On Mon, Jun 26, 2017 at 02:32:48PM +0200, Knut Omang wrote:
> On Mon, 2017-06-26 at 11:22 +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 <knut.om...@oracle.com>
> > > Reviewed-by: Bhavesh Davda <bhavesh.da...@oracle.com>
> > > Reviewed-by: Yuval Shaia <yuval.sh...@oracle.com>
> > > Reviewed-by: Girish Moodalbail <girish.moodalb...@oracle.com>
> > > ---
> > > 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);
> > > + if (s < 0) {
> > > + return errno;
> > > + }
> > > + *socket = s;
> >
> > I don't really like this at all - if we need to close + recreate the
> > socket, IMHO that should remain the job of the caller, since it owns
> > the socket FD ultimately.
>
> Normally I would agree, but this is a very unlikely situation. I considered
> moving the
> complexity out to the caller, even to recreate for every call, but found
> those solutions
> to be inferior as they do not in any way confine the problem, and cause the
> handling of
> the common cases to be much less readable. It's going to be some trade-offs
> here.
>
> As long as the caller is aware of (by the reference call) that the socket in
> use may
> change, this is in my view a clean (as clean as possible) abstraction that
> simplifies the
> logic at the next level. My intention is to make the common, good case as
> readable as
> possible and hide some of the complexity of these
> unlikely error scenarios inside the new functions - divide and conquer..
>
> >
> > > + 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,
> > > @@ -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,
> >
> > Just above this line is the original 'create_fast_reuse_socket' call.
> >
> > I'd suggest that we push that call down into the body of the loop
> > below:
> >
> > > 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);
> >
> > Which would mean try_bind_listen no longer needs the magic to close +
> > recreate the socket.
> >
> > The only cost of doing this is that you end up closing + recreating the
> > socket after bind hits EADDRINUSE, as well as after listen() hits it.
>
> The problem with this approach in my opinion is that one has to understand the
> fix for the problem I am trying to solve here in order to read the main code,
> even though this is a very special case. Everyone reading the code would ask
> themselves
> the question 'why do they recreate the socket here?' and then be forced to
> ready the
> details of try_bind_listen anyway, or we would need additional comments.
That's easily solved by adding a comment
/* We recreate the socket FD on each iteration because
if bind succeeds & listen fails, we can't bind
again on the same socket FD */
> The idea behind the abstractions I have used here is to hide the details
> inside functions,
> but leave them with an as clean as possible (although not ideal) interface
> that
> makes the overall logic more readable.
I think the result is actually harder to understand, because of the
peculiar way the function closes & reopens the socket FD belonging
to the caller, and the error handling is really very unclear and
buggy as a result too.
> > I think that's acceptable tradeoff for simpler code, since this is not
> > a performance critical operation.
>
> Also should we perhaps worry about any side effects of creating and closing a
> lot of
> sockets unnecessary?
What side effects ? I don't think there are any - since this is server
side, not client side, we're not leaving any state around in timed waits
or similar.
Regards,
Daniel
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