[PATCH net v4 0/2] tun: bound the NAPI receive queue

[PATCH net v4 0/2] tun: bound the NAPI receive queue

[Antonio Quartulli]

With IFF_NAPI, tun queues written packets for NAPI on an unbounded
list, which an unprivileged user can grow until the system runs out of
memory. Patch 1 bounds it by netdev_max_backlog; patch 2 adds a
kselftest for the vhost-net paths into that queue.

v4:
- Keep the poll's leftovers on a separate queue, like process_backlog():
  v3 did not bound the queue when the poll ran on another CPU (Sashiko)
- Schedule NAPI when dropping: with netdev_max_backlog below
  NAPI_POLL_WEIGHT, MSG_MORE senders (vhost-net) stalled the queue
  (Sashiko)
- Use goto free_skb for the locked drop (Willem)
- Add a vhost-net kselftest (patch 2); the tun kselftest passes (Willem)
- Cc vhost, kselftest and XDP maintainers and the Fixes author/acker

v3: https://lore.kernel.org/netdev/20261031231444.1528758-1-antonio@mandelbit.com/
- Also bound the sendmsg() paths, which share the queue: zerocopy
  buffers are completed when dropped early, and the batched XDP buffers
  of tun_xdp_one() are covered too (Willem)
- Drop the IFF_NAPI_FRAGS exclusion (Willem)
- Recheck under the queue lock before queueing (Willem)
- Compare with > like enqueue_to_backlog() (Willem)
- Keep the includes sorted (Willem)
- Add the reproducer and a testing statement to the commit message

v2: https://lore.kernel.org/netdev/20261030152540.1409884-1-antonio@mandelbit.com/
- Cc netdev and the maintainers

Testing, in an x86-64 VM with 1 and 2 vCPUs:
- Reproducer from patch 1, 15 seconds, 500 and 3000 iptables rules:
  memory use stays flat, no OOM; forwarding rate unchanged (112962
  packets in 3 s without the patch, 113822 with it).
- tun_vhost (patch 2), with vhost_net experimental_zcopytx=0 and =1:
  passes with patch 1. Without it, every variant fails (nothing is
  dropped); removing the tun_xdp_one() check fails the batch variant,
  and removing the zerocopy completion fails the zerocopy one.
- tun kselftest: pass:91 fail:0 xfail:12, as without the patch.

Notes:
- Affected since 943170998b20 (v4.15). Based on net/main 94cea342e891.
- Mitigation: disallow unprivileged user namespaces or restrict
  access to /dev/net/tun.

Antonio Quartulli (2):
  tun: bound the NAPI receive queue
  selftests: net: add a vhost-net test for the tun NAPI queue bound

 drivers/net/tun.c                       |  82 +++++-
 tools/testing/selftests/net/Makefile    |   1 +
 tools/testing/selftests/net/config      |   1 +
 tools/testing/selftests/net/tun_vhost.c | 361 ++++++++++++++++++++++++
 4 files changed, 431 insertions(+), 14 deletions(-)
 create mode 100644 tools/testing/selftests/net/tun_vhost.c

-- 
2.56.0

[PATCH net v4 1/2] tun: bound the NAPI receive queue

[Antonio Quartulli]

With IFF_NAPI, tun_get_user() queues every written packet for NAPI on
an unbounded list. When the poll runs asynchronously (threaded NAPI)
and is slower than the writer, the queue grows until the system runs
out of memory. An unprivileged user can do this from its own user and
network namespaces, and the queued skbs are not charged to its memcg:
in a test VM, a uid 1000 process limited to 128 MB caused a global OOM
in ~6 seconds. The socket send buffer cannot bound this: tun sets it to
INT_MAX, and the build_skb() and XDP batch paths do not check it.

Drop packets once more than netdev_max_backlog of them are queued, like
enqueue_to_backlog() does, and count them as rx_dropped: before
allocating and copying them where possible, and again under the queue
lock before queueing. This covers write(), sendmsg() and the batches of
XDP buffers vhost-net passes to tun_sendmsg(), which share the queue.
Zerocopy buffers of packets dropped early are completed.

