[Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

[Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

[Richard W.M. Jones]

v2:

 - A few small fixed identified by Dan Berrange.

The original cover letter is below.

Rich.


Socket activation (sometimes known as systemd socket activation)
allows an Internet superserver to pass a pre-opened listening socket
to the process, instead of having qemu-nbd open a socket itself.  This
is done via the LISTEN_FDS and LISTEN_PID environment variables, and a
standard file descriptor range.

This patch partially implements socket activation.

The limitation of this implementation is that qemu-nbd can only listen
on a single file descriptor, and so if LISTEN_FDS > 1 (eg. for
listening on multiple interfaces or ports) socket activation will
fail.  However for the simple case of listening on a single port, and
either all interfaces with IPv4+IPv6, or just a loopback interface,
the current implementation works fine.  Fixing this properly would
require considerable changes throughout qemu, since qemu's currently
handling of getaddrinfo is plainly wrong.

To use qemu-nbd from systemd, you create
/etc/systemd/system/nbd.socket:

  [Unit]
  Description=QEMU Network Block Device server
  [Socket]
  ListenStream=10809
  [Install]
  WantedBy=sockets.target

and /etc/systemd/system/nbd.service:

  [Service]
  ExecStart=/usr/sbin/qemu-nbd -v -t /path/to/file

and enable the socket service (only):

  systemctl enable nbd.socket
  systemctl start nbd.socket

and then connecting to port 10809 will start qemu-nbd and service the
file, with systemd opening the listening socket.

In the ExecStart line, the qemu-nbd -v option is only needed if you
want enhanced debugging.  The -t option is required unless you want to
fiddle with systemd settings for rate-limiting.

If you try to use the -p and similar options with socket activation
then qemu-nbd will give an error.

(I wasn't sure where to document this -- there is no obvious
documentation for qemu-nbd beyond the simple list of command line
arguments)

This is based on the implementations in
libvirt (src/util/virutil.c:virGetListenFDs) and
nbdkit (src/main.c:get_socket_activation), and also on Denis Plotnikov's
implementation of --server-sock-fd
(https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg07781.html).

[Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

[Richard W.M. Jones]

Socket activation (sometimes known as systemd socket activation)
allows an Internet superserver to pass a pre-opened listening socket
to the process, instead of having qemu-nbd open a socket itself.  This
is done via the LISTEN_FDS and LISTEN_PID environment variables, and a
standard file descriptor range.

This change partially implements socket activation for qemu-nbd.  If
the environment variables are set correctly, then socket activation
will happen automatically, otherwise everything works as before.  The
limitation is that LISTEN_FDS must be 1.

Signed-off-by: Richard W.M. Jones.
---
 qemu-nbd.c | 172 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 163 insertions(+), 9 deletions(-)

diff --git a/qemu-nbd.c b/qemu-nbd.c
index c734f62..b3088d0 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -463,6 +463,135 @@ static QCryptoTLSCreds *nbd_get_tls_creds(const char *id, Error **errp)
     return creds;
 }
 
+static void setup_address_and_port(const char **address, const char **port)
+{
+    if (*address == NULL) {
+        *address = "0.0.0.0";
+    }
+
+    if (*port == NULL) {
+        *port = g_strdup_printf("%d", NBD_DEFAULT_PORT);;
+    }
+}
+
+#define FIRST_SOCKET_ACTIVATION_FD 3 /* defined by systemd ABI */
+
+#ifndef _WIN32
+/*
+ * Check if socket activation was requested via use of the
+ * LISTEN_FDS and LISTEN_PID environment variables.
+ *
+ * Returns 0 if no socket activation, or the number of FDs.
+ */
+static unsigned int check_socket_activation(void)
+{
+    const char *s;
+    unsigned long pid;
+    unsigned long nr_fds;
+    unsigned int i;
+    int fd;
+    int err;
+
+    s = getenv("LISTEN_PID");
+    if (s == NULL) {
+        return 0;
+    }
+    err = qemu_strtoul(s, NULL, 10, &pid);
+    if (err) {
+        if (verbose) {
+            fprintf(stderr, "malformed %s environment variable (ignored)\n",
+                    "LISTEN_PID");
+        }
+        return 0;
+    }
+    if (pid != getpid()) {
+        if (verbose) {
+            fprintf(stderr, "%s was not for us (ignored)\n",
+                    "LISTEN_PID");
+        }
+        return 0;
+    }
+
+    s = getenv("LISTEN_FDS");
+    if (s == NULL) {
+        return 0;
+    }
+    err = qemu_strtoul(s, NULL, 10, &nr_fds);
+    if (err) {
+        if (verbose) {
+            fprintf(stderr, "malformed %s environment variable (ignored)\n",
+                    "LISTEN_FDS");
+        }
+        return 0;
+    }
+    assert(nr_fds <= UINT_MAX);
+
+    /* A limitation of current qemu-nbd is that it can only listen on
+     * a single socket.  When that limitation is lifted, we can change
+     * this function to allow LISTEN_FDS > 1, and remove the assertion
+     * in the main function below.
+     */
+    if (nr_fds > 1) {
+        error_report("qemu-nbd does not support socket activation with %s > 1",
+                     "LISTEN_FDS");
+        exit(EXIT_FAILURE);
+    }
+
+    /* So these are not passed to any child processes we might start. */
+    unsetenv("LISTEN_FDS");
+    unsetenv("LISTEN_PID");
+
+    /* So the file descriptors don't leak into child processes. */
+    for (i = 0; i < nr_fds; ++i) {
+        fd = FIRST_SOCKET_ACTIVATION_FD + i;
+        if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
+            /* If we cannot set FD_CLOEXEC then it probably means the file
+             * descriptor is invalid, so socket activation has gone wrong
+             * and we should exit.
+             */
+            error_report("Socket activation failed: "
+                         "invalid file descriptor fd = %d: %m",
+                         fd);
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    return (unsigned int) nr_fds;
+}
+
+#else /* !_WIN32 */
+static unsigned int check_socket_activation(void)
+{
+    return 0;
+}
+#endif
+
+/*
+ * Check socket parameters compatibility when socket activation is used.
+ */
+static const char *socket_activation_validate_opts(const char *device,
+                                                   const char *sockpath,
+                                                   const char *address,
+                                                   const char *port)
+{
+    if (device != NULL) {
+        return "NBD device can't be set when using socket activation";
+    }
+
+    if (sockpath != NULL) {
+        return "Unix socket can't be set when using socket activation";
+    }
+
+    if (address != NULL) {
+        return "The interface can't be set when using socket activation";
+    }
+
+    if (port != NULL) {
+        return "TCP port number can't be set when using socket activation";
+    }
+
+    return NULL;
+}
 
