriptors[0] to memcpy() further below will cause
dereferencing of a garbage pointer read from cdev->qw_sign[] and so (most
probably) a kernel oops. Use list_first_entry_or_null() that returns NULL
if the list is empty; fix the strange indentation below, while at it...

TTBOMK, this situation shouldn't happen with the current gadget drivers --
however there's no guarantee that usb_add_config[_only]() is called by any
gadget driver; and anyway, Svace's complaints would be silenced...

Found by Linux Verification Center (linuxtesting.org) with the Svace static
analysis tool.

Fixes: 53e6242db8d6 ("usb: gadget: composite: add USB_DT_OTG request handling")
Suggested-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>

---
This patch is against the usb-linus branch of Greg KH's usb.git repo.

 drivers/usb/gadget/composite.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: usb/drivers/usb/gadget/composite.c
===================================================================
--- usb.orig/drivers/usb/gadget/composite.c
+++ usb/drivers/usb/gadget/composite.c
@@ -1887,8 +1887,8 @@ composite_setup(struct usb_gadget *gadge
 				if (cdev->config)
 					config = cdev->config;
 				else
-					config = list_first_entry(
-							&cdev->configs,
+					config = list_first_entry_or_null(
+						&cdev->configs,
 						struct usb_configuration, list);
 				if (!config)
 					goto done;