The NAPI poll moves the whole queue out, processes up to its budget and
puts the rest back: while it runs, the queue looks empty to writers, so
with the poll on another CPU the backlog still grows without limit. Keep
the leftovers on a queue of their own and take the next batch only once
it is empty, like process_backlog() does, so that neither queue holds
more than the limit.

Dropping because the queue is full also schedules NAPI, in all three
places. vhost-net sends with MSG_MORE while it has more packets, and
tun_get_user() schedules NAPI only after the last one or once more than
NAPI_POLL_WEIGHT are queued. With a netdev_max_backlog below that, the
queue would not drain until vhost-net ran out of packets; with zerocopy,
vhost-net never would, as it stops sending when too many queued buffers
are pending.

Reproducer, as an unprivileged user in a new user and network
namespace, without kernel modifications or fault injection:
  ip tuntap add t0 mode tun; ip addr add 10.11.0.1/24 dev t0
  ip link add d0 type dummy; ip addr add 10.20.0.1/24 dev d0
  ip link set t0 up; ip link set d0 up; sysctl net.ipv4.ip_forward=1
  install ~500 iptables FORWARD rules and a MASQUERADE rule on d0
  open t0 with IFF_TUN | IFF_NO_PI | IFF_NAPI
  echo 1 > /sys/class/net/t0/threaded
  write() 1400-byte IPv4 packets for 10.20.0.2 in a loop

Tested in an x86-64 VM with 1 and 2 vCPUs: with the reproducer running
for 15 seconds, memory use stays flat and the forwarding rate is
unchanged; the tun kselftest passes, as it does without the patch; and
the tun_vhost kselftest added by the next patch, which drives the
vhost-net batch, copy and zerocopy paths, passes, while it fails without
the patch.

Still open: the limit counts packets rather than bytes and applies per
queue, up to about twice the limit can be in flight (queued plus taken
by the poll), and the queued skbs are still not charged to the writer's
memcg.

Fixes: 943170998b20 ("tun: enable NAPI for TUN/TAP driver")
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
---
 drivers/net/tun.c | 82 +++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 68 insertions(+), 14 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 770b8fe085c3..ea99ee0aee2b 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -58,6 +58,7 @@
 #include <linux/nsproxy.h>
 #include <linux/virtio_net.h>
 #include <linux/rcupdate.h>
+#include <net/hotdata.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
 #include <net/rtnetlink.h>
@@ -140,6 +141,8 @@ struct tun_file {
 		unsigned int ifindex;
 	};
 	struct napi_struct napi;
+	/* NAPI poll only: skbs taken off sk_write_queue, not yet received */
+	struct sk_buff_head napi_process_queue;
 	bool napi_enabled;
 	bool napi_frags_enabled;
 	struct mutex napi_mutex;	/* Protects access to the above napi */
@@ -227,31 +230,40 @@ struct veth {
 static void tun_flow_init(struct tun_struct *tun);
 static void tun_flow_uninit(struct tun_struct *tun);
 
+/* In IFF_NAPI mode written packets wait in the tfile's sk_write_queue for the
+ * NAPI poll, which may run later in a kthread or ksoftirqd. Bound that queue
+ * like the per-CPU backlog of netif_rx().
+ */
+static bool tun_napi_queue_full(const struct sk_buff_head *queue)
+{
+	return skb_queue_len_lockless(queue) >
+	       READ_ONCE(net_hotdata.max_backlog);
+}
+
 static int tun_napi_receive(struct napi_struct *napi, int budget)
 {
 	struct tun_file *tfile = container_of(napi, struct tun_file, napi);
+	struct sk_buff_head *process_queue = &tfile->napi_process_queue;
 	struct sk_buff_head *queue = &tfile->sk.sk_write_queue;
-	struct sk_buff_head process_queue;
 	struct sk_buff *skb;
 	int received = 0;
 
-	__skb_queue_head_init(&process_queue);
-
-	spin_lock(&queue->lock);
-	skb_queue_splice_tail_init(queue, &process_queue);
-	spin_unlock(&queue->lock);
-
-	while (received < budget && (skb = __skb_dequeue(&process_queue))) {
+	while (received < budget) {
+		/* Refill only when empty: leftovers are not put back on the
+		 * queue writers check, so neither queue exceeds the limit.
+		 */
+		if (skb_queue_empty(process_queue)) {
+			spin_lock(&queue->lock);
+			skb_queue_splice_tail_init(queue, process_queue);
+			spin_unlock(&queue->lock);
+			if (skb_queue_empty(process_queue))
+				break;
+		}
+		skb = __skb_dequeue(process_queue);
 		napi_gro_receive(napi, skb);
 		++received;
 	}
 
