[SRU][Q][PATCH 1/1] net: bcmgenet: fix off-by-one in bcmgenet_put_txcb

[Cengiz Can]

From: Justin Chen <justin.chen at broadcom.com>

The write_ptr points to the next open tx_cb. We want to return the
tx_cb that gets rewinded, so we must rewind the pointer first then
return the tx_cb that it points to. That way the txcb can be correctly
cleaned up.

Fixes: 876dbadd53a7 ("net: bcmgenet: Fix unmapping of fragments in bcmgenet_xmit()")
Signed-off-by: Justin Chen <justin.chen at broadcom.com>
Reviewed-by: Nicolai Buchwitz <nb at tipi-net.de>
Link: https://patch.msgid.link/20260406175756.134567-2-justin.chen@broadcom.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(cherry picked from commit 57f3f53d2c9c5a9e133596e2f7bc1c50688a6d38)
CVE-2026-53088
Assisted-by: kybele:claude-opus-4.8
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 drivers/net/ethernet/broadcom/genet/bcmgenet.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
index 98971ae4f87d..9c7d9bfe468b 100644
--- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
+++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
@@ -1822,15 +1822,15 @@ static struct enet_cb *bcmgenet_put_txcb(struct bcmgenet_priv *priv,
 {
 	struct enet_cb *tx_cb_ptr;
 
-	tx_cb_ptr = ring->cbs;
-	tx_cb_ptr += ring->write_ptr - ring->cb_ptr;
-
 	/* Rewinding local write pointer */
 	if (ring->write_ptr == ring->cb_ptr)
 		ring->write_ptr = ring->end_ptr;
 	else
 		ring->write_ptr--;
 
+	tx_cb_ptr = ring->cbs;
+	tx_cb_ptr += ring->write_ptr - ring->cb_ptr;
+
 	return tx_cb_ptr;
 }
 
-- 
2.43.0