[RFC PATCH v3 1/5] Libffi Static Trampolines

[madvenka@linux.microsoft.com]

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

Closure Trampoline Security Issue
=================================

Currently, the trampoline code used in libffi is not statically defined in
a source file (except for MACH). The trampoline is either pre-defined
machine code in a data buffer. Or, it is generated at runtime. In order to
execute a trampoline, it needs to be placed in a page with executable
permissions.

Executable data pages are attack surfaces for attackers who may try to
inject their own code into the page and contrive to have it executed. The
security settings in a system may prevent various tricks used in user land
to write code into a page and to have it executed somehow. On such systems,
libffi trampolines would not be able to run.

Static Trampoline
=================

To solve this problem, the trampoline code needs to be defined statically
in a source file, compiled and placed in the text segment so it can be
mapped and executed naturally without any tricks. However, the trampoline
needs to be able to access the closure pointer at runtime.

PC-relative data referencing
============================

The solution implemented in this patch set uses PC-relative data references.
The trampoline is mapped in a code page. Adjacent to the code page, a data
page is mapped that contains the parameters of the trampoline:

	- the closure pointer
	- pointer to the ABI handler to jump to

The trampoline code uses an offset relative to its current PC to access its
data.

Some architectures support PC-relative data references in the ISA itself.
E.g., X64 supports RIP-relative references. For others, the PC has to
somehow be loaded into a general purpose register to do PC-relative data
referencing. To do this, we need to define a get_pc() kind of function and
call it to load the PC in a desired register.

There are two cases:

1. The call instruction pushes the return address on the stack.

   In this case, get_pc() will extract the return address from the stack
   and load it in the desired register and return.

2. The call instruction stores the return address in a designated register.

   In this case, get_pc() will copy the return address to the desired
   register and return.

Either way, the PC next to the call instruction is obtained.

Scratch register
================

In order to do its job, the trampoline code would need to use a scratch
register. Depending on the ABI, there may not be a register available for
scratch. This problem needs to be solved so that all ABIs will work.

The trampoline will save two values on the stack:

	- the closure pointer
	- the original value of the scratch register

This is what the stack will look like:

	sp before trampoline ------>	--------------------
					| closure pointer  |
					--------------------
					| scratch register |
	sp after trampoline ------->	--------------------

The ABI handler can do the following as needed by the ABI:

	- the closure pointer can be loaded in a desired register

	- the scratch register can be restored to its original value

	- the stack pointer can be restored to its original value
	  (the value when the trampoline was invoked)

To do this, I have defined prolog code for each ABI handler. The legacy
trampoline jumps to the ABI handler directly. But the static trampoline
defined in this patch jumps tp the prolog code which performs the above
actions before jumping to the ABI handler.

NOTE:
	The documentation for this feature will contain information on:

	- the name of the scratch register for each architecture

	- the stack offsets at which the closure and the scratch register
	  will be copied

Trampoline Table
================

In order to reduce the trampoline memory footprint, the trampoline code
would be defined as a code array in the text segment. This array would be
mapped into the address space of the caller. The mapping would, therefore,
contain a trampoline table.

Adjacent to the trampoline table mapping, there will be a data mapping that
contains a parameter table, one parameter block for each trampoline. The
parameter block will contain:

	- a pointer to the closure
	- a pointer to the ABI handler

The static trampoline code would finally look like this:

	- Make space on the stack for the closure and the scratch register
	  by moving the stack pointer down
	- Store the original value of the scratch register on the stack
	- Using PC-relative reference, get the closure pointer
	- Store the closure pointer on the stack
	- Using PC-relative reference, get the ABI handler pointer
	- Jump to the ABI handler

Trampoline API
==============

There is a lot of dynamic code out there. They all have the same security
issue. Dynamic code can be re-written into static code provided the data
required by the static code can be passed to it just like we pass the
closure pointer to an ABI handler.

So, the same trampoline functions used by libffi internally need to be
made available to the rest of the world in the form of an API. The
following API has been defined in this solution:

int ffi_tramp_is_supported(void);

	To support static trampolines, code needs to be added to each
	architecture. This function tells us whether or not the feature is
	supported in the current libffi for the current architecture.

void *ffi_tramp_alloc (int flags);

	Allocate a trampoline. Currently, flags are zero. An opaque
	trampoline structure pointer is returned.

	Internally, libffi manages trampoline tables and individual
	trampolines in each table.

int ffi_tramp_set_parms (void *tramp, void *target, void *data);

	Initialize the parameters of a trampoline. That is, the target code
	that the trampoline should jump to and the data that needs to be
	passed to the target code.

void *ffi_tramp_get_addr (void *tramp);

	Return the address of the trampoline to invoke the trampoline with.
	The trampoline can be invoked in one of two ways:

		- Simply branch to the trampoline address
		- Treat the trampoline address as a function pointer and
		  call it.

	Which method is used depends on the target code.

void ffi_tramp_free (void *tramp);

	Free a trampoline.

Mapping size
============

The size of the code mapping that contains the trampoline table needs to be
determined on a per architecture basis. If a particular architecture
supports multiple base page sizes, then the largest supported base page size
needs to be chosen. E.g., we choose 16K for ARM64.

Trampoline allocation and free
==============================

Static trampolines are allocated in ffi_closure_alloc() and freed in
ffi_closure_free().

Normally, applications use these functions. But there are some cases out
there where the user of libffi allocates and manages its own closure
memory. In such cases, the static trampoline API cannot be used. These
will fall back to using legacy trampolines. The user has to make sure
that the memory is executable.

ffi_closure structure
=====================

I did not want to make any changes to the size of the closure structure for
this feature to guarantee compatibility. But the opaque static trampoline
handle needs to be stored in the closure. I have defined it as follows:

-  char tramp[FFI_TRAMPOLINE_SIZE];
+  union {
+    char tramp[FFI_TRAMPOLINE_SIZE];
+    void *ftramp;
+  };

If static trampolines are used, then tramp[] is not needed to store a
dynamic trampoline. That space can be reused to store the handle. Hence,
the union.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 Makefile.am                                 |   3 +-
 configure.ac                                |   9 +-
 include/ffi.h.in                            |  13 +-
 include/ffi_common.h                        |   4 +
 libffi.map.in                               |   9 +
 src/closures.c                              |  47 +-
 src/tramp.c                                 | 718 ++++++++++++++++++++
 testsuite/libffi.closures/closure_loc_fn0.c |   3 +
 8 files changed, 801 insertions(+), 5 deletions(-)
 create mode 100644 src/tramp.c

diff --git a/Makefile.am b/Makefile.am
index 7654bf5..1b18198 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -38,7 +38,8 @@ toolexeclib_LTLIBRARIES = libffi.la
 noinst_LTLIBRARIES = libffi_convenience.la
 
 libffi_la_SOURCES = src/prep_cif.c src/types.c \
-		src/raw_api.c src/java_raw_api.c src/closures.c
+		src/raw_api.c src/java_raw_api.c src/closures.c \
+		src/tramp.c
 
 if FFI_DEBUG
 libffi_la_SOURCES += src/debug.c
diff --git a/configure.ac b/configure.ac
index 790274e..8bfbabf 100644
--- a/configure.ac
+++ b/configure.ac
@@ -64,7 +64,7 @@ EOF
 AM_MAINTAINER_MODE
 
 AC_CHECK_HEADERS(sys/mman.h)
-AC_CHECK_FUNCS([mmap mkostemp])
+AC_CHECK_FUNCS([mmap mkostemp mkstemp])
 AC_FUNC_MMAP_BLACKLIST
 
 dnl The -no-testsuite modules omit the test subdir.
@@ -360,6 +360,13 @@ AC_ARG_ENABLE(raw-api,
     AC_DEFINE(FFI_NO_RAW_API, 1, [Define this if you do not want support for the raw API.])
   fi)
 
+case "$target" in
+     *-linux*)
+       AC_DEFINE(FFI_EXEC_STATIC_TRAMP, 1,
+                 [Define this if you want statically defined trampolines])
+     ;;
+esac
+
 AC_ARG_ENABLE(purify-safety,
 [  --enable-purify-safety  purify-safe mode],
   if test "$enable_purify_safety" = "yes"; then
diff --git a/include/ffi.h.in b/include/ffi.h.in
index 38885b0..c6a21d9 100644
--- a/include/ffi.h.in
+++ b/include/ffi.h.in
@@ -310,7 +310,10 @@ typedef struct {
   void *trampoline_table;
   void *trampoline_table_entry;
 #else
-  char tramp[FFI_TRAMPOLINE_SIZE];
+  union {
+    char tramp[FFI_TRAMPOLINE_SIZE];
+    void *ftramp;
+  };
 #endif
   ffi_cif   *cif;
   void     (*fun)(ffi_cif*,void*,void**,void*);
@@ -457,6 +460,14 @@ FFI_API void ffi_call_go (ffi_cif *cif, void (*fn)(void), void *rvalue,
 