-	if (!skb_queue_empty(&process_queue)) {
-		spin_lock(&queue->lock);
-		skb_queue_splice(&process_queue, queue);
-		spin_unlock(&queue->lock);
-	}
-
 	return received;
 }
 
@@ -572,6 +584,8 @@ static void tun_queue_purge(struct tun_file *tfile)
 		tun_ptr_free(ptr);
 
 	skb_queue_purge(&tfile->sk.sk_write_queue);
+	/* NAPI is disabled by now */
+	__skb_queue_purge(&tfile->napi_process_queue);
 	skb_queue_purge(&tfile->sk.sk_error_queue);
 }
 
@@ -1856,6 +1870,23 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 			zerocopy = true;
 	}
 
+	/* NAPI queue full: drop before allocating, and schedule NAPI, which a
+	 * MSG_MORE sender may not have done yet. Rechecked under the lock.
+	 */
+	if (tfile->napi_enabled &&
+	    unlikely(tun_napi_queue_full(&tfile->sk.sk_write_queue))) {
+		local_bh_disable();
+		napi_schedule(&tfile->napi);
+		local_bh_enable();
+		if (msg_control) {
+			struct ubuf_info *uarg = msg_control;
+
+			uarg->ops->complete(NULL, uarg, false);
+		}
+		dev_core_stats_rx_dropped_inc(tun->dev);
+		return total_len;
+	}
+
 	if (!frags && tun_can_build_skb(tun, tfile, len, noblock, zerocopy)) {
 		/* For the packet that is not easy to be processed
 		 * (e.g gso or jumbo packet), we will do it at after
@@ -2045,6 +2076,17 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 			goto free_skb;
 		}
 
+		/* NAPI queue full: drop, as above */
+		if (unlikely(tun_napi_queue_full(queue))) {
+			spin_unlock(&queue->lock);
+			napi_schedule(&tfile->napi);
+			local_bh_enable();
+			rcu_read_unlock();
+			dev_core_stats_rx_dropped_inc(tun->dev);
+			drop_reason = SKB_DROP_REASON_FULL_RING;
+			goto free_skb;
+		}
+
 		__skb_queue_tail(queue, skb);
 		queue_len = skb_queue_len(queue);
 		spin_unlock(&queue->lock);
@@ -2649,6 +2691,17 @@ static int tun_xdp_one(struct tun_struct *tun,
 			return -EBUSY;
 		}
 
+		/* NAPI queue full: drop, and schedule NAPI so that it drains
+		 * even if nothing in this batch gets queued
+		 */
+		if (unlikely(tun_napi_queue_full(queue))) {
+			spin_unlock(&queue->lock);
+			napi_schedule(&tfile->napi);
+			kfree_skb_reason(skb, SKB_DROP_REASON_FULL_RING);
+			dev_core_stats_rx_dropped_inc(tun->dev);
+			return 0;
+		}
+
 		__skb_queue_tail(queue, skb);
 		spin_unlock(&queue->lock);
 		ret = 1;
@@ -3571,6 +3624,7 @@ static int tun_chr_open(struct inode *inode, struct file * file)
 	}
 
 	mutex_init(&tfile->napi_mutex);
+	__skb_queue_head_init(&tfile->napi_process_queue);
 	RCU_INIT_POINTER(tfile->tun, NULL);
 	tfile->flags = 0;
 	tfile->ifindex = 0;
-- 
2.56.0

[PATCH net v4 2/2] selftests: net: add a vhost-net test for the tun NAPI queue bound

[Antonio Quartulli]

Send 16k packets from vhost-net into a tap device with IFF_NAPI and
netdev_max_backlog set to 8, in the three ways vhost-net passes packets
to tun: batches of XDP buffers, one packet per sendmsg() with MSG_MORE,
and zerocopy buffers. Check that excess packets are dropped and counted,
that the queue keeps draining while vhost-net keeps sending, that every
packet is either delivered or dropped, and that every buffer comes back
to the ring, which it does not if a dropped zerocopy buffer is never
completed.

