[PATCH v2] elf: Avoid re-initializing already allocated TLS in dlopen (bug 31717)

[PATCH v2] elf: Avoid re-initializing already allocated TLS in dlopen (bug 31717)

[Florian Weimer]

The old code used l_init_called as an indicator for whether TLS
initialization was complete.  However, it is possible that
TLS for an object is initialized, written to, and then dlopen
for this object is called again, and l_init_called is not true at
this point.  Previously, this resulted in TLS being initialized
twice, discarding any interim writes (technically introducing a
use-after-free bug even).

This commit introduces an explicit per-object flag, l_tls_in_slotinfo.
It indicates whether _dl_add_to_slotinfo has been called for this
object.  This flag is used to avoid double-initialization of TLS.
In update_tls_slotinfo, the first_static_tls micro-optimization
is removed because preserving the initalization flag for subsequent
use by the second loop for static TLS is a bit complicated, and
another per-object flag does not seem to be worth it.  Furthermore,
the l_init_called flag is dropped from the second loop (for static
TLS initialization) because l_need_tls_init on its own prevents
double-initialization.

The remaining l_init_called usage in resize_scopes and update_scopes
is just an optimization due to the use of scope_has_map, so it is
not changed in this commit.

Only the dlopen-based test case, elf/tst-dlopen-tlsreinit, fails
before the fix.  The other two (elf/tst-dlopen-tlsreinit2 for
direct linking, elf/tst-dlopen-tlsreinit3 for direct linking
with auditing) are only added for completeness.

Reported-by: Ben Woodard <woodard@redhat.com>

---
v2: Improve compatibility with linkers that default to --as-needed.

 elf/Makefile                   | 27 ++++++++++++
 elf/dl-open.c                  | 35 +++------------
 elf/dl-tls.c                   |  8 +++-
 elf/rtld.c                     |  2 +-
 elf/tst-dlopen-tlsreinit.c     | 36 ++++++++++++++++
 elf/tst-dlopen-tlsreinit2.c    | 35 +++++++++++++++
 elf/tst-dlopen-tlsreinit3.c    |  2 +
 elf/tst-dlopen-tlsreinitmod1.c | 20 +++++++++
 elf/tst-dlopen-tlsreinitmod2.c | 30 +++++++++++++
 elf/tst-dlopen-tlsreinitmod3.c | 96 ++++++++++++++++++++++++++++++++++++++++++
 include/link.h                 |  1 +
 sysdeps/generic/ldsodefs.h     | 21 ++++++---
 12 files changed, 275 insertions(+), 38 deletions(-)

