wireguard: unknown relocation: 102 [ARMv7 Thumb-2]

[Jason A. Donenfeld]

On Wed, Jun 17, 2020 at 02:33:49PM -0600, Jason A. Donenfeld wrote:
> So, some more research: it looks like the R_ARM_THM_JUMP11 symbol is
> actually wg_packet_send_staged_packets, a boring C function with
> nothing fancy about it. That github issue you pointed to suggested
> that it might have something to do with complex crypto functions, but
> it looks like that's not the case. wg_packet_send_staged_packets is
> plain old boring C.
> 
> But there is one interesting thing about
> wg_packet_send_staged_packets: it's defined in send.c, and called from
> send.c, receive.c, device.c, and netlink.c -- four places. What I
> suspect is happening is that the linker can't quite figure out how to
> order the functions in the final executable so that the
> wg_packet_send_staged_packets definition is sufficiently close to all
> of its call sites, so it then needs to add that extra trampoline
> midway to get to it. Stupid linker. I'm playing now if there's some
> manual reordering I can do in the build system so that this isn't a
> problem, but I'm not very optimistic that I'll succeed.

Looks like my explanation there wasn't 100% accurate, but it does seem
like the issue occurs when gcc sees a clear tail call that it can
optimize into a B instruction instead of a BL instruction.

The below patch avoids that, and thus fixes your issue, using a pretty
bad trick that's not really suitable for being committed anywhere, but
it is perhaps leading us in the right direction:

diff --git a/src/send.c b/src/send.c
index 828b086a..4bb6911f 100644
--- a/src/send.c
+++ b/src/send.c
@@ -221,6 +221,8 @@ static bool encrypt_packet(struct sk_buff *skb, struct noise_keypair *keypair,
     simd_context);
 }
 
+volatile char dummy;
+
 void wg_packet_send_keepalive(struct wg_peer *peer)
 {
  struct sk_buff *skb;
@@ -240,6 +242,7 @@ void wg_packet_send_keepalive(struct wg_peer *peer)
  }
 
  wg_packet_send_staged_packets(peer);
+ dummy = -1;
 }
 
 static void wg_packet_create_data_done(struct sk_buff *first,