The zerocopy variant needs vhost_net loaded with experimental_zcopytx=1
and is skipped otherwise; with it, vhost-net does not batch and the
batch variant is skipped instead.

The test drives a minimal split virtqueue itself. It needs root and
CONFIG_VHOST_NET, and restores netdev_max_backlog, which is global, on
exit.

Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
---
 tools/testing/selftests/net/Makefile    |   1 +
 tools/testing/selftests/net/config      |   1 +
 tools/testing/selftests/net/tun_vhost.c | 361 ++++++++++++++++++++++++
 3 files changed, 363 insertions(+)
 create mode 100644 tools/testing/selftests/net/tun_vhost.c

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index df215e1a6262..de4cdaac99a0 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -203,6 +203,7 @@ TEST_GEN_PROGS := \
 	tap \
 	tcp_port_share \
 	tls \
+	tun_vhost \
 	udp_splice_checksum \
 # end of TEST_GEN_PROGS
 
diff --git a/tools/testing/selftests/net/config b/tools/testing/selftests/net/config
index cb8cb4696df7..1dce372caf43 100644
--- a/tools/testing/selftests/net/config
+++ b/tools/testing/selftests/net/config
@@ -136,6 +136,7 @@ CONFIG_TRACEPOINTS=y
 CONFIG_TUN=y
 CONFIG_USER_NS=y
 CONFIG_VETH=y
+CONFIG_VHOST_NET=m
 CONFIG_VLAN_8021Q=y
 CONFIG_VXLAN=m
 CONFIG_XFRM_INTERFACE=m
diff --git a/tools/testing/selftests/net/tun_vhost.c b/tools/testing/selftests/net/tun_vhost.c
new file mode 100644
index 000000000000..dbf76d324a7e
--- /dev/null
+++ b/tools/testing/selftests/net/tun_vhost.c
@@ -0,0 +1,361 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Send packets from vhost-net into a tap device with IFF_NAPI and check
+ * that the NAPI queue is bounded by netdev_max_backlog: excess packets
+ * are dropped and counted, and every buffer is returned to the driver,
+ * zerocopy ones included.
+ *
+ * vhost-net passes packets to tun in three ways:
+ *  - batch: batches of XDP buffers (tun_xdp_one()), the default when the
+ *    socket send buffer is unlimited and zerocopy is disabled
+ *  - copy: one packet per sendmsg(), when the send buffer is limited
+ *  - zerocopy: one packet per sendmsg() with a ubuf_info to complete,
+ *    only with vhost_net experimental_zcopytx=1 (else skipped)
+ *
+ * Tun schedules NAPI only after a batch, or after the last of a series of
+ * MSG_MORE packets, so with a small netdev_max_backlog the queue always
+ * overflows. It must keep draining while the sender keeps sending.
+ *
+ * Author: Antonio Quartulli <antonio@mandelbit.com>
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <poll.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <linux/if.h>
+#include <linux/if_arp.h>
+#include <linux/if_ether.h>
+#include <linux/if_tun.h>
+#include <linux/vhost.h>
+#include <linux/virtio_config.h>
+#include <linux/virtio_net.h>
+#include <linux/virtio_ring.h>
+#include <sys/eventfd.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+
+#include "kselftest_harness.h"
+
+#define TV_IFNAME	"tvh0"
+#define TV_RING_NUM	256
+#define TV_BUF_SIZE	2048
