[PATCH net] sfc: Don't invoke xdp_do_flush() from netpoll.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Yury reported a crash in the sfc driver originated from
netpoll_send_udp(). The netconsole sends a message and then netpoll
invokes the driver's NAPI function with a budget of zero. It is
dedicated to allow driver to free TX resources, that it may have used
while sending the packet.

In the netpoll case the driver invokes xdp_do_flush() unconditionally,
leading to crash because bpf_net_context was never assigned.

Invoke xdp_do_flush() only if budget is not zero.

Fixes: 401cb7dae8130 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Reported-by: Yury Vostrikov <mon@unformed.ru>
Closes: https://lore.kernel.org/5627f6d1-5491-4462-9d75-bc0612c26a22@app.fastmail.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/net/ethernet/sfc/efx_channels.c       | 3 ++-
 drivers/net/ethernet/sfc/siena/efx_channels.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/sfc/efx_channels.c b/drivers/net/ethernet/sfc/efx_channels.c
index c9e17a8208a90..f1723a6fb082b 100644
--- a/drivers/net/ethernet/sfc/efx_channels.c
+++ b/drivers/net/ethernet/sfc/efx_channels.c
@@ -1260,7 +1260,8 @@ static int efx_poll(struct napi_struct *napi, int budget)
 
 	spent = efx_process_channel(channel, budget);
 
-	xdp_do_flush();
+	if (budget)
+		xdp_do_flush();
 
 	if (spent < budget) {
 		if (efx_channel_has_rx_queue(channel) &&
diff --git a/drivers/net/ethernet/sfc/siena/efx_channels.c b/drivers/net/ethernet/sfc/siena/efx_channels.c
index a7346e965bfe7..d120b3c83ac07 100644
--- a/drivers/net/ethernet/sfc/siena/efx_channels.c
+++ b/drivers/net/ethernet/sfc/siena/efx_channels.c
@@ -1285,7 +1285,8 @@ static int efx_poll(struct napi_struct *napi, int budget)
 
 	spent = efx_process_channel(channel, budget);
 
-	xdp_do_flush();
+	if (budget)
+		xdp_do_flush();
 
 	if (spent < budget) {
 		if (efx_channel_has_rx_queue(channel) &&
-- 
2.45.2