Re: [Qemu-devel] [PATCH v3 2/2] sockets: Handle race condition between binds to the same port

[Daniel P. Berrange]

On Wed, Jun 14, 2017 at 06:53:52PM +0200, Knut Omang wrote:
> If an offset of ports is specified to the inet_listen_saddr function(),
> and two or more processes tries to bind from these ports at the same time,
> occasionally more than one process may be able to bind to the same
> port. The condition is detected by listen() but too late to avoid a failure.
> 
> This function is called by socket_listen() and used
> by all socket listening code in QEMU, so all cases where any form of dynamic
> port selection is used should be subject to this issue.
> 
> Add code to close and re-establish the socket when this
> condition is observed, hiding the race condition from the user.
> 
> This has been developed and tested by means of the
> test-listen unit test in the previous commit.
> Enable the test for make check now that it passes.
> 
> Signed-off-by: Knut Omang <address@hidden>
> Reviewed-by: Bhavesh Davda <address@hidden>
> Reviewed-by: Yuval Shaia <address@hidden>
> Reviewed-by: Girish Moodalbail <address@hidden>
> ---
>  tests/Makefile.include |   2 +-
>  util/qemu-sockets.c    | 159 ++++++++++++++++++++++++++++--------------
>  2 files changed, 108 insertions(+), 53 deletions(-)
> 
> diff --git a/tests/Makefile.include b/tests/Makefile.include
> index 22bb97e..c38f94e 100644
> --- a/tests/Makefile.include
> +++ b/tests/Makefile.include
> @@ -127,7 +127,7 @@ check-unit-y += tests/test-bufferiszero$(EXESUF)
>  gcov-files-check-bufferiszero-y = util/bufferiszero.c
>  check-unit-y += tests/test-uuid$(EXESUF)
>  check-unit-y += tests/ptimer-test$(EXESUF)
> -#check-unit-y += tests/test-listen$(EXESUF)
> +check-unit-y += tests/test-listen$(EXESUF)
>  gcov-files-ptimer-test-y = hw/core/ptimer.c
>  check-unit-y += tests/test-qapi-util$(EXESUF)
>  gcov-files-test-qapi-util-y = qapi/qapi-util.c
> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index 852773d..7b118b4 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -149,6 +149,94 @@ int inet_ai_family_from_address(InetSocketAddress *addr,
>      return PF_UNSPEC;
>  }
>  
> +static int create_fast_reuse_socket(struct addrinfo *e, Error **errp)
> +{
> +    int slisten = qemu_socket(e->ai_family, e->ai_socktype, e->ai_protocol);
> +    if (slisten < 0) {
> +        if (!e->ai_next) {
> +            error_setg_errno(errp, errno, "Failed to create socket");
> +        }
> +        return -1;
> +    }
> +
> +    socket_set_fast_reuse(slisten);
> +    return slisten;
> +}
> +
> +static int try_bind(int socket, InetSocketAddress *saddr, struct addrinfo *e)
> +{
> +#ifndef IPV6_V6ONLY
> +    return bind(socket, e->ai_addr, e->ai_addrlen);
> +#else
> +    /*
> +     * Deals with first & last cases in matrix in comment
> +     * for inet_ai_family_from_address().
> +     */
> +    int v6only =
> +        ((!saddr->has_ipv4 && !saddr->has_ipv6) ||
> +         (saddr->has_ipv4 && saddr->ipv4 &&
> +          saddr->has_ipv6 && saddr->ipv6)) ? 0 : 1;
> +    int stat;
> +
> + rebind:
> +    if (e->ai_family == PF_INET6) {
> +        qemu_setsockopt(socket, IPPROTO_IPV6, IPV6_V6ONLY, &v6only,
> +                        sizeof(v6only));
> +    }
> +
> +    stat = bind(socket, e->ai_addr, e->ai_addrlen);
> +    if (!stat) {
> +        return 0;
> +    }
> +
> +    /* If we got EADDRINUSE from an IPv6 bind & v6only is unset,
> +     * it could be that the IPv4 port is already claimed, so retry
> +     * with v6only set
> +     */
> +    if (e->ai_family == PF_INET6 && errno == EADDRINUSE && !v6only) {
> +        v6only = 1;
> +        goto rebind;
> +    }
> +    return stat;
> +#endif
> +}
> +
> +static int try_bind_listen(int *socket, InetSocketAddress *saddr,
> +                           struct addrinfo *e, int port, Error **errp)
> +{
> +    int s = *socket;
> +    int ret;
> +
> +    inet_setport(e, port);
> +    ret = try_bind(s, saddr, e);
> +    if (ret) {
> +        if (errno != EADDRINUSE) {
> +            error_setg_errno(errp, errno, "Failed to bind socket");
> +        }
> +        return errno;
> +    }
> +    if (listen(s, 1) == 0) {
> +            return 0;
> +    }
> +    if (errno == EADDRINUSE) {
> +        /* We got to bind the socket to a port but someone else managed
> +         * to bind to the same port and beat us to listen on it!
> +         * Recreate the socket and return EADDRINUSE to preserve the
