netdev - [PATCH bpf] xsk: Fix handling of invalid descriptors in XSK Tx batching API

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Date:   Tue,  7 Jun 2022 16:22:00 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net
Cc:     netdev@...r.kernel.org, magnus.karlsson@...el.com,
        bjorn@...nel.org, Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Subject: [PATCH bpf] 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: 9349eb3a9d2a ("xsk: Introduce batched Tx descriptor interfaces")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
---
 net/xdp/xsk.c       | 5 +++--
 net/xdp/xsk_queue.h | 8 --------
 2 files changed, 3 insertions(+), 10 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index e0a4526ab66b..19ac872a6624 100644
--- a/net/xdp/xsk.c
+++ b/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);

diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
index a794410989cc..fb20bf7207cf 100644
--- a/net/xdp/xsk_queue.h
+++ b/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.
-- 
2.27.0