Libguestfs - Aug 2020 - [nbdkit PATCH 2/3] server: Expose final thread_model to filter's .get_ready

Patch 3 still needs tests added, but it is at least working from
my simple command line tests.

Eric Blake (3):
  server: Implement nbdkit_is_tls for use during .open
  server: Expose final thread_model to filter's .get_ready
  tlsdummy: New filter

 docs/nbdkit-filter.pod                      |  21 +-
 docs/nbdkit-plugin.pod                      |  34 ++-
 docs/nbdkit-tls.pod                         |   8 +-
 filters/log/nbdkit-log-filter.pod           |   2 +-
 filters/tlsdummy/nbdkit-tlsdummy-filter.pod |  72 ++++++
 plugins/sh/nbdkit-sh-plugin.pod             |   5 +-
 include/nbdkit-filter.h                     |   5 +-
 include/nbdkit-plugin.h                     |   1 +
 configure.ac                                |   2 +
 filters/tlsdummy/Makefile.am                |  63 ++++++
 server/internal.h                           |   3 +-
 server/backend.c                            |   6 +-
 server/filters.c                            |  10 +-
 server/nbdkit.syms                          |   1 +
 server/plugins.c                            |  14 +-
 server/public.c                             |  16 ++
 plugins/sh/methods.c                        |   1 +
 filters/cow/cow.c                           |   2 +-
 filters/exitlast/exitlast.c                 |   2 +-
 filters/ext2/ext2.c                         |   2 +-
 filters/extentlist/extentlist.c             |   3 +-
 filters/gzip/gzip.c                         |   2 +-
 filters/limit/limit.c                       |   2 +-
 filters/log/log.c                           |  18 +-
 filters/partition/partition.c               |   2 +-
 filters/rate/rate.c                         |   4 +-
 filters/retry/retry.c                       |   2 +-
 filters/stats/stats.c                       |   2 +-
 filters/tar/tar.c                           |   2 +-
 filters/tlsdummy/tlsdummy.c                 | 235 ++++++++++++++++++++
 filters/truncate/truncate.c                 |   2 +-
 filters/xz/xz.c                             |   2 +-
 tests/test-layers-filter.c                  |   4 +-
 TODO                                        |  13 +-
 34 files changed, 504 insertions(+), 59 deletions(-)
 create mode 100644 filters/tlsdummy/nbdkit-tlsdummy-filter.pod
 create mode 100644 filters/tlsdummy/Makefile.am
 create mode 100644 filters/tlsdummy/tlsdummy.c

-- 
2.28.0

Now that we can differentiate content based on export name, we also
need to be able to differentiate content for a --tls=on server, since
TLS is optional according to whether the client has authenticated.

For internal code and filters, this means adding a new parameter; the
sh plugin can do likewise.  For plugins, we can't add a parameter
until the V3 protocol, so in the meantime, we add nbdkit_is_tls(),
even though it will be deprecated alongside nbdkit_export_name() when
we do get to V3.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 docs/nbdkit-filter.pod            | 12 ++++++++----
 docs/nbdkit-plugin.pod            | 32 +++++++++++++++++++++++++++----
 docs/nbdkit-tls.pod               |  5 ++++-
 filters/log/nbdkit-log-filter.pod |  2 +-
 plugins/sh/nbdkit-sh-plugin.pod   |  5 +++--
 include/nbdkit-filter.h           |  2 +-
 include/nbdkit-plugin.h           |  1 +
 server/internal.h                 |  3 ++-
 server/backend.c                  |  6 +++---
 server/filters.c                  |  6 ++++--
 server/nbdkit.syms                |  1 +
 server/plugins.c                  | 14 +++++++++-----
 server/public.c                   | 16 ++++++++++++++++
 plugins/sh/methods.c              |  1 +
 filters/cow/cow.c                 |  2 +-
 filters/exitlast/exitlast.c       |  2 +-
 filters/ext2/ext2.c               |  2 +-
 filters/gzip/gzip.c               |  2 +-
 filters/limit/limit.c             |  2 +-
 filters/log/log.c                 | 16 +++++++++-------
 filters/partition/partition.c     |  2 +-
 filters/rate/rate.c               |  2 +-
 filters/retry/retry.c             |  2 +-
 filters/tar/tar.c                 |  2 +-
 filters/truncate/truncate.c       |  2 +-
 filters/xz/xz.c                   |  2 +-
 tests/test-layers-filter.c        |  2 +-
 TODO                              | 13 ++++++-------
 28 files changed, 109 insertions(+), 50 deletions(-)

diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
index 12343dbf..b6ed5504 100644
--- a/docs/nbdkit-filter.pod
+++ b/docs/nbdkit-filter.pod
@@ -403,7 +403,7 @@ Returns a copy of the C<i>'th export.
 =head2 C<.open>

  void * (*open) (nbdkit_next_open *next, void *nxdata,
-                 int readonly, const char *exportname);
+                 int readonly, const char *exportname, int is_tls);

 This is called when a new client connection is opened and can be used
 to allocate any per-connection data structures needed by the filter.
@@ -420,8 +420,9 @@ error message and return C<NULL>.

 This callback is optional, but if provided, it must call C<next>,
 passing C<readonly> and C<exportname> possibly modified according
to
-how the filter plans to use the plugin.  Typically, the filter passes
-the same values as it received, or passes readonly=true to provide a
+how the filter plans to use the plugin (C<is_tls> is not passed,
+because a filter cannot modify it).  Typically, the filter passes the
+same values as it received, or passes readonly=true to provide a
 writable layer on top of a read-only backend.  However, it is also
 acceptable to attempt write access to the plugin even if this filter
 is readonly, such as when a file system mounted read-only still
@@ -430,7 +431,10 @@ to be replayed for consistency as part of the mounting
process.

 The C<exportname> string is only guaranteed to be available during the
 call.  If the filter needs to use it (other than immediately passing
-it down to the next layer) it must take a copy.
+it down to the next layer) it must take a copy.  The C<exportname> and
+C<is_tls> parameters are provided so that filters do not need to use
+the plugin-only interfaces of C<nbdkit_export_name> and
+C<nbdkit_is_tls>.

 The filter should generally call C<next> as its first step, to
 allocate from the plugin outwards, so that C<.close> running from the
diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
index 9341f282..6237b749 100644
--- a/docs/nbdkit-plugin.pod
+++ b/docs/nbdkit-plugin.pod
@@ -728,10 +728,18 @@ connection to be readonly (even if this flag is false) by
returning
 false from the C<.can_write> callback.  So if your plugin can only
 serve read-only, you can ignore this parameter.

-This callback is called after the NBD handshake has completed, which
-includes TLS authentication (if required).  If the plugin defines a
-C<.preconnect> callback, then it must be called and return with
-success before C<.open> is called.
+If the plugin wants to differentiate the content it served based on
+client input, then this is the spot to use C<nbdkit_export_name()> to
+determine which export the client requested.  See also L</EXPORT NAME>
+below.
+
+This callback is called after the NBD handshake has completed; if the
+server requires TLS authentication, then that has occurred as well.
+But if the server is set up to have optional TLS authentication, you
+may check C<nbdkit_is_tls> to learn whether the client has completed
+TLS authentication.  When running the server in a mode that permits
+but not requires TLS, be careful that you do not allow unauthenticated
+clients to cause a denial of service against authentication.

 If there is an error, C<.open> should call C<nbdkit_error> with an
 error message and return C<NULL>.
@@ -1466,6 +1474,22 @@ client data, be cautious when parsing it.>

 On error, C<nbdkit_error> is called and the call returns C<NULL>.