+#define TV_HDR_LEN	sizeof(struct virtio_net_hdr_v1)
+#define TV_PKTS		(64 * TV_RING_NUM)
+#define TV_BACKLOG	"8"
+#define TV_TX_VQ	1
+
+static const unsigned char tv_mac_tap[ETH_ALEN] = { 2, 0, 0, 0, 0, 1 };
+static const unsigned char tv_mac_dst[ETH_ALEN] = { 2, 0, 0, 0, 0, 2 };
+static const unsigned char tv_mac_src[ETH_ALEN] = { 2, 0, 0, 0, 0, 3 };
+
+struct tv_stats {
+	unsigned long long rx_packets;
+	unsigned long long rx_dropped;
+};
+
+static int tv_read_stats(const char *ifname, struct tv_stats *st)
+{
+	char line[512], *p;
+	FILE *f;
+	int ret = -1;
+
+	f = fopen("/proc/net/dev", "r");
+	if (!f)
+		return -1;
+	while (fgets(line, sizeof(line), f)) {
+		p = strchr(line, ':');
+		if (!p)
+			continue;
+		*p = '\0';
+		if (strcmp(line + strspn(line, " "), ifname))
+			continue;
+		if (sscanf(p + 1, "%*u %llu %*u %llu", &st->rx_packets,
+			   &st->rx_dropped) == 2)
+			ret = 0;
+		break;
+	}
+	fclose(f);
+	return ret;
+}
+
+FIXTURE(tun_vhost)
+{
+	int backlog_fd;
+	char backlog[32];
+	int tap;
+	int vhost;
+	int kick;
+	int call;
+	void *ring;
+	struct vring vr;
+	char *bufs;
+	unsigned short avail_idx;
+	unsigned short used_idx;
+};
+
+FIXTURE_VARIANT(tun_vhost)
+{
+	bool batch;
+	bool limit_sndbuf;
+	bool zerocopy;
+	int pktlen;
+};
+
+FIXTURE_VARIANT_ADD(tun_vhost, batch)
+{
+	.batch = true,
+	.pktlen = 64,
+};
+
+FIXTURE_VARIANT_ADD(tun_vhost, copy)
+{
+	.limit_sndbuf = true,
+	.pktlen = 64,
+};
+
+FIXTURE_VARIANT_ADD(tun_vhost, zerocopy)
+{
+	.limit_sndbuf = true,
+	.zerocopy = true,
+	.pktlen = 1000,
+};
+
+static bool tv_zerocopy_enabled(void)
+{
+	char c = '0';
+	int fd;
+
+	fd = open("/sys/module/vhost_net/parameters/experimental_zcopytx",
+		  O_RDONLY);
+	if (fd < 0)
+		return false;
+	if (read(fd, &c, 1) != 1)
+		c = '0';
+	close(fd);
+	return c == '1';
+}
+
+static void tv_vhost_setup(struct __test_metadata *_metadata,
+			   FIXTURE_DATA(tun_vhost) *self)
+{
+	struct vhost_vring_state state = { .index = TV_TX_VQ };
+	struct vhost_vring_file file = { .index = TV_TX_VQ };
+	struct vhost_vring_addr addr = { .index = TV_TX_VQ };
+	size_t memsz = sizeof(struct vhost_memory) +
+		       sizeof(struct vhost_memory_region);
+	unsigned long long features;
+	struct vhost_memory *mem;
+
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_OWNER, NULL), 0);
+	ASSERT_EQ(ioctl(self->vhost, VHOST_GET_FEATURES, &features), 0);
+	ASSERT_TRUE(features & (1ULL << VIRTIO_F_VERSION_1));
+	features = 1ULL << VIRTIO_F_VERSION_1;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_FEATURES, &features), 0);
+
+	/* buffer addresses are passed as is: map them 1:1 */
+	mem = calloc(1, memsz);
+	ASSERT_NE(mem, NULL);
+	mem->nregions = 1;
+	mem->regions[0].guest_phys_addr = (unsigned long)self->bufs;
+	mem->regions[0].userspace_addr = (unsigned long)self->bufs;
+	mem->regions[0].memory_size = TV_RING_NUM * TV_BUF_SIZE;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_MEM_TABLE, mem), 0);
+	free(mem);
+
+	state.num = TV_RING_NUM;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_VRING_NUM, &state), 0);
