Re: [Qemu-devel] [PATCH 1/2] libvhost-user: fix vu_queue_get_avail_bytes() crash

[Dr. David Alan Gilbert]

* Marc-André Lureau (address@hidden) wrote:
> Calling vu_queue_get_avail_bytes() when the queue doesn't yet have
> addresses will result in the following crash:
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x000055c414112ce4 in vring_avail_idx (vq=0x55c41582fd68, vq=0x55c41582fd68)
>     at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
> 940            vq->shadow_avail_idx = vq->vring.avail->idx;
> (gdb) p vq
> $1 = (VuVirtq *) 0x55c41582fd68
> (gdb) p vq->vring
> $2 = {num = 0, desc = 0x0, avail = 0x0, used = 0x0, log_guest_addr = 0, flags 
> = 0}
> 
>     at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
> No locals.
>     at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:960
>         num_heads = <optimized out>
>     address@hidden, address@hidden,
>     address@hidden) at 
> /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:1034
> 
> Check if vring.avail != null before accessing it.
> 
> Signed-off-by: Marc-André Lureau <address@hidden>


This doesn't seem to fully solve it, it's now failing at:

#0  vring_used_idx_set (val=0, vq=0x5570fcb49d68, dev=0x5570fcb49c20)
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:1472
No locals.
#1  vu_queue_flush (dev=0x5570fcb49c20, vq=0x5570fcb49d68, count=0)
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:1494
        old = 0
        new = 0
#2  0x00005570fb8d9639 in vubr_backend_recv_cb (sock=<optimized out>, 
ctx=0x5570fcb49c20)
    at /home/dgilbert/git/qemu/tests/vhost-user-bridge.c:349
        vubr = 0x5570fcb49c20
        dev = 0x5570fcb49c20
        vq = 0x5570fcb49d68
        elem = <optimized out>
        mhdr_sg = {{iov_base = 0x0, iov_len = 0} <repeats 740 times>, {iov_base 
= 0x0, iov_len = 139877843420656}, {

            iov_len = 0}...}
        mhdr = {hdr = {flags = 0 '\000', gso_type = 0 '\000', hdr_len = 0, 
gso_size = 0, csum_start = 0, 
            csum_offset = 0}, num_buffers = 0}
        mhdr_cnt = <optimized out>
        hdrlen = 0
        i = <optimized out>
        hdr = {flags = 0 '\000', gso_type = 0 '\000', hdr_len = 0, gso_size = 
0, csum_start = 0, csum_offset = 0}
        __PRETTY_FUNCTION__ = "vubr_backend_recv_cb"
#3  0x00005570fb8d8ad3 in dispatcher_wait (timeout=200000, dispr=0x5570fcb4a0b8)
    at /home/dgilbert/git/qemu/tests/vhost-user-bridge.c:154
        e = 0x5570fcb4a180
        tv = {tv_sec = 0, tv_usec = 199998}
        fdset = {fds_bits = {48, 0 <repeats 15 times>}}
        rc = <optimized out>
        sock = 4
#4  vubr_run (dev=<optimized out>) at 
/home/dgilbert/git/qemu/tests/vhost-user-bridge.c:624
No locals.
#5  main (argc=<optimized out>, argv=<optimized out>) at 
/home/dgilbert/git/qemu/tests/vhost-user-bridge.c:697
        opt = <optimized out>
        client = <optimized out>

Dave

> ---
>  contrib/libvhost-user/libvhost-user.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/contrib/libvhost-user/libvhost-user.c 
> b/contrib/libvhost-user/libvhost-user.c
> index af4faad60b..f9680b6279 100644
> --- a/contrib/libvhost-user/libvhost-user.c
> +++ b/contrib/libvhost-user/libvhost-user.c
> @@ -1031,6 +1031,10 @@ vu_queue_get_avail_bytes(VuDev *dev, VuVirtq *vq, 
> unsigned int *in_bytes,
>      idx = vq->last_avail_idx;
>  
>      total_bufs = in_total = out_total = 0;
> +    if (!vq->vring.avail) {
> +        goto done;
> +    }
> +
>      while ((rc = virtqueue_num_heads(dev, vq, idx)) > 0) {
>          unsigned int max, num_bufs, indirect = 0;
>          struct vring_desc *desc;
> -- 
> 2.12.0.191.gc5d8de91d
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK