Re: [PATCH v14 7/9] vhost-user: add vhost-user

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 27 Nov 2024 10:09:53 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> On 27/11/2024 05:47, Stefano Brivio wrote:
> > On Fri, 22 Nov 2024 17:43:34 +0100
> > Laurent Vivier <lvivier@redhat.com> wrote:
> > 
> >> +/**
> >> + * tcp_vu_send_flag() - Send segment with flags to vhost-user (no payload)
> >> + * @c:		Execution context
> >> + * @conn:	Connection pointer
> >> + * @flags:	TCP flags: if not set, send segment only if ACK is due
> >> + *
> >> + * Return: negative error code on connection reset, 0 otherwise
> >> + */
> >> +int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
> >> +{
> >> +	struct vu_dev *vdev = c->vdev;
> >> +	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
> >> +	const struct flowside *tapside = TAPFLOW(conn);
> >> +	size_t l2len, l4len, optlen, hdrlen;
> >> +	struct vu_virtq_element flags_elem[2];
> >> +	struct tcp_payload_t *payload;
> >> +	struct ipv6hdr *ip6h = NULL;
> >> +	struct iovec flags_iov[2];
> >> +	struct iphdr *iph = NULL;
> >> +	struct ethhdr *eh;
> >> +	uint32_t seq;
> >> +	int elem_cnt;
> >> +	int nb_ack;
> >> +	int ret;
> >> +
> >> +	hdrlen = tcp_vu_hdrlen(CONN_V6(conn));
> >> +
> >> +	vu_set_element(&flags_elem[0], NULL, &flags_iov[0]);
> >> +
> >> +	elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1,
> >> +			      hdrlen + sizeof(struct tcp_syn_opts), NULL);
> > 
> > Oops, I made this crash, by starting a number of iperf3 client threads
> > on the host:
> > 
> > $ iperf3 -c localhost -p 6001 -Z -l 500 -w 256M -t 600 -P20
> > 
> > with matching server in the guest, then terminating QEMU while the test
> > is running.
> > 
> > Details (I saw it first, then I reproduced it under gdb):
> > 
> > accepted connection from PID 3115463
> > NDP: received RS, sending RA
> > DHCP: offer to discover
> >      from 52:54:00:12:34:56
> > DHCP: ack to request
> >      from 52:54:00:12:34:56
> > NDP: sending unsolicited RA, next in 212s
> > Client connection closed
> > 
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x00005555555884f5 in vring_avail_idx (vq=0x555559343f10 <vdev_storage+1296>) at virtio.c:138
> > 138		vq->shadow_avail_idx = le16toh(vq->vring.avail->idx);
> > (gdb) list
> > 133	 *
> > 134	 * Return: the available ring index of the given virtqueue
> > 135	 */
> > 136	static inline uint16_t vring_avail_idx(struct vu_virtq *vq)
> > 137	{
> > 138		vq->shadow_avail_idx = le16toh(vq->vring.avail->idx);
> > 139	
> > 140		return vq->shadow_avail_idx;
> > 141	}
> > 142	
> > (gdb) bt
> > #0  0x00005555555884f5 in vring_avail_idx (vq=0x555559343f10 <vdev_storage+1296>) at virtio.c:138
> > #1  vu_queue_empty (vq=vq@entry=0x555559343f10 <vdev_storage+1296>) at virtio.c:290
> > #2  vu_queue_pop (dev=dev@entry=0x555559343a00 <vdev_storage>, vq=vq@entry=0x555559343f10 <vdev_storage+1296>, elem=elem@entry=0x7ffffff6f510) at virtio.c:505
> > #3  0x0000555555588c8c in vu_collect (vdev=vdev@entry=0x555559343a00 <vdev_storage>, vq=vq@entry=0x555559343f10 <vdev_storage+1296>, elem=elem@entry=0x7ffffff6f510, max_elem=max_elem@entry=1,
> >      size=size@entry=74, frame_size=frame_size@entry=0x0) at vu_common.c:86
> > #4  0x000055555557e00e in tcp_vu_send_flag (c=0x7ffffff6f7a0, conn=0x5555555bd2d0 <flowtab+2160>, flags=4) at tcp_vu.c:116
> > #5  0x0000555555578125 in tcp_send_flag (flags=4, conn=0x5555555bd2d0 <flowtab+2160>, c=0x7ffffff6f7a0) at tcp.c:1278
> > #6  tcp_rst_do (conn=<optimized out>, c=<optimized out>) at tcp.c:1293
> > #7  tcp_timer_handler (c=c@entry=0x7ffffff6f7a0, ref=..., ref@entry=...) at tcp.c:2266
> > #8  0x0000555555558f26 in main (argc=<optimized out>, argv=<optimized out>) at passt.c:342
> > (gdb) p *vq
> > $1 = {vring = {num = 256, desc = 0x0, avail = 0x0, used = 0x0, log_guest_addr = 4338774592, flags = 0}, last_avail_idx = 35133, shadow_avail_idx = 35133, used_idx = 35133, signalled_used = 0,
> >    signalled_used_valid = false, notification = true, inuse = 0, call_fd = -1, kick_fd = -1, err_fd = -1, enable = 1, started = false, vra = {index = 0, flags = 0, desc_user_addr = 139660501995520,
> >      used_user_addr = 139660502000192, avail_user_addr = 139660501999616, log_guest_addr = 4338774592}}
> > (gdb) p *vq->vring.avail
> > Cannot access memory at address 0x0
> > 
> > ...so we're sending a RST segment to the guest, but the ring doesn't
> > exist anymore.
> > 
> > By the way, I still have the gdb session running, if you need something
> > else out of it.
> > 
> > Now, I guess we should eventually introduce a more comprehensive
> > handling of the case where the guest suddenly terminates (not specific
> > to vhost-user), but given that we have vu_cleanup() working as expected
> > in this case, I wonder if we shouldn't simply avoid calling
> > vring_avail_idx() (it has a single caller) by checking for !vring.avail
> > in the caller, or something like that.
> > 
> 
> Yes, I think it's the lines I removed during the reviews:
> 
>         if (!vq->vring.avail)
>                 return true;

