[PATCH net-next 1/2] stmmac: xsk: fix underflow of budget in zerocopy mode

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

The issue can happen when the budget number of descs are consumed. As
long as the budget is decreased to zero, it will again go into
while (budget-- > 0) statement and get decreased by one, so the
underflow issue can happen. It will lead to returning true whereas the
expected value should be false.

In this case where all the budget are used up, it means zc function
should return false to let the poll run again because normally we
might have more data to process.

Fixes: 132c32ee5bc0 ("net: stmmac: Add TX via XDP zero-copy socket")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 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 f350a6662880..ea5541f9e9a6 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -2596,7 +2596,7 @@ static bool stmmac_xdp_xmit_zc(struct stmmac_priv *priv, u32 queue, u32 budget)
 
 	budget = min(budget, stmmac_tx_avail(priv, queue));
 
-	while (budget-- > 0) {
+	while (budget > 0) {
 		struct stmmac_metadata_request meta_req;
 		struct xsk_tx_metadata *meta = NULL;
 		dma_addr_t dma_addr;
@@ -2681,6 +2681,8 @@ static bool stmmac_xdp_xmit_zc(struct stmmac_priv *priv, u32 queue, u32 budget)
 
 		tx_q->cur_tx = STMMAC_GET_ENTRY(tx_q->cur_tx, priv->dma_conf.dma_tx_size);
 		entry = tx_q->cur_tx;
+
+		budget--;
 	}
 	u64_stats_update_begin(&txq_stats->napi_syncp);
 	u64_stats_add(&txq_stats->napi.tx_set_ic_bit, tx_set_ic_bit);
-- 
2.41.3