 #endif /* FFI_GO_CLOSURES */
 
+/* ---- Static Trampoline Definitions -------------------------------------- */
+
+FFI_API int ffi_tramp_is_supported(void);
+FFI_API void *ffi_tramp_alloc (int flags);
+FFI_API int ffi_tramp_set_parms (void *tramp, void *data, void *code);
+FFI_API void *ffi_tramp_get_addr (void *tramp);
+FFI_API void ffi_tramp_free (void *tramp);
+
 /* ---- Public interface definition -------------------------------------- */
 
 FFI_API 
diff --git a/include/ffi_common.h b/include/ffi_common.h
index 76b9dd6..d3db33a 100644
--- a/include/ffi_common.h
+++ b/include/ffi_common.h
@@ -103,6 +103,10 @@ ffi_status ffi_prep_cif_core(ffi_cif *cif,
    some targets.  */
 void *ffi_data_to_code_pointer (void *data) FFI_HIDDEN;
 
+/* The arch code calls this to determine if a given closure has a
+   static trampoline. */
+int ffi_tramp_is_present (void *closure);
+
 /* Extended cif, used in callback from assembly routine */
 typedef struct
 {
diff --git a/libffi.map.in b/libffi.map.in
index de8778a..9d8a472 100644
--- a/libffi.map.in
+++ b/libffi.map.in
@@ -74,3 +74,12 @@ LIBFFI_GO_CLOSURE_8.0 {
 	ffi_prep_go_closure;
 } LIBFFI_CLOSURE_8.0;
 #endif
+
+LIBFFI_STATIC_TRAMP_8.0 {
+  global:
+	ffi_tramp_is_supported;
+	ffi_tramp_alloc;
+	ffi_tramp_set_parms;
+	ffi_tramp_get_addr;
+	ffi_tramp_free;
+} LIBFFI_BASE_8.0;
diff --git a/src/closures.c b/src/closures.c
index 4fe6158..a006548 100644
--- a/src/closures.c
+++ b/src/closures.c
@@ -109,6 +109,12 @@ ffi_closure_free (void *ptr)
   munmap(dataseg, rounded_size);
   munmap(codeseg, rounded_size);
 }
+
+int
+ffi_tramp_is_present (__attribute__((unused)) void *ptr)
+{
+  return 0;
+}
 #else /* !NetBSD with PROT_MPROTECT */
 
 #if !FFI_MMAP_EXEC_WRIT && !FFI_EXEC_TRAMPOLINE_TABLE
@@ -843,6 +849,12 @@ dlmmap (void *start, size_t length, int prot,
 	  && flags == (MAP_PRIVATE | MAP_ANONYMOUS)
 	  && fd == -1 && offset == 0);
 
+  if (execfd == -1 && ffi_tramp_is_supported ())
+    {
+      ptr = mmap (start, length, prot & ~PROT_EXEC, flags, fd, offset);
+      return ptr;
+    }
+
   if (execfd == -1 && is_emutramp_enabled ())
     {
       ptr = mmap (start, length, prot & ~PROT_EXEC, flags, fd, offset);
@@ -922,7 +934,7 @@ segment_holding_code (mstate m, char* addr)
 void *
 ffi_closure_alloc (size_t size, void **code)
 {
-  void *ptr;
+  void *ptr, *ftramp;
 
   if (!code)
     return NULL;
@@ -934,6 +946,17 @@ ffi_closure_alloc (size_t size, void **code)
       msegmentptr seg = segment_holding (gm, ptr);
 