+=head1 AUTHENTICATION
+
+A server may use C<nbdkit_is_tls> to limit which export names work
+until after a client has completed TLS authentication.  See
+L<nbdkit-tls(1)>.
+
+=head2 C<nbdkit_is_tls>
+
+ int nbdkit_is_tls (void);
+
+Return true if the client has completed TLS authentication, or false
+if the connection is still plaintext.
+
+On error (such as calling this function outside of the context of
+C<.open>), C<nbdkit_error> is called and the call returns
C<-1>.
+
 =head1 PEER NAME

 It is possible to get the address of the client when you are running
diff --git a/docs/nbdkit-tls.pod b/docs/nbdkit-tls.pod
index 628512a9..450e0850 100644
--- a/docs/nbdkit-tls.pod
+++ b/docs/nbdkit-tls.pod
@@ -22,7 +22,10 @@ connect, it will be rejected by the server (in other words,
as if the
 server doesn't support TLS).

 I<--tls=on> means that the client may choose to connect either with or
-without TLS.
+without TLS.  In this mode, a plugin may wish to serve different
+content depending on whether the client has authenticated; the
+function C<nbdkit_is_tls()> can be used during the C<.open>
callback
+for that purpose.

 Because I<--tls=on> is subject to downgrade attacks where a malicious
 proxy pretends not to support TLS in order to force either the client
diff --git a/filters/log/nbdkit-log-filter.pod
b/filters/log/nbdkit-log-filter.pod
index 5f690457..721fc118 100644
--- a/filters/log/nbdkit-log-filter.pod
+++ b/filters/log/nbdkit-log-filter.pod
@@ -74,7 +74,7 @@ before performing a single successful read is:

  2020-08-06 02:07:23.080415 ListExports id=1 readonly=0 default_only=0 ...
  2020-08-06 02:07:23.080502 ...ListExports id=1 exports=[""] return=0
- 2020-08-06 02:07:23.080712 connection=1 Connect export='' size=0x400
write=1 flush=1 rotational=0 trim=1 zero=2 fua=2 extents=1 cache=2 fast_zero=1
+ 2020-08-06 02:07:23.080712 connection=1 Connect export='' tls=0
size=0x400 write=1 flush=1 rotational=0 trim=1 zero=2 fua=2 extents=1 cache=2
fast_zero=1
  2020-08-06 02:07:23.080907 connection=1 Read id=1 offset=0x0 count=0x200 ...
  2020-08-06 02:07:23.080927 connection=1 ...Read id=1 return=0 (Success)
  2020-08-06 02:07:23.081255 connection=1 Disconnect transactions=1
diff --git a/plugins/sh/nbdkit-sh-plugin.pod b/plugins/sh/nbdkit-sh-plugin.pod
index 678116f2..07d90b57 100644
--- a/plugins/sh/nbdkit-sh-plugin.pod
+++ b/plugins/sh/nbdkit-sh-plugin.pod
@@ -320,11 +320,12 @@ as a name and no description.

 =item C<open>

- /path/to/script open <readonly> <exportname>
+ /path/to/script open <readonly> <exportname> <tls>

 The C<readonly> parameter will be C<true> or C<false>.  The
 C<exportname> parameter, if present, is the export name passed to the
-server from the client.
+server from the client.  The C<tls> parameter, if present, will be
+C<true> or C<false> depending on whether the client is using TLS.

 On success this should print the handle (any string) on stdout and
 exit with code C<0>.  If the handle ends with a newline character then
diff --git a/include/nbdkit-filter.h b/include/nbdkit-filter.h
index 08c6abc4..708a1b54 100644
--- a/include/nbdkit-filter.h
+++ b/include/nbdkit-filter.h
@@ -175,7 +175,7 @@ struct nbdkit_filter {
                        struct nbdkit_exports *exports);

   void * (*open) (nbdkit_next_open *next, nbdkit_backend *nxdata,
-                  int readonly, const char *exportname);
+                  int readonly, const char *exportname, int is_tls);
   void (*close) (void *handle);

   int (*prepare) (struct nbdkit_next_ops *next_ops, nbdkit_backend *nxdata,
diff --git a/include/nbdkit-plugin.h b/include/nbdkit-plugin.h
index 86b8565d..e20391b8 100644
--- a/include/nbdkit-plugin.h
+++ b/include/nbdkit-plugin.h
@@ -146,6 +146,7 @@ struct nbdkit_plugin {

 extern void nbdkit_set_error (int err);
 extern const char *nbdkit_export_name (void);
+extern int nbdkit_is_tls (void);

 #define NBDKIT_REGISTER_PLUGIN(plugin)                                  \
   NBDKIT_CXX_LANG_C                                                     \
diff --git a/server/internal.h b/server/internal.h
index f84696ca..d043225a 100644
--- a/server/internal.h
+++ b/server/internal.h
@@ -367,7 +367,8 @@ struct backend {
   int (*preconnect) (struct backend *, int readonly);
   int (*list_exports) (struct backend *, int readonly, int default_only,
                        struct nbdkit_exports *exports);
-  void *(*open) (struct backend *, int readonly, const char *exportname);
+  void *(*open) (struct backend *, int readonly, const char *exportname,
+                 int is_tls);
   int (*prepare) (struct backend *, void *handle, int readonly);
   int (*finalize) (struct backend *, void *handle);
   void (*close) (struct backend *, void *handle);
diff --git a/server/backend.c b/server/backend.c
index 75ca53be..8f4fed9d 100644
--- a/server/backend.c
+++ b/server/backend.c
@@ -186,8 +186,8 @@ backend_open (struct backend *b, int readonly, const char
*exportname)
   GET_CONN;
   struct handle *h = get_handle (conn, b->i);

-  controlpath_debug ("%s: open readonly=%d
exportname=\"%s\"",
-                     b->name, readonly, exportname);
+  controlpath_debug ("%s: open readonly=%d exportname=\"%s\"
tls=%d",
+                     b->name, readonly, exportname, conn->using_tls);

   assert (h->handle == NULL);
   assert ((h->state & HANDLE_OPEN) == 0);
@@ -212,7 +212,7 @@ backend_open (struct backend *b, int readonly, const char
*exportname)
   /* Most filters will call next_open first, resulting in
    * inner-to-outer ordering.
    */
-  h->handle = b->open (b, readonly, exportname);
+  h->handle = b->open (b, readonly, exportname, conn->using_tls);
   controlpath_debug ("%s: open returned handle %p", b->name,
h->handle);

   if (h->handle == NULL) {
diff --git a/server/filters.c b/server/filters.c
index 20518354..90a9a948 100644
--- a/server/filters.c
+++ b/server/filters.c
@@ -250,7 +250,8 @@ filter_list_exports (struct backend *b, int readonly, int
default_only,
 }

 static void *
-filter_open (struct backend *b, int readonly, const char *exportname)
+filter_open (struct backend *b, int readonly, const char *exportname,
+             int is_tls)
 {
   struct backend_filter *f = container_of (b, struct backend_filter, backend);
   void *handle;
@@ -259,7 +260,8 @@ filter_open (struct backend *b, int readonly, const char
*exportname)
    * inner-to-outer ordering.
    */
   if (f->filter.open)
-    handle = f->filter.open (backend_open, b->next, readonly,
exportname);
+    handle = f->filter.open (backend_open, b->next, readonly, exportname,
+                             is_tls);
   else if (backend_open (b->next, readonly, exportname) == -1)
     handle = NULL;
   else
diff --git a/server/nbdkit.syms b/server/nbdkit.syms
index 6cc6ed32..a67669b7 100644
--- a/server/nbdkit.syms
+++ b/server/nbdkit.syms
@@ -53,6 +53,7 @@
     nbdkit_extents_free;
     nbdkit_extents_new;
     nbdkit_get_extent;
+    nbdkit_is_tls;
     nbdkit_nanosleep;
     nbdkit_parse_bool;
     nbdkit_parse_int8_t;
diff --git a/server/plugins.c b/server/plugins.c
index d4364cd2..bc57623a 100644
--- a/server/plugins.c
+++ b/server/plugins.c
@@ -292,7 +292,8 @@ plugin_list_exports (struct backend *b, int readonly, int
default_only,
 }