+	state.num = 0;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_VRING_BASE, &state), 0);
+	addr.desc_user_addr = (unsigned long)self->vr.desc;
+	addr.avail_user_addr = (unsigned long)self->vr.avail;
+	addr.used_user_addr = (unsigned long)self->vr.used;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_VRING_ADDR, &addr), 0);
+	file.fd = self->kick;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_VRING_KICK, &file), 0);
+	file.fd = self->call;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_SET_VRING_CALL, &file), 0);
+	file.fd = self->tap;
+	ASSERT_EQ(ioctl(self->vhost, VHOST_NET_SET_BACKEND, &file), 0);
+}
+
+FIXTURE_SETUP(tun_vhost)
+{
+	struct ifreq ifr = {};
+	int hdrlen = TV_HDR_LEN;
+	int sndbuf = 1 << 20;
+	ssize_t n;
+	int i, fd;
+
+	self->tap = -1;
+	self->vhost = open("/dev/vhost-net", O_RDWR);
+	if (self->vhost < 0)
+		SKIP(return, "no /dev/vhost-net");
+	if (variant->zerocopy && !tv_zerocopy_enabled())
+		SKIP(return, "vhost_net experimental_zcopytx not enabled");
+	if (variant->batch && tv_zerocopy_enabled())
+		SKIP(return, "vhost_net does not batch with zerocopy");
+
+	/* global, and only visible from init_net: keep it open to restore */
+	self->backlog_fd = open("/proc/sys/net/core/netdev_max_backlog",
+				O_RDWR);
+	ASSERT_GE(self->backlog_fd, 0);
+	n = read(self->backlog_fd, self->backlog, sizeof(self->backlog) - 1);
+	ASSERT_GT(n, 0);
+	self->backlog[n] = '\0';
+	ASSERT_EQ(pwrite(self->backlog_fd, TV_BACKLOG, strlen(TV_BACKLOG), 0),
+		  strlen(TV_BACKLOG));
+
+	ASSERT_EQ(unshare(CLONE_NEWNET), 0);
+
+	self->tap = open("/dev/net/tun", O_RDWR);
+	ASSERT_GE(self->tap, 0);
+	ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_VNET_HDR | IFF_NAPI;
+	memcpy(ifr.ifr_name, TV_IFNAME, sizeof(TV_IFNAME));
+	ASSERT_EQ(ioctl(self->tap, TUNSETIFF, &ifr), 0);
+	ASSERT_EQ(ioctl(self->tap, TUNSETVNETHDRSZ, &hdrlen), 0);
+	if (variant->limit_sndbuf)
+		ASSERT_EQ(ioctl(self->tap, TUNSETSNDBUF, &sndbuf), 0);
+
+	fd = socket(AF_INET, SOCK_DGRAM, 0);
+	ASSERT_GE(fd, 0);
+	ifr.ifr_hwaddr.sa_family = ARPHRD_ETHER;
+	memcpy(ifr.ifr_hwaddr.sa_data, tv_mac_tap, ETH_ALEN);
+	ASSERT_EQ(ioctl(fd, SIOCSIFHWADDR, &ifr), 0);
+	ASSERT_EQ(ioctl(fd, SIOCGIFFLAGS, &ifr), 0);
+	ifr.ifr_flags |= IFF_UP;
+	ASSERT_EQ(ioctl(fd, SIOCSIFFLAGS, &ifr), 0);
+	close(fd);
+
+	/* every buffer: zeroed vnet header + IPv4 frame for another host,
+	 * which the stack drops without counting it in rx_dropped
+	 */
+	ASSERT_EQ(posix_memalign((void **)&self->bufs, 4096,
+				 TV_RING_NUM * TV_BUF_SIZE), 0);
+	memset(self->bufs, 0, TV_RING_NUM * TV_BUF_SIZE);
+	for (i = 0; i < TV_RING_NUM; i++) {
+		unsigned char *eth = (unsigned char *)self->bufs +
+				     i * TV_BUF_SIZE + TV_HDR_LEN;
+
+		memcpy(eth, tv_mac_dst, ETH_ALEN);
+		memcpy(eth + ETH_ALEN, tv_mac_src, ETH_ALEN);
+		eth[12] = ETH_P_IP >> 8;
+		eth[13] = ETH_P_IP & 0xff;
+		eth[14] = 0x45;
+	}
+
+	ASSERT_EQ(posix_memalign(&self->ring, 4096,
+				 vring_size(TV_RING_NUM, 4096)), 0);
+	memset(self->ring, 0, vring_size(TV_RING_NUM, 4096));