Ah, right:

  https://archives.passt.top/passt-dev/20241114163859.7eeafa38@elisabeth/

...so, at least in our case, it's more than "sanity checks" after all.
:) Well, I guess it depends on the definition.

> Could you try to checkout virtio.c from v11?

That would take a rather lengthy rebase, but I tried to reintroduce all
the checks you had:

--
diff --git a/virtio.c b/virtio.c
index 6a97435..0598ff4 100644
--- a/virtio.c
+++ b/virtio.c
@@ -284,6 +284,9 @@ static int virtqueue_read_next_desc(const struct vring_desc *desc,
  */
 bool vu_queue_empty(struct vu_virtq *vq)
 {
+	if (!vq->vring.avail)
+		return true;
+
 	if (vq->shadow_avail_idx != vq->last_avail_idx)
 		return false;
 
@@ -327,6 +330,9 @@ static bool vring_can_notify(const struct vu_dev *dev, struct vu_virtq *vq)
  */
 void vu_queue_notify(const struct vu_dev *dev, struct vu_virtq *vq)
 {
+	if (!vq->vring.avail)
+		return;
+
 	if (!vring_can_notify(dev, vq)) {
 		debug("vhost-user: virtqueue can skip notify...");
 		return;
@@ -502,6 +508,9 @@ int vu_queue_pop(struct vu_dev *dev, struct vu_virtq *vq, struct vu_virtq_elemen
 	unsigned int head;
 	int ret;
 
+	if (!vq->vring.avail)
+		return -1;
+
 	if (vu_queue_empty(vq))
 		return -1;
 
@@ -591,6 +600,9 @@ void vu_queue_fill_by_index(struct vu_virtq *vq, unsigned int index,
 {
 	struct vring_used_elem uelem;
 
+	if (!vq->vring.avail)
+		return;
+
 	idx = (idx + vq->used_idx) % vq->vring.num;
 
 	uelem.id = htole32(index);
@@ -633,6 +645,9 @@ void vu_queue_flush(struct vu_virtq *vq, unsigned int count)
 {
 	uint16_t old, new;
 
+	if (!vq->vring.avail)
+		return;
+
 	/* Make sure buffer is written before we update index. */
 	smp_wmb();
 
--

and it's all fine with those, I tried doing a few nasty things and
didn't observe any issue.

Any check I missed? Do you want to submit it as follow-up patch? I can
also do that. I'd rather (still) avoid a re-post of v14 if possible.

-- 
@@ -284,6 +284,9 @@ static int virtqueue_read_next_desc(const struct vring_desc *desc,
  */
 bool vu_queue_empty(struct vu_virtq *vq)
 {
+	if (!vq->vring.avail)
+		return true;
+
 	if (vq->shadow_avail_idx != vq->last_avail_idx)
 		return false;
 
@@ -327,6 +330,9 @@ static bool vring_can_notify(const struct vu_dev *dev, struct vu_virtq *vq)
  */
 void vu_queue_notify(const struct vu_dev *dev, struct vu_virtq *vq)
 {
+	if (!vq->vring.avail)
+		return;
+
 	if (!vring_can_notify(dev, vq)) {
 		debug("vhost-user: virtqueue can skip notify...");
 		return;
@@ -502,6 +508,9 @@ int vu_queue_pop(struct vu_dev *dev, struct vu_virtq *vq, struct vu_virtq_elemen
 	unsigned int head;
 	int ret;
 
+	if (!vq->vring.avail)
+		return -1;
+
 	if (vu_queue_empty(vq))
 		return -1;
 
@@ -591,6 +600,9 @@ void vu_queue_fill_by_index(struct vu_virtq *vq, unsigned int index,
 {
 	struct vring_used_elem uelem;
 
+	if (!vq->vring.avail)
+		return;
+
 	idx = (idx + vq->used_idx) % vq->vring.num;
 
 	uelem.id = htole32(index);
@@ -633,6 +645,9 @@ void vu_queue_flush(struct vu_virtq *vq, unsigned int count)
 {
 	uint16_t old, new;
 
+	if (!vq->vring.avail)
+		return;
+
 	/* Make sure buffer is written before we update index. */
 	smp_wmb();
 
--

and it's all fine with those, I tried doing a few nasty things and
didn't observe any issue.

Any check I missed? Do you want to submit it as follow-up patch? I can
also do that. I'd rather (still) avoid a re-post of v14 if possible.

-- 
Stefano