diff --git a/elf/Makefile b/elf/Makefile
index 280e777c19..4cecb4f382 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -416,6 +416,9 @@ tests += \
   tst-dlmopen4 \
   tst-dlopen-self \
   tst-dlopen-tlsmodid \
+  tst-dlopen-tlsreinit \
+  tst-dlopen-tlsreinit2 \
+  tst-dlopen-tlsreinit3 \
   tst-dlopenfail \
   tst-dlopenfail-2 \
   tst-dlopenrpath \
@@ -846,6 +849,9 @@ modules-names += \
   tst-dlmopen-twice-mod1 \
   tst-dlmopen-twice-mod2 \
   tst-dlmopen1mod \
+  tst-dlopen-tlsreinitmod1 \
+  tst-dlopen-tlsreinitmod2 \
+  tst-dlopen-tlsreinitmod3 \
   tst-dlopenfaillinkmod \
   tst-dlopenfailmod1 \
   tst-dlopenfailmod2 \
@@ -3083,3 +3089,24 @@ CFLAGS-tst-gnu2-tls2mod0.c += -mtls-dialect=$(have-mtls-descriptor)
 CFLAGS-tst-gnu2-tls2mod1.c += -mtls-dialect=$(have-mtls-descriptor)
 CFLAGS-tst-gnu2-tls2mod2.c += -mtls-dialect=$(have-mtls-descriptor)
 endif
+
+# Order matters here.  The test needs the constructor for
+# tst-dlopen-tlsreinitmod2.so to be called first.
+LDFLAGS-tst-dlopen-tlsreinitmod1.so = -Wl,--no-as-needed
+$(objpfx)tst-dlopen-tlsreinitmod1.so: \
+  $(objpfx)tst-dlopen-tlsreinitmod3.so $(objpfx)tst-dlopen-tlsreinitmod2.so
+LDFLAGS-tst-dlopen-tlsreinit2 = -Wl,--no-as-needed
+$(objpfx)tst-dlopen-tlsreinit2: \
+  $(objpfx)tst-dlopen-tlsreinitmod3.so $(objpfx)tst-dlopen-tlsreinitmod2.so
+LDFLAGS-tst-dlopen-tlsreinit3 = -Wl,--no-as-needed
+$(objpfx)tst-dlopen-tlsreinit3: \
+  $(objpfx)tst-dlopen-tlsreinitmod3.so $(objpfx)tst-dlopen-tlsreinitmod2.so
+# tst-dlopen-tlsreinitmod2.so is underlinked and refers to
+# tst-dlopen-tlsreinitmod3.so.  The dependency is provided via
+# $(objpfx)tst-dlopen-tlsreinitmod1.so.
+tst-dlopen-tlsreinitmod2.so-no-z-defs = yes
+$(objpfx)tst-dlopen-tlsreinit.out: $(objpfx)tst-dlopen-tlsreinitmod1.so \
+  $(objpfx)tst-dlopen-tlsreinitmod2.so $(objpfx)tst-dlopen-tlsreinitmod3.so
+$(objpfx)tst-dlopen-tlsreinit3.out: $(objpfx)tst-auditmod1.so
+# Reuse an audit module which provides ample debug logging.
+tst-dlopen-tlsreinit3-ENV = LD_AUDIT=$(objpfx)tst-auditmod1.so
diff --git a/elf/dl-open.c b/elf/dl-open.c
index c378da16c0..8556e7bd2f 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -363,17 +363,8 @@ resize_tls_slotinfo (struct link_map *new)
 {
   bool any_tls = false;
   for (unsigned int i = 0; i < new->l_searchlist.r_nlist; ++i)
-    {
-      struct link_map *imap = new->l_searchlist.r_list[i];
-
-      /* Only add TLS memory if this object is loaded now and
-	 therefore is not yet initialized.  */
-      if (! imap->l_init_called && imap->l_tls_blocksize > 0)
-	{
-	  _dl_add_to_slotinfo (imap, false);
-	  any_tls = true;
-	}
-    }
+    if (_dl_add_to_slotinfo (new->l_searchlist.r_list[i], false))
+      any_tls = true;
   return any_tls;
 }
 
@@ -383,22 +374,8 @@ resize_tls_slotinfo (struct link_map *new)
 static void
 update_tls_slotinfo (struct link_map *new)
 {
-  unsigned int first_static_tls = new->l_searchlist.r_nlist;
   for (unsigned int i = 0; i < new->l_searchlist.r_nlist; ++i)
-    {
-      struct link_map *imap = new->l_searchlist.r_list[i];
-
-      /* Only add TLS memory if this object is loaded now and
-	 therefore is not yet initialized.  */
-      if (! imap->l_init_called && imap->l_tls_blocksize > 0)
-	{
-	  _dl_add_to_slotinfo (imap, true);
-
-	  if (imap->l_need_tls_init
-	      && first_static_tls == new->l_searchlist.r_nlist)
-	    first_static_tls = i;
-	}
-    }
+    _dl_add_to_slotinfo (new->l_searchlist.r_list[i], true);
 
   size_t newgen = GL(dl_tls_generation) + 1;
   if (__glibc_unlikely (newgen == 0))
@@ -410,13 +387,11 @@ TLS generation counter wrapped!  Please report this."));
   /* We need a second pass for static tls data, because
      _dl_update_slotinfo must not be run while calls to
      _dl_add_to_slotinfo are still pending.  */
-  for (unsigned int i = first_static_tls; i < new->l_searchlist.r_nlist; ++i)
+  for (unsigned int i = 0; i < new->l_searchlist.r_nlist; ++i)
     {
       struct link_map *imap = new->l_searchlist.r_list[i];
 