> +         * expected state by the caller:
> +         */
> +        closesocket(s);
> +        s = create_fast_reuse_socket(e, errp);
> +        if (s < 0) {
> +            return errno;
> +        }
> +        *socket = s;
> +        errno = EADDRINUSE;
> +        return errno;
> +    }
> +    error_setg_errno(errp, errno, "Failed to listen on socket");
> +    return errno;
> +}
> +
>  static int inet_listen_saddr(InetSocketAddress *saddr,
>                               int port_offset,
>                               bool update_addr,
> @@ -158,7 +246,9 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
>      char port[33];
>      char uaddr[INET6_ADDRSTRLEN+1];
>      char uport[33];
> -    int slisten, rc, port_min, port_max, p;
> +    int rc, port_min, port_max, p;
> +    int slisten = 0;
> +    int saved_errno = 0;
>      Error *err = NULL;
>  
>      memset(&ai,0, sizeof(ai));
> @@ -210,75 +300,40 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
>          return -1;
>      }
>  
> -    /* create socket + bind */
> +    /* create socket + bind/listen */
>      for (e = res; e != NULL; e = e->ai_next) {
>          getnameinfo((struct sockaddr*)e->ai_addr,e->ai_addrlen,
>                       uaddr,INET6_ADDRSTRLEN,uport,32,
>                       NI_NUMERICHOST | NI_NUMERICSERV);
> -        slisten = qemu_socket(e->ai_family, e->ai_socktype, e->ai_protocol);
> +
> +        slisten = create_fast_reuse_socket(e, &err);
>          if (slisten < 0) {
> -            if (!e->ai_next) {
> -                error_setg_errno(errp, errno, "Failed to create socket");
> -            }
>              continue;
>          }
>  
> -        socket_set_fast_reuse(slisten);
> -
>          port_min = inet_getport(e);
>          port_max = saddr->has_to ? saddr->to + port_offset : port_min;
>          for (p = port_min; p <= port_max; p++) {
> -#ifdef IPV6_V6ONLY
> -            /*
> -             * Deals with first & last cases in matrix in comment
> -             * for inet_ai_family_from_address().
> -             */
> -            int v6only =
> -                ((!saddr->has_ipv4 && !saddr->has_ipv6) ||
> -                 (saddr->has_ipv4 && saddr->ipv4 &&
> -                  saddr->has_ipv6 && saddr->ipv6)) ? 0 : 1;
> -#endif
> -            inet_setport(e, p);
> -#ifdef IPV6_V6ONLY
> -        rebind:
> -            if (e->ai_family == PF_INET6) {
> -                qemu_setsockopt(slisten, IPPROTO_IPV6, IPV6_V6ONLY, &v6only,
> -                                sizeof(v6only));
> -            }
> -#endif
> -            if (bind(slisten, e->ai_addr, e->ai_addrlen) == 0) {
> -                goto listen;
> -            }
> -
> -#ifdef IPV6_V6ONLY
> -            /* If we got EADDRINUSE from an IPv6 bind & V6ONLY is unset,
> -             * it could be that the IPv4 port is already claimed, so retry
> -             * with V6ONLY set
> -             */
> -            if (e->ai_family == PF_INET6 && errno == EADDRINUSE && !v6only) {
> -                v6only = 1;
> -                goto rebind;
> -            }
> -#endif
> -
> -            if (p == port_max) {
> -                if (!e->ai_next) {
> -                    error_setg_errno(errp, errno, "Failed to bind socket");
> -                }
> +            int eno = try_bind_listen(&slisten, saddr, e, p, &err);
> +            if (!eno) {
> +                goto listen_ok;
> +            } else if (eno != EADDRINUSE) {
> +                goto listen_failed;
>              }
>          }
> +    }
> +    error_setg_errno(errp, errno, "Failed to find available port");
> +
> +listen_failed:
> +    saved_errno = errno;
> +    if (slisten >= 0) {
>          closesocket(slisten);
>      }
>      freeaddrinfo(res);
> +    errno = saved_errno;
>      return -1;
>  
> -listen:
> -    if (listen(slisten,1) != 0) {
> -        error_setg_errno(errp, errno, "Failed to listen on socket");
> -        closesocket(slisten);
> -        freeaddrinfo(res);
> -        return -1;
> -    }
> +listen_ok:
>      if (update_addr) {
>          g_free(saddr->host);
>          saddr->host = g_strdup(uaddr);

I find this patch rather hard to review for correctness, because it has
mixed up a huge amount of code movement / refactoring, with the bug
fix. Can you split this up into 2 (or possibly more) patches, so we have
1 (or more) no-functional-change refactoring steps, and then the bug fix
on its own.

Regards,
Daniel
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