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

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 27 Nov 2024 10:48:41 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> On 27/11/2024 10:45, Stefano Brivio wrote:
> > 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.
> 
> As you prefer. Let me know.

It would save me some time if you could... it should be based on v14 as
it is.

I didn't have time to take care of gcc warnings on 32-bit and of the
build failure on musl, yet.

-- 
Stefano