       *code = add_segment_exec_offset (ptr, seg);
+      if (!ffi_tramp_is_supported ())
+        return ptr;
+
+      ftramp = ffi_tramp_alloc (0);
+      if (ftramp == NULL)
+      {
+        dlfree (FFI_RESTORE_PTR (ptr));
+        return NULL;
+      }
+      *code = ffi_tramp_get_addr (ftramp);
+      ((ffi_closure *) ptr)->ftramp = ftramp;
     }
 
   return ptr;
@@ -943,12 +966,17 @@ void *
 ffi_data_to_code_pointer (void *data)
 {
   msegmentptr seg = segment_holding (gm, data);
+
   /* We expect closures to be allocated with ffi_closure_alloc(), in
      which case seg will be non-NULL.  However, some users take on the
      burden of managing this memory themselves, in which case this
      we'll just return data. */
   if (seg)
-    return add_segment_exec_offset (data, seg);
+    {
+      if (!ffi_tramp_is_supported ())
+        return add_segment_exec_offset (data, seg);
+      return ffi_tramp_get_addr (((ffi_closure *) data)->ftramp);
+    }
   else
     return data;
 }
@@ -966,10 +994,19 @@ ffi_closure_free (void *ptr)
   if (seg)
     ptr = sub_segment_exec_offset (ptr, seg);
 #endif
+  if (ffi_tramp_is_supported ())
+    ffi_tramp_free (((ffi_closure *) ptr)->ftramp);
 
   dlfree (FFI_RESTORE_PTR (ptr));
 }
 
+int
+ffi_tramp_is_present (void *ptr)
+{
+  msegmentptr seg = segment_holding (gm, ptr);
+  return seg != NULL && ffi_tramp_is_supported();
+}
+
 # else /* ! FFI_MMAP_EXEC_WRIT */
 
 /* On many systems, memory returned by malloc is writable and
@@ -998,6 +1035,12 @@ ffi_data_to_code_pointer (void *data)
   return data;
 }
 
+int
+ffi_tramp_is_present (__attribute__((unused)) void *ptr)
+{
+  return 0;
+}
+
 # endif /* ! FFI_MMAP_EXEC_WRIT */
 #endif /* FFI_CLOSURES */
 
