linux-kernel - [PATCH bpf 2/2] selftests/xsk: fix number of Tx frags in invalid packet

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Message-ID: <20260203155103.2305816-3-larysa.zaremba@intel.com>
Date: Tue,  3 Feb 2026 16:50:58 +0100
From: Larysa Zaremba <larysa.zaremba@...el.com>
To: bpf@...r.kernel.org
Cc: Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
	"Bastien Curutchet (eBPF Foundation)" <bastien.curutchet@...tlin.com>,
	Aleksandr Loktionov <aleksandr.loktionov@...el.com>,
	Larysa Zaremba <larysa.zaremba@...el.com>,
	Tushar Vyavahare <tushar.vyavahare@...el.com>,
	Ricardo B. Marlière <rbm@...e.com>,
	Jason Xing <kernelxing@...cent.com>,
	Magnus Karlsson <magnus.karlsson@...el.com>,
	linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	Alexander Lobakin <aleksander.lobakin@...el.com>
Subject: [PATCH bpf 2/2] selftests/xsk: fix number of Tx frags in invalid packet

The issue occurs in TOO_MANY_FRAGS test case when xdp_zc_max_segs is set to
an odd number.

TOO_MANY_FRAGS test case contains an invalid packet consisting of
(xdp_zc_max_segs) frags. Every frag, even the last one has XDP_PKT_CONTD
flag set. This packet is expected to be dropped. After that, there is a
valid linear packet, which is expected to be received back.

Once (xdp_zc_max_segs) is an odd number, the last packet cannot be
received, if packet forwarding between Rx and Tx interfaces relies on
the ethernet header, e.g. checks for ETH_P_LOOPBACK. Packet is malformed,
if all traffic is looped.

Turns out, sending function processes multiple invalid frags as if they
were in 2-frag packets. So once the invalid mbuf packet contains an odd
number of those, the valid packet after gets paired with the previous
invalid descriptor, and hence does not get an ethernet header generated, so
it is either dropped or malformed.

Make invalid packets in verbatim mode always have only a single frag. For
such packets, number of frags is otherwise meaningless, as descriptor flags
are pre-configured in verbatim mode and packet data is not generated for
invalid descriptors.

Fixes: 697604492b64 ("selftests/xsk: add invalid descriptor test for multi-buffer")
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@...el.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@...el.com>
---
 tools/testing/selftests/bpf/prog_tests/test_xsk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/test_xsk.c b/tools/testing/selftests/bpf/prog_tests/test_xsk.c
index 69a5a9a5189b..bab4a31621c7 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_xsk.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_xsk.c
@@ -433,7 +433,7 @@ static u32 pkt_nb_frags(u32 frame_size, struct pkt_stream *pkt_stream, struct pk
 	}

 	/* Search for the end of the packet in verbatim mode */
-	if (!pkt_continues(pkt->options))
+	if (!pkt_continues(pkt->options) || !pkt->valid)
 		return nb_frags;

 	next_frag = pkt_stream->current_pkt_nb;
-- 
2.52.0