Re: [Qemu-devel] [PATCH v4 4/4] sockets: Handle race condition between binds to the same port

[Knut Omang]

On Mon, 2017-06-26 at 11:34 +0100, Daniel P. Berrange wrote:
> On Fri, Jun 23, 2017 at 12:31:08PM +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    | 68 ++++++++++++++++++++++++++++++++-----------
> >  2 files changed, 53 insertions(+), 17 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 48b9319..7b118b4 100644
> > --- a/util/qemu-sockets.c
> > +++ b/util/qemu-sockets.c
> > @@ -201,6 +201,42 @@ static int try_bind(int socket, InetSocketAddress
> > *saddr, struct addrinfo *e)
> >  #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);
> 
> This usage scenario for create_fast_reuse_socket() makes its error
> reporting behaviour even more wrong. Recall that create_fast_reuse_socket
> is reporting an error if e->ai_next is NULL, which is a way of determining
> this is the last call to create_fast_reuse_socket in the loop. That
> assumption is violated though now that we're calling the method from
> inside the inner loop. Even when e->ai_next is NULL, we may be calling
> create_fast_reuse_socket many many times due to the port  'to' range.

I agree that the error reporting should go out of create_fast_reuse_socket().
Note however that this code will only be called when the race condition occurs,
which I think is very unlikely to happen more than once for each call to
inet_listen_saddr (except in my test of course..)

> 
> > 
> > +        if (s < 0) {
> > +            return errno;
> > +        }
> > +        *socket = s;
> > +        errno = EADDRINUSE;
> > +        return errno;
> > +    }
> > +    error_setg_errno(errp, errno, "Failed to listen on socket");
> > +    return errno;
> > +}
> 
> This method is both preserving the global errno, and returning the
> global errno. The caller expects global errno to be preserved, so
> I think we can just return '-1' from this method.

will do,

Thanks,
Knut

> 
> > 
> > +
> >  static int inet_listen_saddr(InetSocketAddress *saddr,
> >                               int port_offset,
> >                               bool update_addr,
> > @@ -210,7 +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));
> > @@ -276,28 +314,26 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
> >          port_min = inet_getport(e);
> >          port_max = saddr->has_to ? saddr->to + port_offset : port_min;
> >          for (p = port_min; p <= port_max; p++) {
> > -            inet_setport(e, p);
> > -            if (try_bind(slisten, saddr, e) >= 0) {
> > -                goto listen;
> > -            }
> > -            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);
> > -- 
> > git-series 0.9.1
> 
> Regards,
> Daniel