diff --git a/src/tramp.c b/src/tramp.c
new file mode 100644
index 0000000..4151cc9
--- /dev/null
+++ b/src/tramp.c
@@ -0,0 +1,718 @@
+/* -----------------------------------------------------------------------
+   tramp.c - Copyright (c) 2020 Madhavan T. Venkataraman
+
+   API and support functions for managing statically defined closure
+   trampolines.
+
+   Permission is hereby granted, free of charge, to any person obtaining
+   a copy of this software and associated documentation files (the
+   ``Software''), to deal in the Software without restriction, including
+   without limitation the rights to use, copy, modify, merge, publish,
+   distribute, sublicense, and/or sell copies of the Software, and to
+   permit persons to whom the Software is furnished to do so, subject to
+   the following conditions:
+
+   The above copyright notice and this permission notice shall be included
+   in all copies or substantial portions of the Software.
+
+   THE SOFTWARE IS PROVIDED ``AS IS'', WITHOUT WARRANTY OF ANY KIND,
+   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+   NONINFRINGEMENT.  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+   OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+   DEALINGS IN THE SOFTWARE.
+   ----------------------------------------------------------------------- */
+
+#include <fficonfig.h>
+
+#ifdef FFI_EXEC_STATIC_TRAMP
+
+/* -------------------------- Headers and Definitions ---------------------*/
+/*
+ * Add support for other OSes later. For now, it is just Linux.
+ */
+
+#if defined __linux__
+#ifdef __linux__
+#define _GNU_SOURCE 1
+#endif
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <sys/mman.h>
+#ifdef __linux__
+#include <linux/limits.h>
+#include <linux/types.h>
+#endif
+#endif /* __linux__ */
+
+/*
+ * Each architecture defines static code for a trampoline code table. The
+ * trampoline code table is mapped into the address space of a process.
+ *
+ * The following architecture specific function returns:
+ *
+ *	- the address of the trampoline code table in the text segment
+ *	- the size of each trampoline in the trampoline code table
+ *	- the size of the mapping for the whole trampoline code table
+ */
+void __attribute__((weak)) *ffi_tramp_arch (size_t *tramp_size,
+  size_t *map_size);
+
+/* ------------------------- Trampoline Data Structures --------------------*/
+
+struct tramp;
+
+/*
+ * Trampoline table. Manages one trampoline code table and one trampoline
+ * parameter table.
+ *
+ * prev, next	Links in the global trampoline table list.
+ * code_table	Trampoline code table mapping.
+ * parm_table	Trampoline parameter table mapping.
+ * array	Array of trampolines malloced.
+ * free		List of free trampolines.
+ * nfree	Number of free trampolines.
+ */
+struct tramp_table
+{
+  struct tramp_table *prev;
+  struct tramp_table *next;
+  void *code_table;
+  void *parm_table;
+  struct tramp *array;
+  struct tramp *free;
+  int nfree;
+};
+
+/*
+ * Parameters for each trampoline.
+ *
+ * data
+ *	Data for the target code that the trampoline jumps to.
+ * target
+ *	Target code that the trampoline jumps to.
+ */
+struct tramp_parm
+{
+  void *data;
+  void *target;
+};
+
+/*
+ * Trampoline structure for each trampoline.
+ *
+ * prev, next	Links in the trampoline free list of a trampoline table.
+ * table	Trampoline table to which this trampoline belongs.
+ * code		Address of this trampoline in the code table mapping.
+ * parm		Address of this trampoline's parameters in the parameter
+ *		table mapping.
+ */
+struct tramp
+{
+  struct tramp *prev;
+  struct tramp *next;
+  struct tramp_table *table;
+  void *code;
+  struct tramp_parm *parm;
+};
+
+enum gtramp_status {
+	GTRAMP_UNINITIALIZED = 0,
+	GTRAMP_PASSED,
+	GTRAMP_FAILED,
+};
+
+/*
+ * Trampoline globals.
+ *
+ * fd
+ *	File descriptor of binary file that contains the trampoline code table.
+ * offset
+ *	Offset of the trampoline code table in that file.
+ * text
+ *	Address of the trampoline code table in the text segment.
+ * map_size
+ *	Size of the trampoline code table mapping.
+ * size
+ *	Size of one trampoline in the trampoline code table.
+ * ntramp
+ *	Total number of trampolines in the trampoline code table.
+ * tables
+ *	List of trampoline tables that contain free trampolines.
+ * ntables
+ *	Number of trampoline tables that contain free trampolines.
