For the benefit and technical details of static trampoline, see
libffi#624.  As a new architecture, let's
be "safer" from the start.

The change survived libffi testsuite on loongarch64-linux-gnu.

For the benefit and technical details of static trampoline, see
#624.  As a new architecture, let's
be "safer" from the start.

The change survived libffi testsuite on loongarch64-linux-gnu.

* Static Trampolines

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.

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

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, static trampolines 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.

Architecture Support
====================

Support has been added for aarch64. Support for other architectures can be
added very easily in the future.

OS Support
==========

Support has been added for Linux. Support for other OSes can be added very
easily.

* arm64: Support for Static Trampolines

	- Define the arch-specific initialization function ffi_tramp_arch ()
	  that returns trampoline size information to common code.

	- Define the trampoline code mapping and data mapping sizes.

	- Define the trampoline code table statically.

	- Introduce a tiny prolog for each ABI handling function. The ABI
	  handlers addressed are:

	  	- ffi_closure_SYSV
		- ffi_closure_SYSV_V

	  The prolog functions are called:

	  	- ffi_closure_SYSV_alt
		- ffi_closure_SYSV_V_alt

	  The legacy trampoline jumps to the ABI handler. The static
	  trampoline jumps to the prolog function. The prolog function uses
	  the information provided by the static trampoline, sets things up
	  for the ABI handler and then jumps to the ABI handler.

	- Call ffi_tramp_set_parms () in ffi_prep_closure_loc () to
	  initialize static trampoline parameters.

Signed-off-by: Madhavan T. Venkataraman <[email protected]>

For the benefit and technical details of static trampoline, see
libffi/libffi#624.  As a new architecture, let's
be "safer" from the start.

The change survived libffi testsuite on loongarch64-linux-gnu.