-      if (imap->l_need_tls_init
-	  && ! imap->l_init_called
-	  && imap->l_tls_blocksize > 0)
+      if (imap->l_need_tls_init && imap->l_tls_blocksize > 0)
 	{
 	  /* For static TLS we have to allocate the memory here and
 	     now, but we can delay updating the DTV.  */
diff --git a/elf/dl-tls.c b/elf/dl-tls.c
index 7b3dd9ab60..4d55ddd6c9 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -1012,9 +1012,12 @@ _dl_tls_get_addr_soft (struct link_map *l)
 }
 
 
-void
+bool
 _dl_add_to_slotinfo (struct link_map *l, bool do_add)
 {
+  if (l->l_tls_blocksize == 0 || l->l_tls_in_slotinfo)
+    return false;
+
   /* Now that we know the object is loaded successfully add
      modules containing TLS data to the dtv info table.  We
      might have to increase its size.  */
@@ -1068,7 +1071,10 @@ cannot create TLS data structures"));
       atomic_store_relaxed (&listp->slotinfo[idx].map, l);
       atomic_store_relaxed (&listp->slotinfo[idx].gen,
 			    GL(dl_tls_generation) + 1);
+      l->l_tls_in_slotinfo = true;
     }
+
+  return true;
 }
 
 #if PTHREAD_IN_LIBC
diff --git a/elf/rtld.c b/elf/rtld.c
index e9525ea987..0b0fbcdbfb 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -2313,7 +2313,7 @@ dl_main (const ElfW(Phdr) *phdr,
 			       consider_profiling);
 