 static void *
-plugin_open (struct backend *b, int readonly, const char *exportname)
+plugin_open (struct backend *b, int readonly, const char *exportname,
+             int is_tls)
 {
   GET_CONN;
   struct backend_plugin *p = container_of (b, struct backend_plugin, backend);
@@ -302,11 +303,14 @@ plugin_open (struct backend *b, int readonly, const char
*exportname)
   /* Save the exportname since the lifetime of the string passed in
    * here is likely to be brief.  In addition this provides a place
    * for nbdkit_export_name to retrieve it if called from the plugin.
+   * While readonly and the export name can be altered in plugins, the
+   * tls mode cannot.
    *
-   * In API V3 we propose to pass the exportname as an extra parameter
-   * to the (new) plugin.open and deprecate nbdkit_export_name for V3
-   * users.  Even then we will still need to save it in the handle
-   * because of the lifetime issue.
+   * In API V3 we propose to pass the exportname and tls mode as extra
+   * parameters to the (new) plugin.open and deprecate
+   * nbdkit_export_name and nbdkit_is_tls for V3 users.  Even then we
+   * will still need to save the export name in the handle because of
+   * the lifetime issue.
    */
   if (conn->exportname == NULL) {
     conn->exportname = strdup (exportname);
diff --git a/server/public.c b/server/public.c
index c00a5cb6..f682d732 100644
--- a/server/public.c
+++ b/server/public.c
@@ -670,6 +670,22 @@ nbdkit_export_name (void)
   return conn->exportname;
 }

+/* This function will be deprecated for API V3 users.  The preferred
+ * approach will be to get the tls mode from .open().
+ */
+int
+nbdkit_is_tls (void)
+{
+  struct connection *conn = threadlocal_get_conn ();
+
+  if (!conn) {
+    nbdkit_error ("no connection in this thread");
+    return -1;
+  }
+
+  return conn->using_tls;
+}
+
 int
 nbdkit_peer_name (struct sockaddr *addr, socklen_t *addrlen)
 {
diff --git a/plugins/sh/methods.c b/plugins/sh/methods.c
index 9f247524..c59198ca 100644
--- a/plugins/sh/methods.c
+++ b/plugins/sh/methods.c
@@ -341,6 +341,7 @@ sh_open (int readonly)
     { script, method,
       readonly ? "true" : "false",
       nbdkit_export_name () ? : "",
+      nbdkit_is_tls () ? "true" : "false",
       NULL };
   struct sh_handle *h = malloc (sizeof *h);

diff --git a/filters/cow/cow.c b/filters/cow/cow.c
index 0faf2726..51ca64a4 100644
--- a/filters/cow/cow.c
+++ b/filters/cow/cow.c
@@ -94,7 +94,7 @@ cow_config (nbdkit_next_config *next, void *nxdata,

 static void *
 cow_open (nbdkit_next_open *next, void *nxdata,
-          int readonly, const char *exportname)
+          int readonly, const char *exportname, int is_tls)
 {
   /* Always pass readonly=1 to the underlying plugin. */
   if (next (nxdata, 1, exportname) == -1)
diff --git a/filters/exitlast/exitlast.c b/filters/exitlast/exitlast.c
index 378cda75..4c879cc9 100644
--- a/filters/exitlast/exitlast.c
+++ b/filters/exitlast/exitlast.c
@@ -51,7 +51,7 @@ static _Atomic unsigned connections;

 static void *
 exitlast_open (nbdkit_next_open *next, nbdkit_backend *nxdata,
-               int readonly, const char *exportname)
+               int readonly, const char *exportname, int is_tls)
 {
   if (next (nxdata, readonly, exportname) == -1)
     return NULL;
diff --git a/filters/ext2/ext2.c b/filters/ext2/ext2.c
index 72b7ac9f..75ac2c4c 100644
--- a/filters/ext2/ext2.c
+++ b/filters/ext2/ext2.c
@@ -122,7 +122,7 @@ struct handle {
 /* Create the per-connection handle. */
 static void *
 ext2_open (nbdkit_next_open *next, void *nxdata,
-           int readonly, const char *exportname)
+           int readonly, const char *exportname, int is_tls)
 {
   struct handle *h;

diff --git a/filters/gzip/gzip.c b/filters/gzip/gzip.c
index 8323b882..d92e00d9 100644
--- a/filters/gzip/gzip.c
+++ b/filters/gzip/gzip.c
@@ -75,7 +75,7 @@ gzip_thread_model (void)

 static void *
 gzip_open (nbdkit_next_open *next, nbdkit_backend *nxdata,
-           int readonly, const char *exportname)
+           int readonly, const char *exportname, int is_tls)
 {
   /* Always pass readonly=1 to the underlying plugin. */
   if (next (nxdata, 1, exportname) == -1)
diff --git a/filters/limit/limit.c b/filters/limit/limit.c
index 7c4477eb..fb862df7 100644
--- a/filters/limit/limit.c
+++ b/filters/limit/limit.c
@@ -91,7 +91,7 @@ limit_preconnect (nbdkit_next_preconnect *next, nbdkit_backend
*nxdata,

 static void *
 limit_open (nbdkit_next_open *next, nbdkit_backend *nxdata,
-            int readonly, const char *exportname)
+            int readonly, const char *exportname, int is_tls)
 {
   if (next (nxdata, readonly, exportname) == -1)
     return NULL;
diff --git a/filters/log/log.c b/filters/log/log.c
index c89f5001..f8da9ad8 100644
--- a/filters/log/log.c
+++ b/filters/log/log.c
@@ -135,6 +135,7 @@ struct handle {
   uint64_t connection;
   uint64_t id;
   char *exportname;
+  int tls;
 };

 /* Compute the next id number on the current connection. */
@@ -283,7 +284,7 @@ log_list_exports (nbdkit_next_list_exports *next, void
*nxdata,
 /* Open a connection. */
 static void *
 log_open (nbdkit_next_open *next, void *nxdata,
-          int readonly, const char *exportname)
+          int readonly, const char *exportname, int is_tls)
 {
   struct handle *h;

@@ -296,9 +297,9 @@ log_open (nbdkit_next_open *next, void *nxdata,
     return NULL;
   }

-  /* Save the exportname in the handle so we can display it in
-   * log_prepare.  We must take a copy because this string has a short
-   * lifetime.
+  /* Save the exportname and tls state in the handle so we can display
+   * it in log_prepare.  We must take a copy because this string has a
+   * short lifetime.
    */
   h->exportname = strdup (exportname);
   if (h->exportname == NULL) {
@@ -306,6 +307,7 @@ log_open (nbdkit_next_open *next, void *nxdata,
     free (h);
     return NULL;
   }
+  h->tls = is_tls;

   ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock);
   h->connection = ++connections;
@@ -343,9 +345,9 @@ log_prepare (struct nbdkit_next_ops *next_ops, void *nxdata,
void *handle,
       e < 0 || c < 0 || Z < 0)
     return -1;

