Re: [Qemu-devel] [PATCH v5 4/4] sockets: Handle race condition between binds to the same port

[Knut Omang]

On Tue, 2017-07-25 at 10:54 +0100, Daniel P. Berrange wrote:
> On Sat, Jul 22, 2017 at 09:49:33AM +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.
> > 
> > Also clean up some issues with error handling to allow more
> > accurate reporting of the cause of an error.
> > 
> > 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>
> > Signed-off-by: Knut Omang <address@hidden>
> > ---
> >  tests/Makefile.include |  2 +-
> >  util/qemu-sockets.c    | 51 ++++++++++++++++++++++++++++++++-----------
> >  2 files changed, 39 insertions(+), 14 deletions(-)
> > 
> > diff --git a/tests/Makefile.include b/tests/Makefile.include
> > index b37c0c8..9d2131d 100644
> > --- a/tests/Makefile.include
> > +++ b/tests/Makefile.include
> > @@ -128,7 +128,7 @@ 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 39f207c..8ca3ba6 100644
> > --- a/util/qemu-sockets.c
> > +++ b/util/qemu-sockets.c
> > @@ -210,7 +210,9 @@ 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));
> > @@ -277,27 +279,50 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
> >          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) {
> > +            rc = try_bind(slisten, saddr, e);
> > +            if (rc) {
> > +                if (errno == EADDRINUSE) {
> 
> Add in  '&& e->ai_next' here, so we trigger the error handling
> on the else branch if this was the last address we had.

See my comment on patch 2 - with that reporting semantics, inability to
create a socket will only be reported if none of the addrinfo structs
allowed socket creation so this reporting case goes away.

> > +                    continue;
> > +                } else {
> >                      error_setg_errno(errp, errno, "Failed to bind socket");
> > +                    goto listen_failed;
> > +                }
> > +            }
> > +            rc = listen(slisten, 1);
> > +            if (!rc) {
> 
> Assigning to rc seems redundant here.

removed.

> > +                goto listen_ok;
> > +            } else if (errno == EADDRINUSE) {
> > +                /* Someone else managed to bind to the same port and beat 
> > us
> > +                 * to listen on it! Socket semantics does not allow us to
> > +                 * recover from this situation, so we need to recreate the
> > +                 * socket to allow bind attempts for subsequent ports:
> > +                 */
> > +                closesocket(slisten);
> > +                slisten = create_fast_reuse_socket(e, errp);
> 
> create_fast_reuse_socket may report an error here
> 
> > +                if (slisten >= 0) {
> > +                    continue;
> >                  }
> >              }
> > +            error_setg_errno(errp, errno, "Failed to listen on socket");
> 
> at which point you report another error on top of it with a stale
> errno value here. This bug would be more obvious if error reporting
> was pulled out of create_fast_reuse_socket() as described in the
> earlier patch.
> 
> I think it'd be clearer if we reduced the nesting here. eg
> 
>      if (listen() == 0) {
>         goto listen_ok;
>      }
>      if (errno != EADDRINUSE) {
>          error_setg_error(errp, errno, "....");
>        goto listen_failed;
>      }
>      closesocket(listen);
>      slisten = create_fast_reuse_socket(e);
>      if (slisten < 0 && !e->ai_next) {
>          error_setg_error(errp, errno, "....");
>        goto listen_failed;
>      }

Good idea to push the != case up, done!

> > +            goto listen_failed;
> >          }
> > +    }
> > +    if (err) {
> > +        error_propagate(errp, err);
> > +    } else {
> > +        error_setg_errno(errp, errno, "Failed to find an available port");
> > +    }
> 
> if we tidied up the first call to create_fast_reuse_socket() too, it would
> remove the need for this.

I think v6 now solves this in a better way by separating between 
whether one or more ports have actually been tried and the case where no 
valid/working addrinfo records have been supplied.

Thanks,
Knut
> 
> > +
> > +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);
> 
> Regards,
> Daniel