 	/* Add object to slot information data if necessasy.  */
-	if (l->l_tls_blocksize != 0 && __rtld_tls_init_tp_called)
+	if (__rtld_tls_init_tp_called)
 	  _dl_add_to_slotinfo (l, true);
       }
   }
diff --git a/elf/tst-dlopen-tlsreinit.c b/elf/tst-dlopen-tlsreinit.c
new file mode 100644
index 0000000000..7b8611fbe3
--- /dev/null
+++ b/elf/tst-dlopen-tlsreinit.c
@@ -0,0 +1,36 @@
+/* Test that dlopen preserves already accessed TLS (bug 31717).
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdbool.h>
+#include <support/check.h>
+#include <support/xdlfcn.h>
+
+static int
+do_test (void)
+{
+  void *handle = xdlopen ("tst-dlopen-tlsreinitmod1.so", RTLD_NOW);
+
+  bool *tlsreinitmod3_tested = xdlsym (handle, "tlsreinitmod3_tested");
+  TEST_VERIFY (*tlsreinitmod3_tested);
+
+  xdlclose (handle);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-dlopen-tlsreinit2.c b/elf/tst-dlopen-tlsreinit2.c
new file mode 100644
index 0000000000..7d32c35e4f
--- /dev/null
+++ b/elf/tst-dlopen-tlsreinit2.c
@@ -0,0 +1,35 @@
+/* Test that dlopen preserves already accessed TLS (bug 31717).
+   Variant with initially-linked modules.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdbool.h>
+#include <support/check.h>
+#include <support/xdlfcn.h>
+
+
+static int
+do_test (void)
+{
+  /* Defined in tst-dlopen-tlsreinitmod3.so.  */
+  extern bool tlsreinitmod3_tested;
+  TEST_VERIFY (tlsreinitmod3_tested);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-dlopen-tlsreinit3.c b/elf/tst-dlopen-tlsreinit3.c
new file mode 100644
index 0000000000..344c9211ab
--- /dev/null
+++ b/elf/tst-dlopen-tlsreinit3.c
@@ -0,0 +1,2 @@
+/* Same code, but run with LD_AUDIT=tst-auditmod1.so.  */
+#include "tst-dlopen-tlsreinit2.c"
diff --git a/elf/tst-dlopen-tlsreinitmod1.c b/elf/tst-dlopen-tlsreinitmod1.c
new file mode 100644
index 0000000000..354cc3de51
--- /dev/null
+++ b/elf/tst-dlopen-tlsreinitmod1.c
@@ -0,0 +1,20 @@
+/* Test that dlopen preserves already accessed TLS (bug 31717), module 1.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* This module triggers loading of tst-dlopen-tlsreinitmod2.so and
+   tst-dlopen-tlsreinitmod3.so.  */
diff --git a/elf/tst-dlopen-tlsreinitmod2.c b/elf/tst-dlopen-tlsreinitmod2.c
new file mode 100644
index 0000000000..677e69bd35
--- /dev/null
+++ b/elf/tst-dlopen-tlsreinitmod2.c
@@ -0,0 +1,30 @@
+/* Test that dlopen preserves already accessed TLS (bug 31717), module 2.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+
+/* Defined in tst-dlopen-tlsreinitmod3.so.  This an underlinked symbol
+   dependency.  */
+extern void call_tlsreinitmod3 (void);
+
+static void __attribute__ ((constructor))
+tlsreinitmod2_init (void)
+{
+  puts ("info: constructor of tst-dlopen-tlsreinitmod2.so invoked");
+  call_tlsreinitmod3 ();
+}
diff --git a/elf/tst-dlopen-tlsreinitmod3.c b/elf/tst-dlopen-tlsreinitmod3.c
new file mode 100644
index 0000000000..1a5fda0a14
--- /dev/null
+++ b/elf/tst-dlopen-tlsreinitmod3.c
@@ -0,0 +1,96 @@
+/* Test that dlopen preserves already accessed TLS (bug 31717), module 3.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <dlfcn.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <unistd.h>
+
+/* Used to verify from the main program that the test ran.  */
+bool tlsreinitmod3_tested;
+
+/* This TLS variable must not revert back to the initial state after
+   dlopen.  */
+static __thread int tlsreinitmod3_state = 1;
+
+/* Set from the ELF constructor during dlopen.  */
+static bool tlsreinitmod3_constructed;
+
+/* Second half of test, behind a compiler barrier.  */
+void call_tlsreinitmod3_tail (void *self) __attribute__ ((weak));
+
+/* Called from tst-dlopen-tlsreinitmod2.so.  */
+void
+call_tlsreinitmod3 (void)
+{
+  printf ("info: call_tlsreinitmod3 invoked (state=%d)\n",
+          tlsreinitmod3_state);
+
+  if (tlsreinitmod3_constructed)
+    {
+      puts ("error: call_tlsreinitmod3 called after ELF constructor");
+      /* Cannot rely on test harness due to dynamic linking.  */
+      _exit (1);
+    }
+
+  tlsreinitmod3_state = 2;
+
+  /* Self-dlopen.  This will run the ELF constructor.   */
+  void *self = dlopen ("tst-dlopen-tlsreinitmod3.so", RTLD_NOW);
+  if (self == NULL)
+    {
+      printf ("error: dlopen: %s\n", dlerror ());
+      /* Cannot rely on test harness due to dynamic linking.  */
+      _exit (1);
+    }
+
+  call_tlsreinitmod3_tail (self);
+}
+
+void
+call_tlsreinitmod3_tail (void *self)
+{
+  printf ("info: dlopen returned in tlsreinitmod3 (state=%d)\n",
+          tlsreinitmod3_state);
+
+  if (!tlsreinitmod3_constructed)
+    {
+      puts ("error: dlopen did not call tlsreinitmod3 ELF constructor");
+      /* Cannot rely on test harness due to dynamic linking.  */
+      _exit (1);
+    }
+
+  if (tlsreinitmod3_state != 2)
+    {
+      puts ("error: TLS state reverted in tlsreinitmod3");
+      /* Cannot rely on test harness due to dynamic linking.  */
+      _exit (1);
+    }
+
+  dlclose (self);
+
+  /* Signal test completion to the main program.  */
+  tlsreinitmod3_tested = true;
+}
+
+static void __attribute__ ((constructor))
+tlsreinitmod3_init (void)
+{
+  puts ("info: constructor of tst-dlopen-tlsreinitmod3.so invoked");
+  tlsreinitmod3_constructed = true;
+}
diff --git a/include/link.h b/include/link.h
index cb0d7d8e2f..5ed445d5a6 100644
--- a/include/link.h
+++ b/include/link.h
@@ -212,6 +212,7 @@ struct link_map
     unsigned int l_find_object_processed:1; /* Zero if _dl_find_object_update
 					       needs to process this
 					       lt_library map.  */
+    unsigned int l_tls_in_slotinfo:1; /* TLS slotinfo updated in dlopen.  */
 