-  output (h, "Connect", 0, "export='%s' size=0x%"
PRIx64 " write=%d flush=%d "
-          "rotational=%d trim=%d zero=%d fua=%d extents=%d cache=%d "
-          "fast_zero=%d", exportname, size, w, f, r, t, z, F, e, c,
Z);
+  output (h, "Connect", 0, "export='%s' tls=%d
size=0x%" PRIx64 " write=%d "
+          "flush=%d rotational=%d trim=%d zero=%d fua=%d extents=%d
cache=%d "
+          "fast_zero=%d", exportname, h->tls, size, w, f, r, t, z,
F, e, c, Z);
   return 0;
 }

diff --git a/filters/partition/partition.c b/filters/partition/partition.c
index c348664b..849f0cc7 100644
--- a/filters/partition/partition.c
+++ b/filters/partition/partition.c
@@ -88,7 +88,7 @@ struct handle {
 /* Open a connection. */
 static void *
 partition_open (nbdkit_next_open *next, void *nxdata,
-                int readonly, const char *exportname)
+                int readonly, const char *exportname, int is_tls)
 {
   struct handle *h;

diff --git a/filters/rate/rate.c b/filters/rate/rate.c
index 6e99fcc7..32c47fdf 100644
--- a/filters/rate/rate.c
+++ b/filters/rate/rate.c
@@ -163,7 +163,7 @@ rate_get_ready (nbdkit_next_get_ready *next, void *nxdata)
 /* Create the per-connection handle. */
 static void *
 rate_open (nbdkit_next_open *next, void *nxdata,
-           int readonly, const char *exportname)
+           int readonly, const char *exportname, int is_tls)
 {
   struct rate_handle *h;

diff --git a/filters/retry/retry.c b/filters/retry/retry.c
index be334c39..a2e57d77 100644
--- a/filters/retry/retry.c
+++ b/filters/retry/retry.c
@@ -113,7 +113,7 @@ struct retry_handle {

 static void *
 retry_open (nbdkit_next_open *next, void *nxdata,
-            int readonly, const char *exportname)
+            int readonly, const char *exportname, int is_tls)
 {
   struct retry_handle *h;

diff --git a/filters/tar/tar.c b/filters/tar/tar.c
index c04f09dd..ab041153 100644
--- a/filters/tar/tar.c
+++ b/filters/tar/tar.c
@@ -113,7 +113,7 @@ struct handle {

 static void *
 tar_open (nbdkit_next_open *next, nbdkit_backend *nxdata,
-          int readonly, const char *exportname)
+          int readonly, const char *exportname, int is_tls)
 {
   struct handle *h;

diff --git a/filters/truncate/truncate.c b/filters/truncate/truncate.c
index ee384871..00b5d8ce 100644
--- a/filters/truncate/truncate.c
+++ b/filters/truncate/truncate.c
@@ -125,7 +125,7 @@ struct handle {
 /* Open a connection. */
 static void *
 truncate_open (nbdkit_next_open *next, void *nxdata,
-               int readonly, const char *exportname)
+               int readonly, const char *exportname, int is_tls)
 {
   struct handle *h;

diff --git a/filters/xz/xz.c b/filters/xz/xz.c
index 9154dbeb..26cfa959 100644
--- a/filters/xz/xz.c
+++ b/filters/xz/xz.c
@@ -90,7 +90,7 @@ struct xz_handle {
 /* Create the per-connection handle. */
 static void *
 xz_open (nbdkit_next_open *next, void *nxdata,
-         int readonly, const char *exportname)
+         int readonly, const char *exportname, int is_tls)
 {
   struct xz_handle *h;

diff --git a/tests/test-layers-filter.c b/tests/test-layers-filter.c
index 397af575..5c5b3f0f 100644
--- a/tests/test-layers-filter.c
+++ b/tests/test-layers-filter.c
@@ -117,7 +117,7 @@ test_layers_filter_list_exports (nbdkit_next_list_exports
*next, void *nxdata,

 static void *
 test_layers_filter_open (nbdkit_next_open *next, void *nxdata,
-                         int readonly, const char *exportname)
+                         int readonly, const char *exportname, int is_tls)
 {
   struct handle *h = malloc (sizeof *h);

diff --git a/TODO b/TODO
index 46f92443..262327fb 100644
--- a/TODO
+++ b/TODO
@@ -62,11 +62,11 @@ General ideas for improvements
   continue to keep their non-standard handshake while utilizing nbdkit
   to prototype new behaviors in serving the kernel.

-* When using --tls=on (encryption is supported but not required),
-  plugins need to be able to differentiate which content they serve to
-  plaintext clients vs. secure clients.  This may require an
-  alteration of the signature for .list_exports and .open, or a new
-  utility function nbdkit_tls_mode() similar to nbdkit_export_name().
+* The current signature of .list_exports is awkward; it overloads the
+  list response to NBD_OPT_LIST with the resolution of the default
+  export name in .open, and it is missing a tls parameter.  It is
+  probably worth splitting out a new .default_export callback for the
+  latter purpose, as well as fixing the signature for the former.

 * Background thread for filters.  Some filters (readahead, cache and
   proposed scan filter - see below) could be more effective if they
@@ -308,7 +308,6 @@ using ‘#define NBDKIT_API_VERSION <version>’.
   and filters, and the type of the values, and both check and parse
   them for the plugin.

-* Modify open() API so it takes an export name parameter, and maybe a
-  tls mode.
+* Modify open() API so it takes an export name and tls parameter.

 * Change config_help from a variable to a function.
-- 
2.28.0

The next patch wants to add a filter that will prevent DoS attacks
from a plaintext client; to be successful, the filter must guarantee
that nbdkit did not settle on SERIALIZE_CONNECTIONS.  The easiest way
to solve this is to expose the final thread model to .get_ready, which
is after the point where .config_complete may have altered it, and
before any connections are permitted.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 docs/nbdkit-filter.pod          | 9 ++++++++-
 include/nbdkit-filter.h         | 3 ++-
 server/filters.c                | 4 ++--
 filters/extentlist/extentlist.c | 3 ++-
 filters/log/log.c               | 2 +-
 filters/rate/rate.c             | 2 +-
 filters/stats/stats.c           | 2 +-
 tests/test-layers-filter.c      | 2 +-
 8 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
index b6ed5504..32db0938 100644
--- a/docs/nbdkit-filter.pod
+++ b/docs/nbdkit-filter.pod
@@ -298,11 +298,18 @@ with an error message and return C<-1>.

 =head2 C<.get_ready>

- int (*get_ready) (nbdkit_next_get_ready *next, void *nxdata);
+ int (*get_ready) (nbdkit_next_get_ready *next, void *nxdata,
+                   int thread_model);

 This intercepts the plugin C<.get_ready> method and can be used by the
 filter to get ready to serve requests.

