Be bug compatible with other operating systems by allowing non-sequential

interface numbering for USB descriptors in userspace. Else certain USB
control requests using the interface number, won't be recognized by the
USB firmware.

Refer to section 9.2.3 in the USB 2.0 specification:
Interfaces are numbered from zero to one less than the number of concurrent interfaces
supported by the configuration.

PR:		251784
MFC after:	1 week
Sponsored by:	Mellanox Technologies // NVIDIA Networking

1 files changed, 12 insertions, 8 deletions

diff --git a/lib/libusb/libusb20_desc.c b/lib/libusb/libusb20_desc.c
index 517a2e5972c3..3052af09e9c2 100644
--- a/lib/libusb/libusb20_desc.c
+++ b/lib/libusb/libusb20_desc.c
@@ -81,9 +81,10 @@ libusb20_parse_config_desc(const void *config_desc)
 	if (ptr[1] != LIBUSB20_DT_CONFIG) {
 		return (NULL);		/* not config descriptor */
 	}
+
 	/*
-	 * The first "bInterfaceNumber" should never have the value 0xff.
-	 * Then it is corrupt.
+	 * The first "bInterfaceNumber" cannot start at 0xFFFF
+	 * because the field is 8-bit.
 	 */
 	niface_no_alt = 0;
 	nendpoint = 0;
@@ -206,12 +207,15 @@ libusb20_parse_config_desc(const void *config_desc)
 			if (libusb20_me_decode(ptr, ptr[0], &last_if->desc)) {
 				/* ignore */
 			}
-			/*
-			 * Sometimes USB devices have corrupt interface
-			 * descriptors and we need to overwrite the provided
-			 * interface number!
-			 */
-			last_if->desc.bInterfaceNumber = niface - 1;
+
+			/* detect broken USB descriptors when USB debugging is enabled */
+			if (last_if->desc.bInterfaceNumber != (uint8_t)(niface - 1)) {
+				const char *str = getenv("LIBUSB_DEBUG");
+				if (str != NULL && str[0] != '\0' && str[0] != '0') {
+					printf("LIBUSB_DEBUG: bInterfaceNumber(%u) is not sequential(%u)\n",
+					    last_if->desc.bInterfaceNumber, niface - 1);
+				}
+			}
 			last_if->extra.ptr = LIBUSB20_ADD_BYTES(ptr, ptr[0]);
 			last_if->extra.len = 0;
 			last_if->extra.type = LIBUSB20_ME_IS_RAW;