[PATCH net v3 2/2] net: stmmac: fix a divide by zero in stmmac_xdp_xmit_xdpf()

[Linkui Xiao <xiaolinkui@126.com>]

From: Linkui Xiao <xiaolinkui@kylinos.cn>

tx-frames 0 is a valid coalescing request: it only stops the frame count
from raising TX completion interrupts, while the coalescing timer armed
by stmmac_tx_timer_arm() keeps reclaiming the descriptors. That is why
stmmac_xmit(), stmmac_tso_xmit() and stmmac_xdp_xmit_zc() all test
priv->tx_coal_frames[queue] before taking the modulo, and why
__stmmac_set_coalesce() rejects the request only when tx-usecs is zero
as well, since then nothing would complete the transmissions.

stmmac_xdp_xmit_xdpf() is the one transmit path that takes the modulo
without the test, so

	ethtool -C eth0 tx-usecs 10 tx-frames 0

followed by an XDP_TX or an ndo_xdp_xmit frame divides by zero, which
oopses in softirq context on the architectures that trap on a zero
divisor.

Add the missing test, which leaves set_ic false exactly like the other
transmit paths do.

Fixes: be8b38a722e6 ("net: stmmac: Add support for XDP_TX action")
Cc: stable@vger.kernel.org
Signed-off-by: Linkui Xiao <xiaolinkui@kylinos.cn>
---
v3:
- New patch. stmmac_xdp_xmit_xdpf() is the only transmit path that divides
  by tx_coal_frames[queue] without a zero test; add the missing test the
  way stmmac_xdp_xmit_zc() has it. (Sashiko review)

 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index e2e680dd980c..276187f50ee3 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5256,7 +5256,9 @@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
 
 	tx_q->tx_count_frames++;
 
-	if (tx_q->tx_count_frames % priv->tx_coal_frames[queue] == 0)
+	if (!priv->tx_coal_frames[queue])
+		set_ic = false;
+	else if (tx_q->tx_count_frames % priv->tx_coal_frames[queue] == 0)
 		set_ic = true;
 	else
 		set_ic = false;
-- 
2.25.1