+The C<thread_model> parameter informs the filter about the final
+thread model chosen by nbdkit after considering the results of
+C<.thread_model> of all filters in the chain after C<.config>. 
This
+does not need to be passed on to C<next>, as the model can no longer
+be altered at this point.
+
 If there is an error, C<.get_ready> should call C<nbdkit_error>
with
 an error message and return C<-1>.

diff --git a/include/nbdkit-filter.h b/include/nbdkit-filter.h
index 708a1b54..6aba1aec 100644
--- a/include/nbdkit-filter.h
+++ b/include/nbdkit-filter.h
@@ -166,7 +166,8 @@ struct nbdkit_filter {
                           nbdkit_backend *nxdata);
   const char *config_help;
   int (*thread_model) (void);
-  int (*get_ready) (nbdkit_next_get_ready *next, nbdkit_backend *nxdata);
+  int (*get_ready) (nbdkit_next_get_ready *next, nbdkit_backend *nxdata,
+                    int thread_model);
   int (*after_fork) (nbdkit_next_after_fork *next, nbdkit_backend *nxdata);
   int (*preconnect) (nbdkit_next_preconnect *next, nbdkit_backend *nxdata,
                      int readonly);
diff --git a/server/filters.c b/server/filters.c
index 90a9a948..0cfae344 100644
--- a/server/filters.c
+++ b/server/filters.c
@@ -183,10 +183,10 @@ filter_get_ready (struct backend *b)
 {
   struct backend_filter *f = container_of (b, struct backend_filter, backend);

-  debug ("%s: get_ready", b->name);
+  debug ("%s: get_ready thread_model=%d", b->name, thread_model);

   if (f->filter.get_ready) {
-    if (f->filter.get_ready (next_get_ready, b->next) == -1)
+    if (f->filter.get_ready (next_get_ready, b->next, thread_model) ==
-1)
       exit (EXIT_FAILURE);
   }
   else
diff --git a/filters/extentlist/extentlist.c b/filters/extentlist/extentlist.c
index 3005b790..dfb5e808 100644
--- a/filters/extentlist/extentlist.c
+++ b/filters/extentlist/extentlist.c
@@ -260,7 +260,8 @@ parse_extentlist (void)
 }

 static int
-extentlist_get_ready (nbdkit_next_get_ready *next, void *nxdata)
+extentlist_get_ready (nbdkit_next_get_ready *next, void *nxdata,
+                      int thread_model)
 {
   parse_extentlist ();

diff --git a/filters/log/log.c b/filters/log/log.c
index f8da9ad8..6a3a9b14 100644
--- a/filters/log/log.c
+++ b/filters/log/log.c
@@ -100,7 +100,7 @@ log_config_complete (nbdkit_next_config_complete *next, void
*nxdata)

 /* Open the logfile. */
 static int
-log_get_ready (nbdkit_next_get_ready *next, void *nxdata)
+log_get_ready (nbdkit_next_get_ready *next, void *nxdata, int thread_model)
 {
   int fd;

diff --git a/filters/rate/rate.c b/filters/rate/rate.c
index 32c47fdf..325f5657 100644
--- a/filters/rate/rate.c
+++ b/filters/rate/rate.c
@@ -145,7 +145,7 @@ rate_config (nbdkit_next_config *next, void *nxdata,
 }

 static int
-rate_get_ready (nbdkit_next_get_ready *next, void *nxdata)
+rate_get_ready (nbdkit_next_get_ready *next, void *nxdata, int thread_model)
 {
   /* Initialize the global buckets. */
   bucket_init (&read_bucket, rate, BUCKET_CAPACITY);
diff --git a/filters/stats/stats.c b/filters/stats/stats.c
index 688078ec..687dd05b 100644
--- a/filters/stats/stats.c
+++ b/filters/stats/stats.c
@@ -210,7 +210,7 @@ stats_config_complete (nbdkit_next_config_complete *next,
void *nxdata)
 }

 static int
-stats_get_ready (nbdkit_next_get_ready *next, void *nxdata)
+stats_get_ready (nbdkit_next_get_ready *next, void *nxdata, int thread_model)
 {
   int fd;

diff --git a/tests/test-layers-filter.c b/tests/test-layers-filter.c
index 5c5b3f0f..3f295588 100644
--- a/tests/test-layers-filter.c
+++ b/tests/test-layers-filter.c
@@ -84,7 +84,7 @@ test_layers_filter_config_complete
(nbdkit_next_config_complete *next,

 static int
 test_layers_filter_get_ready (nbdkit_next_get_ready *next,
-                              void *nxdata)
+                              void *nxdata, int thread_model)
 {
   DEBUG_FUNCTION;
   return next (nxdata);
-- 
2.28.0

Take advantage of the fact that we can now detect the type of client
during --tls=on in order to provide safe dummy content for plaintext
clients without having to rewrite plugins to do so.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 docs/nbdkit-plugin.pod                      |   4 +-
 docs/nbdkit-tls.pod                         |   5 +-
 filters/tlsdummy/nbdkit-tlsdummy-filter.pod |  72 ++++++
 configure.ac                                |   2 +
 filters/tlsdummy/Makefile.am                |  63 ++++++
 filters/tlsdummy/tlsdummy.c                 | 235 ++++++++++++++++++++
 6 files changed, 379 insertions(+), 2 deletions(-)
 create mode 100644 filters/tlsdummy/nbdkit-tlsdummy-filter.pod
 create mode 100644 filters/tlsdummy/Makefile.am
 create mode 100644 filters/tlsdummy/tlsdummy.c

diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
index 6237b749..43b0f6f9 100644
--- a/docs/nbdkit-plugin.pod
+++ b/docs/nbdkit-plugin.pod
@@ -1478,7 +1478,9 @@ On error, C<nbdkit_error> is called and the call
returns C<NULL>.

 A server may use C<nbdkit_is_tls> to limit which export names work
 until after a client has completed TLS authentication.  See
-L<nbdkit-tls(1)>.
+L<nbdkit-tls(1)>.  It is also possible to use
+L<nbdkit-tlsdummy-filter(1)> to automatically ensure that the plugin
+is only used with authentication.

 =head2 C<nbdkit_is_tls>

diff --git a/docs/nbdkit-tls.pod b/docs/nbdkit-tls.pod
index 450e0850..fd78c21b 100644
--- a/docs/nbdkit-tls.pod
+++ b/docs/nbdkit-tls.pod
@@ -25,7 +25,9 @@ I<--tls=on> means that the client may choose to connect
either with or
 without TLS.  In this mode, a plugin may wish to serve different
 content depending on whether the client has authenticated; the
 function C<nbdkit_is_tls()> can be used during the C<.open>
callback
-for that purpose.
+for that purpose.  Or, you can opt to use L<nbdkit-tlsdummy-filter(1)>
+to provide safe dummy content to plaintext connections, saving the
+plugin content only for secure connections.

 Because I<--tls=on> is subject to downgrade attacks where a malicious
 proxy pretends not to support TLS in order to force either the client
@@ -275,6 +277,7 @@ More information can be found in
L<gnutls_priority_init(3)>.
 =head1 SEE ALSO

