xsk: Fix handling of invalid descriptors in XSK TX batching API

xdpxceiver run on a AF_XDP ZC enabled driver revealed a problem with XSK
Tx batching API. There is a test that checks how invalid Tx descriptors
are handled by AF_XDP. Each valid descriptor is followed by invalid one
on Tx side whereas the Rx side expects only to receive a set of valid
descriptors.

In current xsk_tx_peek_release_desc_batch() function, the amount of
available descriptors is hidden inside xskq_cons_peek_desc_batch(). This
can be problematic in cases where invalid descriptors are present due to
the fact that xskq_cons_peek_desc_batch() returns only a count of valid
descriptors. This means that it is impossible to properly update XSK
ring state when calling xskq_cons_release_n().

To address this issue, pull out the contents of
xskq_cons_peek_desc_batch() so that callers (currently only
xsk_tx_peek_release_desc_batch()) will always be able to update the
state of ring properly, as total count of entries is now available and
use this value as an argument in xskq_cons_release_n(). By
doing so, xskq_cons_peek_desc_batch() can be dropped altogether.

Fixes: 9349eb3a9d ("xsk: Introduce batched Tx descriptor interfaces")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220607142200.576735-1-maciej.fijalkowski@intel.com

net/xdp/xsk.c

@@ -373,7 +373,8 @@ u32 xsk_tx_peek_release_desc_batch(struct xsk_buff_pool *pool, u32 max_entries)
 		goto out;
 	}
 
-	nb_pkts = xskq_cons_peek_desc_batch(xs->tx, pool, max_entries);
+	max_entries = xskq_cons_nb_entries(xs->tx, max_entries);
+	nb_pkts = xskq_cons_read_desc_batch(xs->tx, pool, max_entries);
 	if (!nb_pkts) {
 		xs->tx->queue_empty_descs++;
 		goto out;
@@ -389,7 +390,7 @@ u32 xsk_tx_peek_release_desc_batch(struct xsk_buff_pool *pool, u32 max_entries)
 	if (!nb_pkts)
 		goto out;
 
-	xskq_cons_release_n(xs->tx, nb_pkts);
+	xskq_cons_release_n(xs->tx, max_entries);
 	__xskq_cons_release(xs->tx);
 	xs->sk.sk_write_space(&xs->sk);
 

net/xdp/xsk_queue.h

@@ -282,14 +282,6 @@ static inline bool xskq_cons_peek_desc(struct xsk_queue *q,
 	return xskq_cons_read_desc(q, desc, pool);
 }
 
-static inline u32 xskq_cons_peek_desc_batch(struct xsk_queue *q, struct xsk_buff_pool *pool,
-					    u32 max)
-{
-	u32 entries = xskq_cons_nb_entries(q, max);
-
-	return xskq_cons_read_desc_batch(q, pool, entries);
-}
-
 /* To improve performance in the xskq_cons_release functions, only update local state here.
  * Reflect this to global state when we get new entries from the ring in
  * xskq_cons_get_entries() and whenever Rx or Tx processing are completed in the NAPI loop.