[PATCH net-next] net: stmmac: fix rx limit check in stmmac_rx_zc()

[Alexey Kodanev]

The extra "count >= limit" check in stmmac_rx_zc() is redundant and
has no effect because the value of "count" doesn't change after the
while condition at this point.

However, it can change after "read_again:" label:

        while (count < limit) {
            ...

            if (count >= limit)
                break;
    read_again:
            ...
            /* XSK pool expects RX frame 1:1 mapped to XSK buffer */
            if (likely(status & rx_not_ls)) {
                xsk_buff_free(buf->xdp);
                buf->xdp = NULL;
                dirty++;
                count++;
                goto read_again;
            }
            ...

This patch addresses the same issue previously resolved in stmmac_rx()
by commit fa02de9e7588 ("net: stmmac: fix rx budget limit check").
The fix is the same: move the check after the label to ensure that it
bounds the goto loop.

Detected using the static analysis tool - Svace.
Fixes: bba2556efad6 ("net: stmmac: Enable RX via AF_XDP zero-copy")
Signed-off-by: Alexey Kodanev <aleksei.kodanev at bell-sw.com>
---

After creating the patch, I also found the previous attempt to fix this issue
from 2023, but I'm not sure what went wrong or why it wasn't applied:
https://lore.kernel.org/netdev/ZBRd2HyZdz52eXyz@nimitz/

 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 7b90ecd3a55e..86e912471dea 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5258,10 +5258,10 @@ static int stmmac_rx_zc(struct stmmac_priv *priv, int limit, u32 queue)
 			len = 0;
 		}
 
+read_again:
 		if (count >= limit)
 			break;
 
-read_again:
 		buf1_len = 0;
 		entry = next_entry;
 		buf = &rx_q->buf_pool[entry];
-- 
2.25.1