 L<nbdkit(1)>,
+L<nbdkit-tlsdummy-filter(1)>,
 L<gnutls_priority_init(3)>,
 L<psktool(1)>,
 L<https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md>,
diff --git a/filters/tlsdummy/nbdkit-tlsdummy-filter.pod
b/filters/tlsdummy/nbdkit-tlsdummy-filter.pod
new file mode 100644
index 00000000..f8eef69f
--- /dev/null
+++ b/filters/tlsdummy/nbdkit-tlsdummy-filter.pod
@@ -0,0 +1,72 @@
+=head1 NAME
+
+nbdkit-tlsdummy-filter - nbdkit TLS protection filter
+
+=head1 SYNOPSIS
+
+ nbdkit --tls=on --filter=tlsdummy plugin [plugin-args...] [tlsreadme=MESSAGE]
+
+=head1 DESCRIPTION
+
+C<nbdkit-tlsdummy-filter> is designed to be used when offering a
+connection that allows but does not require TLS from clients, in order
+to offer safe alternative content to plaintext clients, only exposing
+the underlying plugin to authenticated users.  This may provide a
+nicer failure mode for plaintext clients than the harsher C<nbdkit
+--tls=require>.
+
+When this filter detects a plaintext connection, it ignores the
+client's export name, and provides a single read-only export with 512
+bytes of data and content that defaults to the message "This NBD
+server requires TLS authentication before it will serve useful data."
+
+When using this filter, it is recommended to place this filter first
+in the command line, to reduce the chance that any work done by
+C<.open> in later filters or the plugin can be exploited by plaintext
+connections as a denial of service attack to starve further
+authenticated connections.  Note that this plugin will fail to load if
+the plugin requests the C<SERIALIZE_CONNECTIONS> thread model, as a
+plaintext client holding its connection open indefinitely would be
+such a starvation.
+
+=head1 PARAMETERS
+
+=over 4
+
+=item B<tlsreadme=>MESSAGE
+
+This optional parameter can be used to use C<MESSAGE> as the contents
+of the dummy export exposed to plaintext clients, using trailing NUL
+bytes to round the size up to 512 bytes.
+
+=back
+
+=head1 FILES
+
+=over 4
+
+=item F<$filterdir/nbdkit-tlsdummy-filter.so>
+
+The filter.
+
+Use C<nbdkit --dump-config> to find the location of C<$filterdir>.
+
+=back
+
+=head1 VERSION
+
+C<nbdkit-tlsdummy-filter> first appeared in nbdkit 1.22.
+
+=head1 SEE ALSO
+
+L<nbdkit(1)>,
+L<nbdkit-tls(1)>,
+L<nbdkit-filter(3)>.
+
+=head1 AUTHORS
+
+Eric Blake
+
+=head1 COPYRIGHT
+
+Copyright (C) 2020 Red Hat Inc.
diff --git a/configure.ac b/configure.ac
index 28f4a3cd..34730000 100644
--- a/configure.ac
+++ b/configure.ac
@@ -124,6 +124,7 @@ filters="\
         stats \
         swab \
         tar \
+	tlsdummy \
         truncate \
         xz \
         "
@@ -1165,6 +1166,7 @@ AC_CONFIG_FILES([Makefile
                  filters/stats/Makefile
                  filters/swab/Makefile
                  filters/tar/Makefile
+                 filters/tlsdummy/Makefile
                  filters/truncate/Makefile
                  filters/xz/Makefile
                  fuzzing/Makefile
diff --git a/filters/tlsdummy/Makefile.am b/filters/tlsdummy/Makefile.am
new file mode 100644
index 00000000..a1577511
--- /dev/null
+++ b/filters/tlsdummy/Makefile.am
@@ -0,0 +1,63 @@
+# nbdkit
+# Copyright (C) 2020 Red Hat Inc.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+include $(top_srcdir)/common-rules.mk
+
+EXTRA_DIST = nbdkit-tlsdummy-filter.pod
+
+filter_LTLIBRARIES = nbdkit-tlsdummy-filter.la
+
+nbdkit_tlsdummy_filter_la_SOURCES = \
+	tlsdummy.c \
+	$(top_srcdir)/include/nbdkit-filter.h \
+	$(NULL)
+
+nbdkit_tlsdummy_filter_la_CPPFLAGS = \
+	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/include \
+	$(NULL)
+nbdkit_tlsdummy_filter_la_CFLAGS = $(WARNINGS_CFLAGS)
+nbdkit_tlsdummy_filter_la_LDFLAGS = \
+	-module -avoid-version -shared $(SHARED_LDFLAGS) \
+	-Wl,--version-script=$(top_srcdir)/filters/filters.syms \
+	$(NULL)
+
+if HAVE_POD
+
+man_MANS = nbdkit-tlsdummy-filter.1
+CLEANFILES += $(man_MANS)
+
+nbdkit-tlsdummy-filter.1: nbdkit-tlsdummy-filter.pod
+	$(PODWRAPPER) --section=1 --man $@ \
+	    --html $(top_builddir)/html/$@.html \
+	    $<
+
+endif HAVE_POD
diff --git a/filters/tlsdummy/tlsdummy.c b/filters/tlsdummy/tlsdummy.c
new file mode 100644
index 00000000..6e1a7414
--- /dev/null
+++ b/filters/tlsdummy/tlsdummy.c
@@ -0,0 +1,235 @@
+/* nbdkit
+ * Copyright (C) 2020 Red Hat Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <config.h>
+
+#include <string.h>
+#include <assert.h>
+
+#include <nbdkit-filter.h>
+
+/* Needed to shut up newer gcc about use of strncpy on our message buffer */
+#if __GNUC__ >= 8
+#define NONSTRING __attribute__ ((nonstring))
+#else
+#define NONSTRING
+#endif
+
+static char message[512] NONSTRING = "This NBD server requires TLS "
+  "authentication before it will serve useful data.\n";
+
+/* Called for each key=value passed on the command line. */
+static int
+tlsdummy_config (nbdkit_next_config *next, void *nxdata,
+                 const char *key, const char *value)
+{
+  if (strcmp (key, "tlsreadme") == 0) {
+    strncpy (message, value, sizeof message); /* Yes, we really mean strncpy */
+    return 0;
+  }
+  return next (nxdata, key, value);
+}
+
+#define tlsdummy_config_help \
+  "tlsreadme=<MESSAGE>  Alternative contents for the plaintext dummy
export.\n"
+
+int
+tlsdummy_get_ready (nbdkit_next_get_ready *next, void *nxdata,
+                    int thread_model)
+{
+  if (thread_model == NBDKIT_THREAD_MODEL_SERIALIZE_CONNECTIONS) {
+    nbdkit_error ("the tlsdummy filter requires parallel connection
support");
+    return -1;
+  }
+  return next (nxdata);
+}
+
+/* TODO: init_exports needs is_tls parameter */
+
+/* Helper for determining if this connection is insecure.  This works
+ * because we can treat all handles on a binary basis: secure or
+ * insecure, which lets .open get away without allocating a more
+ * complex handle.
+ */
+#define NOT_TLS (handle == &message)
+
+static void *
+tlsdummy_open (nbdkit_next_open *next, void *nxdata, int readonly,
+               const char *exportname, int is_tls)
+{
+  /* Bummer - we really do NOT want to call next() when insecure,
+   * because we don't know how long it will take.  But that requires a
+   * fix to nbdkit; right now, it asserts if we skip this :(
+   */
+  if (next (nxdata, readonly, exportname) == -1)
+    return NULL;
+  if (!is_tls)
+    return &message; /* See NOT_TLS for this choice of handle */
+  return NBDKIT_HANDLE_NOT_NEEDED;
+}
+
+/* When insecure, override ALL plugin .can_FOO to avoid an information leak */
+
+static int64_t
+tlsdummy_get_size (struct nbdkit_next_ops *next_ops, void *nxdata,
+                   void *handle)
+{
+  if (NOT_TLS)
+    return sizeof message;
+  return next_ops->get_size (nxdata);
+}
+
+static int
+tlsdummy_can_write (struct nbdkit_next_ops *next_ops, void *nxdata,
+                    void *handle)
+{
+  if (NOT_TLS)
+    return 0;
+  return next_ops->can_write (nxdata);
+}
+
+static int
+tlsdummy_can_flush (struct nbdkit_next_ops *next_ops, void *nxdata,
+                    void *handle)
+{
+  if (NOT_TLS)
+    return 0;
+  return next_ops->can_flush (nxdata);
+}
+
+static int
+tlsdummy_is_rotational (struct nbdkit_next_ops *next_ops, void *nxdata,
+                        void *handle)
+{
+  if (NOT_TLS)
+    return 0;
+  return next_ops->is_rotational (nxdata);
+}
+
+static int
+tlsdummy_can_trim (struct nbdkit_next_ops *next_ops, void *nxdata,
+                   void *handle)
+{
+  if (NOT_TLS)
+    return 0;
+  return next_ops->can_trim (nxdata);
+}
+
+static int
+tlsdummy_can_zero (struct nbdkit_next_ops *next_ops, void *nxdata,
+                   void *handle)
+{
+  if (NOT_TLS)
+    return NBDKIT_ZERO_NONE;
+  return next_ops->can_zero (nxdata);
+}
+
+static int
+tlsdummy_can_fast_zero (struct nbdkit_next_ops *next_ops, void *nxdata,
+                        void *handle)
+{
+  if (NOT_TLS)
+    return 0;
+  return next_ops->can_fast_zero (nxdata);
+}
+
+static int
+tlsdummy_can_extents (struct nbdkit_next_ops *next_ops, void *nxdata,
+                      void *handle)
+{
+  if (NOT_TLS)
+    return 0;
+  return next_ops->can_extents (nxdata);
+}
+
+static int
+tlsdummy_can_fua (struct nbdkit_next_ops *next_ops, void *nxdata,
+                  void *handle)
+{
+  if (NOT_TLS)
+    return NBDKIT_FUA_NONE;
+  return next_ops->can_fua (nxdata);
+}
+
+static int
+tlsdummy_can_multi_conn (struct nbdkit_next_ops *next_ops, void *nxdata,
+                         void *handle)
+{
+  if (NOT_TLS)
+    return 0;
+  return next_ops->can_multi_conn (nxdata);
+}
+
+static int
+tlsdummy_can_cache (struct nbdkit_next_ops *next_ops, void *nxdata,
+                    void *handle)
+{
+  if (NOT_TLS)
+    return NBDKIT_CACHE_NONE;
+  return next_ops->can_cache (nxdata);
+}
+
+static int
+tlsdummy_pread (struct nbdkit_next_ops *next_ops, void *nxdata,
+                void *handle, void *b, uint32_t count, uint64_t offs,
+                uint32_t flags, int *err)
+{
+  if (NOT_TLS) {
+    memcpy (b, message + offs, count);
+    return 0;
+  }
+  return next_ops->pread (nxdata, b, count, offs, flags, err);
+}
+
+static struct nbdkit_filter filter = {
+  .name              = "tlsdummy",
+  .longname          = "nbdkit tlsdummy filter",
+  .config            = tlsdummy_config,
+  .config_help       = tlsdummy_config_help,
+  .get_ready         = tlsdummy_get_ready,
+  /* XXX .init_exports needs is_tls parameter */
+  .open              = tlsdummy_open,
+  .get_size          = tlsdummy_get_size,
+  .can_write         = tlsdummy_can_write,
+  .can_flush         = tlsdummy_can_flush,
+  .is_rotational     = tlsdummy_is_rotational,
+  .can_trim          = tlsdummy_can_trim,
+  .can_zero          = tlsdummy_can_zero,
+  .can_fast_zero     = tlsdummy_can_fast_zero,
+  .can_extents       = tlsdummy_can_extents,
+  .can_fua           = tlsdummy_can_fua,
+  .can_multi_conn    = tlsdummy_can_multi_conn,
+  .can_cache         = tlsdummy_can_cache,
+  .pread             = tlsdummy_pread,
+};
+
+NBDKIT_REGISTER_FILTER(filter)
-- 
2.28.0