+ * status
+ *	Initialization status.
+ */
+struct tramp_global
+{
+  int fd;
+  off_t offset;
+  void *text;
+  size_t map_size;
+  size_t size;
+  int ntramp;
+  struct tramp_table *tables;
+  int ntables;
+  enum gtramp_status status;
+};
+
+static struct tramp_global gtramp;
+
+/* --------------------- Trampoline File Initialization --------------------*/
+
+/*
+ * The trampoline file is the file used to map the trampoline code table into
+ * the address space of a process. There are two ways to get this file:
+ *
+ * - From the OS. E.g., on Linux, /proc/<pid>/maps lists all the memory
+ *   mappings for <pid>. For file-backed mappings, maps supplies the file name
+ *   and the file offset. Using this, we can locate the mapping that maps
+ *   libffi and get the path to the libffi binary. And, we can compute the
+ *   offset of the trampoline code table within that binary.
+ *
+ * - Else, if we can create a temporary file, we can write the trampoline code
+ *   table from the text segment into the temporary file.
+ *
+ * The first method is the preferred one. If the OS security subsystem
+ * disallows mapping unsigned files with PROT_EXEC, then the second method
+ * will fail.
+ *
+ * If an OS allows the trampoline code table in the text segment to be
+ * directly remapped (e.g., MACH vm_remap ()), then we don't need the
+ * trampoline file.
+ */
+static int tramp_table_alloc (void);
+
+#if defined __linux__
+
+static int
+ffi_tramp_get_libffi (void)
+{
+  FILE *fp;
+  char file[PATH_MAX], line[PATH_MAX+100], perm[10], dev[10];
+  unsigned long start, end, offset, inode;
+  uintptr_t addr = (uintptr_t) gtramp.text;
+  int nfields, found;
+
+  snprintf (file, PATH_MAX, "/proc/%d/maps", getpid());
+  fp = fopen (file, "r");
+  if (fp == NULL)
+    return 0;
+
+  found = 0;
+  while (feof (fp) == 0) {
+    if (fgets (line, sizeof (line), fp) == 0)
+      break;
+
+    nfields = sscanf (line, "%lx-%lx %s %lx %s %ld %s",
+      &start, &end, perm, &offset, dev, &inode, file);
+    if (nfields != 7)
+      continue;
+
+    if (addr >= start && addr < end) {
+      gtramp.offset = offset + (addr - start);
+      found = 1;
+      break;
+    }
+  }
+  fclose (fp);
+
+  if (!found)
+    return 0;
+
+  gtramp.fd = open (file, O_RDONLY);
+  if (gtramp.fd == -1)
+    return 0;
+
+  /*
+   * Allocate a trampoline table just to make sure that the trampoline code
+   * table can be mapped.
+   */
+  if (!tramp_table_alloc ())
+    {
+      close (gtramp.fd);
+      gtramp.fd = -1;
+      return 0;
+    }
+  return 1;
+}
+
+#endif /* __linux__ */
+
+#if defined __linux__
+
+#if defined HAVE_MKSTEMP
+
+static int
+ffi_tramp_get_temp_file (void)
+{
+  char template[12] = "/tmp/XXXXXX";
+  ssize_t count;
+
+  gtramp.offset = 0;
+  gtramp.fd = mkstemp (template);
+  if (gtramp.fd == -1)
+    return 0;
+
+  unlink (template);
+  /*
+   * Write the trampoline code table into the temporary file and allocate a
+   * trampoline table to make sure that the temporary file can be mapped.
+   */
+  count = write(gtramp.fd, gtramp.text, gtramp.map_size);
+  if (count == gtramp.map_size && tramp_table_alloc ())
+    return 1;
+
+  close (gtramp.fd);
+  gtramp.fd = -1;
+  return 0;
+}
+
+#else /* !defined HAVE_MKSTEMP */
+
+/*
+ * TODO:
+ * Perhaps, libffi can supply its own version of mkstemp() if it is
+ * not natively available.
+ */
+static int
+ffi_tramp_get_temp_file (void)
+{
+  gtramp.offset = 0;
+  gtramp.fd = -1;
+  return 0;
+}
+
+#endif /* defined HAVE_MKSTEMP */
+
+#endif /* __linux__ */
+
+/* ------------------------ OS-specific Initialization ----------------------*/
+
+#if defined __linux__
+
+static int
+ffi_tramp_init_os (void)
+{
+  if (ffi_tramp_get_libffi ())
+    return 1;
+  return ffi_tramp_get_temp_file ();
+}
+
+#endif /* __linux__ */
+
+/* --------------------------- OS-specific Locking -------------------------*/
+
+#if defined __linux__
+
+static pthread_mutex_t gtramp_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static void
+ffi_tramp_lock(void)
+{
+  pthread_mutex_lock (&gtramp_mutex);
+}
+
+static void
+ffi_tramp_unlock()
+{
+  pthread_mutex_unlock (&gtramp_mutex);
+}
+
+#endif /* __linux__ */
+
+/* ------------------------ OS-specific Memory Mapping ----------------------*/
+
+/*
+ * Create a trampoline code table mapping and a trampoline parameter table