 int main(int argc, char **argv)
 {
@@ -471,7 +600,7 @@ int main(int argc, char **argv)
     off_t dev_offset = 0;
     uint16_t nbdflags = 0;
     bool disconnect = false;
-    const char *bindto = "0.0.0.0";
+    const char *bindto = NULL;
     const char *port = NULL;
     char *sockpath = NULL;
     char *device = NULL;
@@ -533,6 +662,7 @@ int main(int argc, char **argv)
     char *trace_file = NULL;
     bool fork_process = false;
     int old_stderr = -1;
+    unsigned socket_activation;
 
     /* The client thread uses SIGTERM to interrupt the server.  A signal
      * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
@@ -751,6 +881,19 @@ int main(int argc, char **argv)
     trace_init_file(trace_file);
     qemu_set_log(LOG_TRACE);
 
+    socket_activation = check_socket_activation();
+    if (socket_activation == 0) {
+        setup_address_and_port(&bindto, &port);
+    } else {
+        /* Using socket activation - check user didn't use -p etc. */
+        const char *err_msg = socket_activation_validate_opts(device, sockpath,
+                                                              bindto, port);
+        if (err_msg != NULL) {
+            error_report("%s", err_msg);
+            exit(EXIT_FAILURE);
+        }
+    }
+
     if (tlscredsid) {
         if (sockpath) {
             error_report("TLS is only supported with IPv4/IPv6");
@@ -855,7 +998,25 @@ int main(int argc, char **argv)
         snprintf(sockpath, 128, SOCKET_PATH, basename(device));
     }
 
-    saddr = nbd_build_socket_address(sockpath, bindto, port);
+    if (socket_activation == 0) {
+        server_ioc = qio_channel_socket_new();
+        saddr = nbd_build_socket_address(sockpath, bindto, port);
+        if (qio_channel_socket_listen_sync(server_ioc, saddr, &local_err) < 0) {
+            object_unref(OBJECT(server_ioc));
+            error_report_err(local_err);
+            return 1;
+        }
+    } else {
+        /* See comment in check_socket_activation above. */
+        assert(socket_activation == 1);
+        server_ioc = qio_channel_socket_new_fd(FIRST_SOCKET_ACTIVATION_FD,
+                                               &local_err);
+        if (server_ioc == NULL) {
+            error_report("Failed to use socket activation: %s",
+                         error_get_pretty(local_err));
+            exit(EXIT_FAILURE);
+        }
+    }
 
     if (qemu_init_main_loop(&local_err)) {
         error_report_err(local_err);
@@ -950,13 +1111,6 @@ int main(int argc, char **argv)
         exit(EXIT_FAILURE);
     }
 
-    server_ioc = qio_channel_socket_new();
-    if (qio_channel_socket_listen_sync(server_ioc, saddr, &local_err) < 0) {
-        object_unref(OBJECT(server_ioc));
-        error_report_err(local_err);
-        return 1;
-    }
-
     if (device) {
         int ret;
 
-- 
2.10.2

Re: [Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

[Daniel P. Berrange]