On 8/7/20 5:00 PM, Eric Blake wrote:
> Take advantage of the fact that we can now detect the type of client
> during --tls=on in order to provide safe dummy content for plaintext
> clients without having to rewrite plugins to do so.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
I got a test working (although it still shows that we are bit awkward 
until nbdkit makes it easier for filters to skip calling next_open when 
it wants to)

diff --git c/tests/Makefile.am i/tests/Makefile.am
index b5ef96a7..dd756723 100644
--- c/tests/Makefile.am
+++ i/tests/Makefile.am
@@ -1501,6 +1501,10 @@ EXTRA_DIST += \
  	test-truncate-extents.sh \
  	$(NULL)

+# tlsdummy filter test.
+TESTS += test-tlsdummy.sh
+EXTRA_DIST += test-tlsdummy.sh
+
  # xz filter test.
  LIBGUESTFS_TESTS += test-xz

diff --git c/tests/test-tlsdummy.sh i/tests/test-tlsdummy.sh
new file mode 100755
index 00000000..40c29602
--- /dev/null
+++ i/tests/test-tlsdummy.sh
@@ -0,0 +1,98 @@
+#!/usr/bin/env bash
+# nbdkit
+# Copyright (C) 2019-2020 Red Hat Inc.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+source ./functions.sh
+set -e
+set -x
+
+requires nbdsh -c 'exit (not h.supports_tls ())'
+
+# Does the nbdkit binary support TLS?
+if ! nbdkit --dump-config | grep -sq tls=yes; then
+    echo "$0: nbdkit built without TLS support"
+    exit 77
+fi
+
+# Did we create the PSK keys file?
+# Probably 'certtool' is missing.
+if [ ! -s keys.psk ]; then
+    echo "$0: PSK keys file was not created by the test harness"
+    exit 77
+fi
+
+export sock=`mktemp -u`
+pid="tlsdummy.pid"
+
+files="$sock $pid"
+rm -f $files
+cleanup_fn rm -f $files
+
+# Run dual-mode server
+start_nbdkit -P $pid -U $sock --tls=on --tls-psk=keys.psk \
+    --filter=tlsdummy sh - tlsreadme=$'dummy\n' <<\EOF
+if test "$2" = insecure; then
+    echo 'EINVAL insecure handle' 2>&1; exit 1
+fi
+case $1 in
+  list_exports) echo NAMES; echo hello; echo world ;;
+  open) if test "$4" != true; then
+      # nbdkit needs a fix to let tlsdummy skip .open when insecure...
+      # echo 'EINVAL unexpected tls mode' 2>&1; exit 1
+      echo insecure; exit 0
+    fi
+    echo $3 ;;
+  get_size) echo 6 ;;
+  pread) echo "$2" | dd skip=$4 count=$3 iflag=skip_bytes,count_bytes
;;
+  *) exit 2;
+esac
+EOF
+
+# Plaintext client sees only dummy volume
+nbdsh -c '
+import os
+h.set_export_name ("hello")
+h.connect_unix (os.environ["sock"])
+assert h.get_size () == 512
+assert h.pread (5, 0) == b"dummy"
+'
+
+# Encrypted client sees desired volumes
+nbdsh -c '
+import os
+h.set_export_name ("hello")
+h.set_tls (nbd.TLS_REQUIRE)
+h.set_tls_psk_file ("keys.psk")
+h.set_tls_username ("qemu")
+h.connect_unix (os.environ["sock"])
+assert h.get_size () == 6
+assert h.pread (5, 0) == b"hello"
+'


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Largely a mechanical patch, and the changes at the beginning adding
the new API and updating documentation were fine, so ACK.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

On Fri, Aug 07, 2020 at 05:00:52PM -0500, Eric Blake
wrote:
> The next patch wants to add a filter that will prevent DoS attacks
> from a plaintext client; to be successful, the filter must guarantee
> that nbdkit did not settle on SERIALIZE_CONNECTIONS.  The easiest way
> to solve this is to expose the final thread model to .get_ready, which
> is after the point where .config_complete may have altered it, and
> before any connections are permitted.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  docs/nbdkit-filter.pod          | 9 ++++++++-
>  include/nbdkit-filter.h         | 3 ++-
>  server/filters.c                | 4 ++--
>  filters/extentlist/extentlist.c | 3 ++-
>  filters/log/log.c               | 2 +-
>  filters/rate/rate.c             | 2 +-
>  filters/stats/stats.c           | 2 +-
>  tests/test-layers-filter.c      | 2 +-
>  8 files changed, 18 insertions(+), 9 deletions(-)
This is fine.  Is this something that we would also with to add to
plugin->get_ready in V3 API?  If so it would be a good idea to add
this to TODO.

