[PATCH AUTOSEL 6.18] xsk: fix XDP_UMEM_SG_FLAG issues

[Sasha Levin]

From: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>

[ Upstream commit 93e84fe45b752d17a5a46b306ed78f0133bbc719 ]

Currently xp_assign_dev_shared() is missing XDP_USE_SG being propagated
to flags so set it in order to preserve mtu check that is supposed to be
done only when no multi-buffer setup is in picture.

Also, this flag has the same value as XDP_UMEM_TX_SW_CSUM so we could
get unexpected SG setups for software Tx checksums. Since csum flag is
UAPI, modify value of XDP_UMEM_SG_FLAG.

Fixes: d609f3d228a8 ("xsk: add multi-buffer support for sockets sharing umem")
Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxx>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>
Link: https://patch.msgid.link/20260402154958.562179-4-maciej.fijalkowski@xxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---

LLM Generated explanations, may be completely bogus:

Error: Failed to generate final synthesis

 include/net/xdp_sock.h  | 2 +-
 net/xdp/xsk_buff_pool.c | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index ce587a2256618..7c2bc46c67050 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -14,7 +14,7 @@
 #include <linux/mm.h>
 #include <net/sock.h>
 
-#define XDP_UMEM_SG_FLAG (1 << 1)
+#define XDP_UMEM_SG_FLAG BIT(3)
 
 struct net_device;
 struct xsk_queue;
diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index aa9788f20d0db..677c7d00f8c32 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -259,6 +259,10 @@ int xp_assign_dev_shared(struct xsk_buff_pool *pool, struct xdp_sock *umem_xs,
 		return -EINVAL;
 
 	flags = umem->zc ? XDP_ZEROCOPY : XDP_COPY;
+
+	if (umem->flags & XDP_UMEM_SG_FLAG)
+		flags |= XDP_USE_SG;
+
 	if (umem_xs->pool->uses_need_wakeup)
 		flags |= XDP_USE_NEED_WAKEUP;
 
-- 
2.53.0