+ * mapping. The two mappings must be adjacent to each other for PC-relative
+ * access.
+ *
+ * For each trampoline in the code table, there is a corresponding parameter
+ * block in the parameter table. The size of the parameter block is the same
+ * as the size of the trampoline. This means that the parameter block is at
+ * a fixed offset from its trampoline making it easy for a trampoline to find
+ * its parameters using PC-relative access.
+ *
+ * The parameter block will contain a struct tramp_parm. This means that
+ * sizeof (struct tramp_parm) cannot exceed the size of a parameter block.
+ */
+
+#if defined __linux__
+
+static int
+tramp_table_map (struct tramp_table *table)
+{
+  char *addr;
+
+  /*
+   * Create an anonymous mapping twice the map size. The top half will be used
+   * for the code table. The bottom half will be used for the parameter table.
+   */
+  addr = mmap (NULL, gtramp.map_size * 2, PROT_READ | PROT_WRITE,
+    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  if (addr == MAP_FAILED)
+    return 0;
+
+  /*
+   * Replace the top half of the anonymous mapping with the code table mapping.
+   */
+  table->code_table = mmap (addr, gtramp.map_size, PROT_READ | PROT_EXEC,
+    MAP_PRIVATE | MAP_FIXED, gtramp.fd, gtramp.offset);
+  if (table->code_table == MAP_FAILED)
+    {
+      (void) munmap (addr, gtramp.map_size * 2);
+      return 0;
+    }
+  table->parm_table = table->code_table + gtramp.map_size;
+  return 1;
+}
+
+static void
+tramp_table_unmap (struct tramp_table *table)
+{
+  (void) munmap (table->code_table, gtramp.map_size);
+  (void) munmap (table->parm_table, gtramp.map_size);
+}
+
+#endif /* __linux__ */
+
+/* ------------------------ Trampoline Initialization ----------------------*/
+
+/*
+ * Initialize the static trampoline feature.
+ */
+static int
+ffi_tramp_init (void)
+{
+  if (gtramp.status == GTRAMP_PASSED)
+    return 1;
+
+  if (gtramp.status == GTRAMP_FAILED)
+    return 0;
+
+  if (ffi_tramp_arch == NULL)
+    {
+      gtramp.status = GTRAMP_FAILED;
+      return 0;
+    }
+
+  gtramp.tables = NULL;
+  gtramp.ntables = 0;
+
+  /*
+   * Get trampoline code table information from the architecture.
+   */
+  gtramp.text = ffi_tramp_arch (&gtramp.size, &gtramp.map_size);
+  gtramp.ntramp = gtramp.map_size / gtramp.size;
+
+  if (sysconf (_SC_PAGESIZE) > gtramp.map_size)
+    return 0;
+
+  if (ffi_tramp_init_os ())
+    {
+      gtramp.status = GTRAMP_PASSED;
+      return 1;
+    }
+
+  gtramp.status = GTRAMP_FAILED;
+  return 0;
+}
+
+/* ---------------------- Trampoline Table functions ---------------------- */
+
+/* This code assumes that malloc () is available on all OSes. */
+
+static void tramp_add (struct tramp *tramp);
+
+/*
+ * Allocate and initialize a trampoline table.
+ */
+static int
+tramp_table_alloc (void)
+{
+  struct tramp_table *table;
+  struct tramp *tramp_array, *tramp;
+  size_t size;
+  char *code, *parm;
+  int i;
+
+  /*
+   * If we already have tables with free trampolines, there is no need to
+   * allocate a new table.
+   */
+  if (gtramp.ntables > 0)
+    return 1;
+
+  /*
+   * Allocate a new trampoline table structure.
+   */
+  table = malloc (sizeof (*table));
+  if (table == NULL)
+    return 0;
+
+  /*
+   * Allocate new trampoline structures.
+   */
+  tramp_array = malloc (sizeof (*tramp) * gtramp.ntramp);
+  if (tramp_array == NULL)
+    goto free_table;
+
+  /*
+   * Map a code table and a parameter table into the caller's address space.
+   */
+  if (!tramp_table_map (table))
+    goto free_tramp_array;
+
+  /*
+   * Initialize the trampoline table.
+   */
+  table->array = tramp_array;
+  table->free = NULL;
+  table->nfree = 0;
+
+  /*
+   * Populate the trampoline table free list. This will also add the trampoline
+   * table to the global list of trampoline tables.
+   */
+  size = gtramp.size;
+  code = table->code_table;
+  parm = table->parm_table;
+  for (i = 0; i < gtramp.ntramp; i++)
+    {
+      tramp = &tramp_array[i];
+      tramp->table = table;
+      tramp->code = code;
+      tramp->parm = (struct tramp_parm *) parm;
+      tramp_add (tramp);
+
+      code += size;
+      parm += size;
+    }
+  return 1;
+
+free_tramp_array:
+  free (tramp_array);
+free_table:
+  free (table);
+  return 0;
+}
+
+/*
+ * Free a trampoline table.
+ */
+static void