ACK

Rich.
> diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
> index b6ed5504..32db0938 100644
> --- a/docs/nbdkit-filter.pod
> +++ b/docs/nbdkit-filter.pod
> @@ -298,11 +298,18 @@ with an error message and return C<-1>.
> 
>  =head2 C<.get_ready>
> 
> - int (*get_ready) (nbdkit_next_get_ready *next, void *nxdata);
> + int (*get_ready) (nbdkit_next_get_ready *next, void *nxdata,
> +                   int thread_model);
> 
>  This intercepts the plugin C<.get_ready> method and can be used by
the
>  filter to get ready to serve requests.
> 
> +The C<thread_model> parameter informs the filter about the final
> +thread model chosen by nbdkit after considering the results of
> +C<.thread_model> of all filters in the chain after C<.config>.
This
> +does not need to be passed on to C<next>, as the model can no longer
> +be altered at this point.
> +
>  If there is an error, C<.get_ready> should call
C<nbdkit_error> with
>  an error message and return C<-1>.
> 
> diff --git a/include/nbdkit-filter.h b/include/nbdkit-filter.h
> index 708a1b54..6aba1aec 100644
> --- a/include/nbdkit-filter.h
> +++ b/include/nbdkit-filter.h
> @@ -166,7 +166,8 @@ struct nbdkit_filter {
>                            nbdkit_backend *nxdata);
>    const char *config_help;
>    int (*thread_model) (void);
> -  int (*get_ready) (nbdkit_next_get_ready *next, nbdkit_backend *nxdata);
> +  int (*get_ready) (nbdkit_next_get_ready *next, nbdkit_backend *nxdata,
> +                    int thread_model);
>    int (*after_fork) (nbdkit_next_after_fork *next, nbdkit_backend
*nxdata);
>    int (*preconnect) (nbdkit_next_preconnect *next, nbdkit_backend *nxdata,
>                       int readonly);
> diff --git a/server/filters.c b/server/filters.c
> index 90a9a948..0cfae344 100644
> --- a/server/filters.c
> +++ b/server/filters.c
> @@ -183,10 +183,10 @@ filter_get_ready (struct backend *b)
>  {
>    struct backend_filter *f = container_of (b, struct backend_filter,
backend);
> 
> -  debug ("%s: get_ready", b->name);
> +  debug ("%s: get_ready thread_model=%d", b->name,
thread_model);
> 
>    if (f->filter.get_ready) {
> -    if (f->filter.get_ready (next_get_ready, b->next) == -1)
> +    if (f->filter.get_ready (next_get_ready, b->next, thread_model)
== -1)
>        exit (EXIT_FAILURE);
>    }
>    else
> diff --git a/filters/extentlist/extentlist.c
b/filters/extentlist/extentlist.c
> index 3005b790..dfb5e808 100644
> --- a/filters/extentlist/extentlist.c
> +++ b/filters/extentlist/extentlist.c
> @@ -260,7 +260,8 @@ parse_extentlist (void)
>  }
> 
>  static int
> -extentlist_get_ready (nbdkit_next_get_ready *next, void *nxdata)
> +extentlist_get_ready (nbdkit_next_get_ready *next, void *nxdata,
> +                      int thread_model)
>  {
>    parse_extentlist ();
> 
> diff --git a/filters/log/log.c b/filters/log/log.c
> index f8da9ad8..6a3a9b14 100644
> --- a/filters/log/log.c
> +++ b/filters/log/log.c
> @@ -100,7 +100,7 @@ log_config_complete (nbdkit_next_config_complete *next,
void *nxdata)
> 
>  /* Open the logfile. */
>  static int
> -log_get_ready (nbdkit_next_get_ready *next, void *nxdata)
> +log_get_ready (nbdkit_next_get_ready *next, void *nxdata, int
thread_model)
>  {
>    int fd;
> 
> diff --git a/filters/rate/rate.c b/filters/rate/rate.c
> index 32c47fdf..325f5657 100644
> --- a/filters/rate/rate.c
> +++ b/filters/rate/rate.c
> @@ -145,7 +145,7 @@ rate_config (nbdkit_next_config *next, void *nxdata,
>  }
> 
>  static int
> -rate_get_ready (nbdkit_next_get_ready *next, void *nxdata)
> +rate_get_ready (nbdkit_next_get_ready *next, void *nxdata, int
thread_model)
>  {
>    /* Initialize the global buckets. */
>    bucket_init (&read_bucket, rate, BUCKET_CAPACITY);
> diff --git a/filters/stats/stats.c b/filters/stats/stats.c
> index 688078ec..687dd05b 100644
> --- a/filters/stats/stats.c
> +++ b/filters/stats/stats.c
> @@ -210,7 +210,7 @@ stats_config_complete (nbdkit_next_config_complete
*next, void *nxdata)
>  }
> 
>  static int
> -stats_get_ready (nbdkit_next_get_ready *next, void *nxdata)
> +stats_get_ready (nbdkit_next_get_ready *next, void *nxdata, int
thread_model)
>  {
>    int fd;
> 
> diff --git a/tests/test-layers-filter.c b/tests/test-layers-filter.c
> index 5c5b3f0f..3f295588 100644
> --- a/tests/test-layers-filter.c
> +++ b/tests/test-layers-filter.c
> @@ -84,7 +84,7 @@ test_layers_filter_config_complete
(nbdkit_next_config_complete *next,
> 
>  static int
>  test_layers_filter_get_ready (nbdkit_next_get_ready *next,
> -                              void *nxdata)
> +                              void *nxdata, int thread_model)
>  {
>    DEBUG_FUNCTION;
>    return next (nxdata);
> -- 
> 2.28.0
> 
> _______________________________________________
> Libguestfs mailing list
> Libguestfs@redhat.com
> https://www.redhat.com/mailman/listinfo/libguestfs
-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

On 8/7/20 5:00 PM, Eric Blake wrote:
> Take advantage of the fact that we can now detect the type of client
> during --tls=on in order to provide safe dummy content for plaintext
> clients without having to rewrite plugins to do so.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
After playing with this a bit more, I'm thinking:

nbdkit --tls=on --filter=tls-fallback PLUGIN ...

reads better than tlsdummy.  Renaming the filter is not too difficult of 
a search-and-replace, so I'll go ahead and make that change before 
pushing this.
>   docs/nbdkit-plugin.pod                      |   4 +-
>   docs/nbdkit-tls.pod                         |   5 +-
>   filters/tlsdummy/nbdkit-tlsdummy-filter.pod |  72 ++++++
>   configure.ac                                |   2 +
>   filters/tlsdummy/Makefile.am                |  63 ++++++
>   filters/tlsdummy/tlsdummy.c                 | 235 ++++++++++++++++++++
>   6 files changed, 379 insertions(+), 2 deletions(-)
>   create mode 100644 filters/tlsdummy/nbdkit-tlsdummy-filter.pod
>   create mode 100644 filters/tlsdummy/Makefile.am
>   create mode 100644 filters/tlsdummy/tlsdummy.c
> 
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org