Re: [RFC PATCH v1 2/7] char-socket: return -1 in case of disconnect during tcp_chr_write

[Marc-André Lureau]

Hi

On Thu, Apr 23, 2020 at 8:43 PM Dima Stepanov <address@hidden> wrote:
>
> During testing of the vhost-user-blk reconnect functionality the qemu
> SIGSEGV was triggered:
>  start qemu as:
>  x86_64-softmmu/qemu-system-x86_64 -m 1024M -M q35 \
>    -object 
> memory-backend-file,id=ram-node0,size=1024M,mem-path=/dev/shm/qemu,share=on \
>    -numa node,cpus=0,memdev=ram-node0 \
>    -chardev socket,id=chardev0,path=./vhost.sock,noserver,reconnect=1 \
>    -device vhost-user-blk-pci,chardev=chardev0,num-queues=4 --enable-kvm
>  start vhost-user-blk daemon:
>  ./vhost-user-blk -s ./vhost.sock -b test-img.raw
>
> If vhost-user-blk will be killed during the vhost initialization
> process, for instance after getting VHOST_SET_VRING_CALL command, then
> QEMU will fail with the following backtrace:
>
> Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
> 0x00005555559272bb in vhost_user_read (dev=0x7fffef2d53e0, msg=0x7fffffffd5b0)
>     at ./hw/virtio/vhost-user.c:260
> 260         CharBackend *chr = u->user->chr;
>
>  #0  0x00005555559272bb in vhost_user_read (dev=0x7fffef2d53e0, 
> msg=0x7fffffffd5b0)
>     at ./hw/virtio/vhost-user.c:260
>  #1  0x000055555592acb8 in vhost_user_get_config (dev=0x7fffef2d53e0, 
> config=0x7fffef2d5394 "", config_len=60)
>     at ./hw/virtio/vhost-user.c:1645
>  #2  0x0000555555925525 in vhost_dev_get_config (hdev=0x7fffef2d53e0, 
> config=0x7fffef2d5394 "", config_len=60)
>     at ./hw/virtio/vhost.c:1490
>  #3  0x00005555558cc46b in vhost_user_blk_device_realize (dev=0x7fffef2d51a0, 
> errp=0x7fffffffd8f0)
>     at ./hw/block/vhost-user-blk.c:429
>  #4  0x0000555555920090 in virtio_device_realize (dev=0x7fffef2d51a0, 
> errp=0x7fffffffd948)
>     at ./hw/virtio/virtio.c:3615
>  #5  0x0000555555a9779c in device_set_realized (obj=0x7fffef2d51a0, 
> value=true, errp=0x7fffffffdb88)
>     at ./hw/core/qdev.c:891
>  ...
>
> The problem is that vhost_user_write doesn't get an error after
> disconnect and try to call vhost_user_read(). The tcp_chr_write()
> routine should return -1 in case of disconnect. Indicate the EIO error
> if this routine is called in the disconnected state.
>
> Signed-off-by: Dima Stepanov <address@hidden>
> ---
>  chardev/char-socket.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/chardev/char-socket.c b/chardev/char-socket.c
> index 185fe38..c128cca 100644
> --- a/chardev/char-socket.c
> +++ b/chardev/char-socket.c
> @@ -175,14 +175,16 @@ static int tcp_chr_write(Chardev *chr, const uint8_t 
> *buf, int len)
>          if (ret < 0 && errno != EAGAIN) {
>              if (tcp_chr_read_poll(chr) <= 0) {
>                  tcp_chr_disconnect_locked(chr);
> -                return len;
> +                /* Return an error since we made a disconnect. */
> +                return ret;

Looks ok, but this return was introduced in commit
b0a335e351103bf92f3f9d0bd5759311be8156ac ("qemu-char: socket backend:
disconnect on write error"). It doesn't say why it didn't return -1
though. Anton, could you review? thanks

>              } /* else let the read handler finish it properly */
>          }
>
>          return ret;
>      } else {
> -        /* XXX: indicate an error ? */
> -        return len;
> +        /* Indicate an error. */
> +        errno = EIO;
> +        return -1;
>      }
>  }
>
> --
> 2.7.4
>
>


-- 
Marc-André Lureau