+	vring_init(&self->vr, TV_RING_NUM, self->ring, 4096);
+
+	self->kick = eventfd(0, EFD_NONBLOCK);
+	self->call = eventfd(0, EFD_NONBLOCK);
+	ASSERT_GE(self->kick, 0);
+	ASSERT_GE(self->call, 0);
+
+	tv_vhost_setup(_metadata, self);
+}
+
+FIXTURE_TEARDOWN(tun_vhost)
+{
+	if (self->tap < 0)
+		return;
+	/* waits for outstanding zerocopy buffers */
+	close(self->vhost);
+	close(self->tap);
+	if (pwrite(self->backlog_fd, self->backlog, strlen(self->backlog),
+		   0) < 0)
+		TH_LOG("cannot restore netdev_max_backlog to %s",
+		       self->backlog);
+	close(self->backlog_fd);
+}
+
+/* queue up to @n buffers, return how many */
+static int tv_post(FIXTURE_DATA(tun_vhost) *self, int pktlen, int n)
+{
+	int inflight = (unsigned short)(self->avail_idx - self->used_idx);
+	unsigned long long one = 1;
+	int i;
+
+	for (i = 0; i < n && inflight < TV_RING_NUM; i++, inflight++) {
+		unsigned int id = self->avail_idx % TV_RING_NUM;
+
+		self->vr.desc[id].addr = (unsigned long)self->bufs +
+					 id * TV_BUF_SIZE;
+		self->vr.desc[id].len = TV_HDR_LEN + pktlen;
+		self->vr.desc[id].flags = 0;
+		self->vr.avail->ring[id] = id;
+		self->avail_idx++;
+	}
+	__atomic_store_n(&self->vr.avail->idx, self->avail_idx,
+			 __ATOMIC_RELEASE);
+	if (i && write(self->kick, &one, sizeof(one)) != sizeof(one))
+		return -1;
+	return i;
+}
+
+/* reap returned buffers, waiting up to @ms for at least one */
+static int tv_reap(FIXTURE_DATA(tun_vhost) *self, int ms)
+{
+	struct pollfd pfd = { .fd = self->call, .events = POLLIN };
+	unsigned short used;
+	unsigned long long v;
+
+	for (;;) {
+		used = __atomic_load_n(&self->vr.used->idx, __ATOMIC_ACQUIRE);
+		if (used != self->used_idx) {
+			int n = (unsigned short)(used - self->used_idx);
+
+			self->used_idx = used;
+			return n;
+		}
+		if (ms <= 0 || poll(&pfd, 1, 100) < 0)
+			return 0;
+		if (pfd.revents & POLLIN && read(self->call, &v, sizeof(v)) < 0)
+			return -1;
+		ms -= 100;
+	}
+}
+
+TEST_F(tun_vhost, napi_queue_bounded)
+{
+	struct tv_stats before, after;
+	unsigned long long delivered, dropped;
+	int posted = 0, reaped = 0, n;
+
+	ASSERT_EQ(tv_read_stats(TV_IFNAME, &before), 0);
+
+	while (reaped < TV_PKTS) {
+		n = tv_post(self, variant->pktlen, TV_PKTS - posted);
+		ASSERT_GE(n, 0);
+		posted += n;
+		n = tv_reap(self, 5000);
+		ASSERT_GE(n, 0);
+		if (!n) {
+			TH_LOG("%d of %d buffers not returned",
+			       posted - reaped, posted);
+			ASSERT_GT(n, 0);
+		}
+		reaped += n;
+	}
+
+	ASSERT_EQ(tv_read_stats(TV_IFNAME, &after), 0);
+	delivered = after.rx_packets - before.rx_packets;
+	dropped = after.rx_dropped - before.rx_dropped;
+	TH_LOG("sent %d, delivered %llu, dropped %llu", TV_PKTS, delivered,
+	       dropped);
+	/* bounded, */
+	EXPECT_GT(dropped, 0);
+	/* still draining, */
+	EXPECT_GE(delivered, TV_PKTS / 64);
+	/* and every packet accounted for */
+	EXPECT_EQ(delivered + dropped, TV_PKTS);
+}
+
+TEST_HARNESS_MAIN
-- 
2.56.0

Re: [PATCH net v4 0/2] tun: bound the NAPI receive queue

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.