     /* NODELETE status of the map.  Only valid for maps of type
        lt_loaded.  Lazy binding sets l_nodelete_active directly,
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 50f58a60e3..b07c3c082a 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1242,12 +1242,21 @@ extern int _dl_scope_free (void *) attribute_hidden;
 
 
 /* Add module to slot information data.  If DO_ADD is false, only the
-   required memory is allocated.  Must be called with GL
-   (dl_load_tls_lock) acquired.  If the function has already been called
-   for the link map L with !do_add, then this function will not raise
-   an exception, otherwise it is possible that it encounters a memory
-   allocation failure.  */
-extern void _dl_add_to_slotinfo (struct link_map *l, bool do_add)
+   required memory is allocated.  Must be called with
+   GL (dl_load_tls_lock) acquired.  If the function has already been
+   called for the link map L with !DO_ADD, then this function will not
+   raise an exception, otherwise it is possible that it encounters a
+   memory allocation failure.
+
+   Return false if L has already been added to the slotinfo data, or
+   if L has no TLS data.  If the returned value is true, L has been
+   added with this call (DO_ADD), or has been added in a previous call
+   (!DO_ADD).
+
+   If DO_ADD is true and the return value is true, the TLS generation
+   counter must be incremented afterwards, by calling
+   _dl_update_slotinfo.  */
+extern bool _dl_add_to_slotinfo (struct link_map *l, bool do_add)
   attribute_hidden;
 
 /* Update slot information data for at least the generation of the

base-commit: a07e000e82cb71238259e674529c37c12dc7d423

Re: [PATCH v2] elf: Avoid re-initializing already allocated TLS in dlopen (bug 31717)

[Andreas Schwab]

On Mai 14 2024, Florian Weimer wrote:

> +   If DO_ADD is true and the return value is true, the TLS generation
> +   counter must be incremented afterwards, by calling
> +   _dl_update_slotinfo.  */

This is confusing.  The only uses that check the return value call it
with !DO_ADD.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."

Re: [PATCH v2] elf: Avoid re-initializing already allocated TLS in dlopen (bug 31717)

[Florian Weimer]

* Andreas Schwab:

> On Mai 14 2024, Florian Weimer wrote:
>
>> +   If DO_ADD is true and the return value is true, the TLS generation
>> +   counter must be incremented afterwards, by calling
>> +   _dl_update_slotinfo.  */
>
> This is confusing.  The only uses that check the return value call it
> with !DO_ADD.

I'll assume this is criticism of the description, not the interface
itself.

What about this?

/* Add module to slot information data.  If DO_ADD is false, only the
   required memory is allocated.  Must be called with
   GL (dl_load_tls_lock) acquired.  If the function has already been
   called for the link map L with !DO_ADD, then this function will not
   raise an exception, otherwise it is possible that it encounters a
   memory allocation failure.

   Return false if L has already been added to the slotinfo data, or
   if L has no TLS data.  If the returned value is true, L has been
   added with this call (DO_ADD), or has been added in a previous call
   (!DO_ADD).  

   The expected usage is as follows: Call _dl_add_to_slotinfo for
   several link maps with DO_ADD set to false, and record if any calls
   result in a true result.  If there was a true result, call
   _dl_add_to_slotinfo again, this time with DO_ADD set to true.  (For
   simplicity, it's possible to call the function for link maps where
   the previous result was false.)  The return value from the second
   round of calls can be ignored.  If there was true result initially,
   call _dl_update_slotinfo to update the TLS generation counter.  */
extern bool _dl_add_to_slotinfo (struct link_map *l, bool do_add)
  attribute_hidden;

Thanks,
Florian

Re: [PATCH v2] elf: Avoid re-initializing already allocated TLS in dlopen (bug 31717)

[Andreas Schwab]

Looks good.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."