On Thu, Feb 02, 2017 at 05:16:25PM +0000, Richard W.M. Jones wrote:
> Socket activation (sometimes known as systemd socket activation)
> allows an Internet superserver to pass a pre-opened listening socket
> to the process, instead of having qemu-nbd open a socket itself.  This
> is done via the LISTEN_FDS and LISTEN_PID environment variables, and a
> standard file descriptor range.
> 
> This change partially implements socket activation for qemu-nbd.  If
> the environment variables are set correctly, then socket activation
> will happen automatically, otherwise everything works as before.  The
> limitation is that LISTEN_FDS must be 1.
> 
> Signed-off-by: Richard W.M. Jones.
> ---
>  qemu-nbd.c | 172 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 163 insertions(+), 9 deletions(-)

Reviewed-by: Daniel P. Berrange <berrange@redhat.com>


Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://entangle-photo.org       -o-    http://search.cpan.org/~danberr/ :|

Re: [Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 2166 bytes --]

On Thu, Feb 02, 2017 at 05:16:25PM +0000, Richard W.M. Jones wrote:
> Socket activation (sometimes known as systemd socket activation)
> allows an Internet superserver to pass a pre-opened listening socket
> to the process, instead of having qemu-nbd open a socket itself.  This
> is done via the LISTEN_FDS and LISTEN_PID environment variables, and a
> standard file descriptor range.
> 
> This change partially implements socket activation for qemu-nbd.  If
> the environment variables are set correctly, then socket activation
> will happen automatically, otherwise everything works as before.  The
> limitation is that LISTEN_FDS must be 1.
> 
> Signed-off-by: Richard W.M. Jones.
> ---
>  qemu-nbd.c | 172 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 163 insertions(+), 9 deletions(-)
> 
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index c734f62..b3088d0 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -463,6 +463,135 @@ static QCryptoTLSCreds *nbd_get_tls_creds(const char *id, Error **errp)
>      return creds;
>  }
>  
> +static void setup_address_and_port(const char **address, const char **port)
> +{
> +    if (*address == NULL) {
> +        *address = "0.0.0.0";
> +    }
> +
> +    if (*port == NULL) {
> +        *port = g_strdup_printf("%d", NBD_DEFAULT_PORT);;

Please stringify NBD_DEFAULT_PORT instead of using g_strdup_printf().
That avoids the memory leak.

> @@ -751,6 +881,19 @@ int main(int argc, char **argv)
>      trace_init_file(trace_file);
>      qemu_set_log(LOG_TRACE);
>  
> +    socket_activation = check_socket_activation();
> +    if (socket_activation == 0) {
> +        setup_address_and_port(&bindto, &port);
> +    } else {
> +        /* Using socket activation - check user didn't use -p etc. */
> +        const char *err_msg = socket_activation_validate_opts(device, sockpath,
> +                                                              bindto, port);

Daemonizing is also incompatible with socket activation.  We've marked
the fds O_CLOEXEC so the child won't have access to them.  Please add an
error in case a user tries this.

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Re: [Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

[Richard W.M. Jones]

On Fri, Feb 03, 2017 at 03:16:43PM +0000, Stefan Hajnoczi wrote:
> On Thu, Feb 02, 2017 at 05:16:25PM +0000, Richard W.M. Jones wrote:
> > +    if (*port == NULL) {
> > +        *port = g_strdup_printf("%d", NBD_DEFAULT_PORT);;
> 
> Please stringify NBD_DEFAULT_PORT instead of using g_strdup_printf().
> That avoids the memory leak.

Oops.

Do we have a macro for this already?  I couldn't see one, and the
best I could come up with is:

#define MACRO_EXPAND_STRINGIFY(x) STRINGIFY(x)
#define STRINGIFY(x) #x

static void setup_address_and_port(const char **address, const char **port)
{
    if (*address == NULL) {
        *address = "0.0.0.0";
    }

    if (*port == NULL) {
        *port = MACRO_EXPAND_STRINGIFY(NBD_DEFAULT_PORT);
    }
}

It works, but it's a bit of a mouthful.

Rich.

-- 
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Re: [Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

[Markus Armbruster]

"Richard W.M. Jones" <rjones@redhat.com> writes:

> On Fri, Feb 03, 2017 at 03:16:43PM +0000, Stefan Hajnoczi wrote:
>> On Thu, Feb 02, 2017 at 05:16:25PM +0000, Richard W.M. Jones wrote:
>> > +    if (*port == NULL) {
>> > +        *port = g_strdup_printf("%d", NBD_DEFAULT_PORT);;
>> 
>> Please stringify NBD_DEFAULT_PORT instead of using g_strdup_printf().
>> That avoids the memory leak.
>
> Oops.
>
> Do we have a macro for this already?  I couldn't see one, and the
> best I could come up with is:
>
> #define MACRO_EXPAND_STRINGIFY(x) STRINGIFY(x)
> #define STRINGIFY(x) #x

Check out stringify() in compiler.h.

(Yes, lower-case macros that don't behave like functions are bad style)

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