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

[Laurent Vivier <lvivier@redhat.com>]

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;

Could you try to checkout virtio.c from v11?

Thanks,
Laurent