Re: [PATCH] sockets: update SOCKET_ADDRESS_TYPE_FD listen(2) backlog

[Stefan Hajnoczi]

On Wed, Mar 10, 2021 at 05:30:04PM +0000, Stefan Hajnoczi wrote:
> socket_get_fd() fails with the error "socket_get_fd: too many
> connections" if the given listen backlog value is not 1.
> 
> Not all callers set the backlog to 1. For example, commit
> 582d4210eb2f2ab5baac328fe4b479cd86da1647 ("qemu-nbd: Use SOMAXCONN for
> socket listen() backlog") uses SOMAXCONN. This will always fail with in
> socket_get_fd().
> 
> This patch calls listen(2) on the fd to update the backlog value. The
> socket may already be in the listen state. I have tested that this works
> on Linux 5.10 and macOS Catalina.
> 
> As a bonus this allows us to detect when the fd cannot listen. Now we'll
> be able to catch unbound or connected fds in socket_listen().
> 
> Drop the num argument from socket_get_fd() since this function is also
> called by socket_connect() where a listen backlog value does not make
> sense.
> 
> Fixes: e5b6353cf25c99c3f08bf51e29933352f7140e8f ("socket: Add backlog 
> parameter to socket_listen")
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Cc: Juan Quintela <quintela@redhat.com>
> Cc: Eric Blake <eblake@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  util/qemu-sockets.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)

Dan and Gerd: Can this go via one of your trees?

Thanks,
Stefan

> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index 8af0278f15..2463c49773 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -1116,14 +1116,10 @@ fail:
>      return NULL;
>  }
>  
> -static int socket_get_fd(const char *fdstr, int num, Error **errp)
> +static int socket_get_fd(const char *fdstr, Error **errp)
>  {
>      Monitor *cur_mon = monitor_cur();
>      int fd;
> -    if (num != 1) {
> -        error_setg_errno(errp, EINVAL, "socket_get_fd: too many 
> connections");
> -        return -1;
> -    }
>      if (cur_mon) {
>          fd = monitor_get_fd(cur_mon, fdstr, errp);
>          if (fd < 0) {
> @@ -1159,7 +1155,7 @@ int socket_connect(SocketAddress *addr, Error **errp)
>          break;
>  
>      case SOCKET_ADDRESS_TYPE_FD:
> -        fd = socket_get_fd(addr->u.fd.str, 1, errp);
> +        fd = socket_get_fd(addr->u.fd.str, errp);
>          break;
>  
>      case SOCKET_ADDRESS_TYPE_VSOCK:
> @@ -1187,7 +1183,26 @@ int socket_listen(SocketAddress *addr, int num, Error 
> **errp)
>          break;
>  
>      case SOCKET_ADDRESS_TYPE_FD:
> -        fd = socket_get_fd(addr->u.fd.str, num, errp);
> +        fd = socket_get_fd(addr->u.fd.str, errp);
> +        if (fd < 0) {
> +            return -1;
> +        }
> +
> +        /*
> +         * If the socket is not yet in the listen state, then transition it 
> to
> +         * the listen state now.
> +         *
> +         * If it's already listening then this updates the backlog value as
> +         * requested.
> +         *
> +         * If this socket cannot listen because it's already in another state
> +         * (e.g. unbound or connected) then we'll catch the error here.
> +         */
> +        if (listen(fd, num) != 0) {
> +            error_setg_errno(errp, errno, "Failed to listen on fd socket");
> +            closesocket(fd);
> +            return -1;
> +        }
>          break;
>  
>      case SOCKET_ADDRESS_TYPE_VSOCK:
> -- 
> 2.29.2
>

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