[PATCH net] xsk: restore UMEM flags after failed pool assignment

[Paul Moses <p@1g4.org>]

xp_assign_dev() sets XDP_UMEM_SG_FLAG before checking whether the
device supports multibuffer zero-copy. If assignment fails, xsk_bind()
destroys the pool but retains the registered UMEM. A subsequent bind
without XDP_USE_SG inherits the stale flag.

The flag also changes RX buffer sizing after DMA mapping. On igc, a
failed SG bind followed by a non-SG retry with 4096-byte chunks and
1792 bytes of headroom passes admission with 2048 bytes, but programs
a 1024-byte RX buffer. On I225-V hardware, an ordinary 1514-byte frame
is truncated to 1024 bytes or 1008 with RX timestamps enabled.
Restoring the original flags eliminates the truncation in both cases.

Save the UMEM flags before assignment and restore them on failure.

Fixes: d609f3d228a8 ("xsk: add multi-buffer support for sockets sharing umem")
Cc: stable@vger.kernel.org
Signed-off-by: Paul Moses <p@1g4.org>
---
 net/xdp/xsk_buff_pool.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index 0ebd4d46f7ab..7750d2ec4ee2 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -166,6 +166,7 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
 {
 	u32 needed = netdev->mtu + ETH_PAD_LEN;
 	u32 segs = netdev->xdp_zc_max_segs;
+	u8 umem_flags = pool->umem->flags;
 	bool mbuf = flags & XDP_USE_SG;
 	bool force_zc, force_copy;
 	struct netdev_bpf bpf;
@@ -268,6 +269,7 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
 	if (!force_zc)
 		err = 0; /* fallback to copy mode */
 	if (err) {
+		pool->umem->flags = umem_flags;
 		xsk_clear_pool_at_qid(netdev, queue_id);
 		hlist_del(&pool->dev_node);
 		pool->netdev = NULL;

-- 
2.56.GIT