generic: ipeth: fix iOS 14 tethering

This fixes tethering with devices using iOS 14. Prior to this patch,
connections to remote endpoints were not possible while data transfers
between the OpenWrt device and the iOS endpoints worked fine.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit f64496f30f2ef97124dc4e13a48ee0de9d51832e)

target/linux/generic/backport-4.14/315-v5.10-usbnet-ipeth-fix-connectivity-with-ios-14.patch

@@ -0,0 +1,44 @@
+From:   Yves-Alexis Perez <corsac@corsac.net>
+Subject: [PATCH] usbnet: ipheth: fix connectivity with iOS 14
+Date:   Thu, 19 Nov 2020 18:24:39 +0100
+Archived-At: <https://lore.kernel.org/stable/20201119172439.94988-1-corsac@corsac.net/>
+List-Post: <mailto:stable@vger.kernel.org>
+
+Starting with iOS 14 released in September 2020, connectivity using the
+personal hotspot USB tethering function of iOS devices is broken.
+
+Communication between the host and the device (for example ICMP traffic
+or DNS resolution using the DNS service running in the device itself)
+works fine, but communication to endpoints further away doesn't work.
+
+Investigation on the matter shows that UDP and ICMP traffic from the
+tethered host is reaching the Internet at all. For TCP traffic there are
+exchanges between tethered host and server but packets are modified in
+transit leading to impossible communication.
+
+After some trials Matti Vuorela discovered that reducing the URB buffer
+size by two bytes restored the previous behavior. While a better
+solution might exist to fix the issue, since the protocol is not
+publicly documented and considering the small size of the fix, let's do
+that.
+
+Tested-by: Matti Vuorela <matti.vuorela@bitfactor.fi>
+Signed-off-by: Yves-Alexis Perez <corsac@corsac.net>
+Link: https://lore.kernel.org/linux-usb/CAAn0qaXmysJ9vx3ZEMkViv_B19ju-_ExN8Yn_uSefxpjS6g4Lw@mail.gmail.com/
+Link: https://github.com/libimobiledevice/libimobiledevice/issues/1038
+Cc: stable@vger.kernel.org
+---
+ drivers/net/usb/ipheth.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/drivers/net/usb/ipheth.c
++++ b/drivers/net/usb/ipheth.c
+@@ -70,7 +70,7 @@
+ #define IPHETH_USBINTF_SUBCLASS 253
+ #define IPHETH_USBINTF_PROTO    1
+ 
+-#define IPHETH_BUF_SIZE         1516
++#define IPHETH_BUF_SIZE         1514
+ #define IPHETH_IP_ALIGN		2	/* padding at front of URB */
+ #define IPHETH_TX_TIMEOUT       (5 * HZ)
+