From: Sven Eckelmann <sven@narfation.org>
Date: Fri, 15 Jan 2021 23:38:31 +0100
Subject: batctl: Don't stop when create_interface detected existing interface

"batctl meshif bat0 interface add ..." should never be called in parallel.
But when something still does this, it could be that the code first detects
the missing meshif and then tries to create it - which fails when another
process requested the creation of the same interface slightly before batctl
did it.

But this should not prevent batctl to add the lower interface to the
meshif. It is not really important that the batctl process was the one
which created it - only that it exists is important.

Fixes: 25022e0b154d ("batctl: Use rtnl to add/remove interfaces")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Origin: upstream, https://git.open-mesh.org/batctl.git/commit/5d465bfd9a19c4bda20fb4e36c6e4b22c56ceb17

diff --git a/interface.c b/interface.c
index 138a6cd45744081a04f986fe4be67901b3305b74..2c6a78ad17cb716fbb7f6a1c78e0076494397b01 100644
--- a/interface.c
+++ b/interface.c
@@ -479,7 +479,7 @@ static int interface(struct state *state, int argc, char **argv)
 	ifmaster = if_nametoindex(state->mesh_iface);
 	if (!manual_mode && !ifmaster && rest_argv[0][0] == 'a') {
 		ret = create_interface(state->mesh_iface);
-		if (ret < 0) {
+		if (ret < 0 && ret != -EEXIST) {
 			fprintf(stderr,
 				"Error - failed to create batman-adv interface: %s\n",
 				strerror(-ret));