+tramp_table_free (struct tramp_table *table)
+{
+  tramp_table_unmap (table);
+  free (table->array);
+  free (table);
+}
+
+/*
+ * Add a new trampoline table to the global table list.
+ */
+static void
+tramp_table_add (struct tramp_table *table)
+{
+  table->next = gtramp.tables;
+  table->prev = NULL;
+  if (gtramp.tables != NULL)
+    gtramp.tables->prev = table;
+  gtramp.tables = table;
+  gtramp.ntables++;
+}
+
+/*
+ * Delete a trampoline table from the global table list.
+ */
+static void
+tramp_table_del (struct tramp_table *table)
+{
+  gtramp.ntables--;
+  if (table->prev != NULL)
+    table->prev->next = table->next;
+  if (table->next != NULL)
+    table->next->prev = table->prev;
+  if (gtramp.tables == table)
+    gtramp.tables = table->next;
+}
+
+/* ------------------------- Trampoline functions ------------------------- */
+
+/*
+ * Add a trampoline to its trampoline table.
+ */
+static void
+tramp_add (struct tramp *tramp)
+{
+  struct tramp_table *table = tramp->table;
+
+  tramp->next = table->free;
+  tramp->prev = NULL;
+  if (table->free != NULL)
+    table->free->prev = tramp;
+  table->free = tramp;
+  table->nfree++;
+
+  if (table->nfree == 1)
+    tramp_table_add (table);
+
+  /*
+   * We don't want to keep too many free trampoline tables lying around.
+   */
+  if (table->nfree == gtramp.ntramp && gtramp.ntables > 1)
+    {
+      tramp_table_del (table);
+      tramp_table_free (table);
+    }
+}
+
+/*
+ * Remove a trampoline from its trampoline table.
+ */
+static void
+tramp_del (struct tramp *tramp)
+{
+  struct tramp_table *table = tramp->table;
+
+  table->nfree--;
+  if (tramp->prev != NULL)
+    tramp->prev->next = tramp->next;
+  if (tramp->next != NULL)
+    tramp->next->prev = tramp->prev;
+  if (table->free == tramp)
+    table->free = tramp->next;
+
+  if (table->nfree == 0)
+    tramp_table_del (table);
+}
+
+/* ------------------------ Trampoline API functions ------------------------ */
+
+int
+ffi_tramp_is_supported(void)
+{
+  int ret;
+
+  ffi_tramp_lock();
+  ret = ffi_tramp_init ();
+  ffi_tramp_unlock();
+  return ret;
+}
+
+/*
+ * Allocate a trampoline and return its opaque address.
+ */
+void *
+ffi_tramp_alloc (int flags)
+{
+  struct tramp *tramp;
+
+  ffi_tramp_lock();
+
+  if (!ffi_tramp_init () || flags != 0)
+    {
+      ffi_tramp_unlock();
+      return NULL;
+    }
+
+  if (!tramp_table_alloc ())
+    {
+      ffi_tramp_unlock();
+      return NULL;
+    }
+
+  tramp = gtramp.tables->free;
+  tramp_del (tramp);
+
+  ffi_tramp_unlock();
+
+  return tramp;
+}
+
+/*
+ * Set the parameters for a trampoline.
+ */
+void
+ffi_tramp_set_parms (void *arg, void *target, void *data)
+{
+  struct tramp *tramp = arg;
+
+  ffi_tramp_lock();
+  tramp->parm->target = target;
+  tramp->parm->data = data;
+  ffi_tramp_unlock();
+}
+
+/*
+ * Get the invocation address of a trampoline.
+ */
+void *
+ffi_tramp_get_addr (void *arg)
+{
+  struct tramp *tramp = arg;
+  void *addr;
+
+  ffi_tramp_lock();
+  addr = tramp->code;
+  ffi_tramp_unlock();
+
+  return addr;
+}
+
+/*
+ * Free a trampoline.
+ */
+void
+ffi_tramp_free (void *arg)
+{
+  struct tramp *tramp = arg;
+
+  ffi_tramp_lock();
+  tramp_add (tramp);
+  ffi_tramp_unlock();
+}
+
+/* ------------------------------------------------------------------------- */
+
+#else /* !FFI_EXEC_STATIC_TRAMP */
+
+#include <stddef.h>
+
+int
+ffi_tramp_is_supported(void)
+{
+  return 0;
+}
+
+void *
+ffi_tramp_alloc (int flags)
+{
+  return NULL;
+}
+
+void
+ffi_tramp_set_parms (void *arg, void *target, void *data)
+{
+}
+
+void *
+ffi_tramp_get_addr (void *arg)
+{
+  return NULL;
+}
+
+void
+ffi_tramp_free (void *arg)
+{
+}
+
+#endif /* FFI_EXEC_STATIC_TRAMP */
diff --git a/testsuite/libffi.closures/closure_loc_fn0.c b/testsuite/libffi.closures/closure_loc_fn0.c
index b3afa0b..ad488ac 100644
--- a/testsuite/libffi.closures/closure_loc_fn0.c
+++ b/testsuite/libffi.closures/closure_loc_fn0.c
@@ -83,7 +83,10 @@ int main (void)
   CHECK(ffi_prep_closure_loc(pcl, &cif, closure_loc_test_fn0,
 			 (void *) 3 /* userdata */, codeloc) == FFI_OK);
   
+#ifndef FFI_EXEC_STATIC_TRAMP
+  /* With static trampolines, the codeloc does not point to closure */
   CHECK(memcmp(pcl, codeloc, sizeof(*pcl)) == 0);
+#endif
 
   res = (*((closure_loc_test_type0)codeloc))
     (1LL, 2, 3LL, 4, 127, 429LL, 7, 8, 9.5, 10, 11, 12, 13,
-- 
2.25.1