On Tue, Sep 22, 2020 at 09:46:16PM -0400, Arvind Sankar wrote:

On Thu, Sep 17, 2020 at 10:36:02AM -0500, Madhavan T. Venkataraman wrote:

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On 9/16/20 8:04 PM, Florian Weimer wrote:

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* madvenka:

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Examples of trampolines
=======================

libffi (A Portable Foreign Function Interface Library):

libffi allows a user to define functions with an arbitrary list of
arguments and return value through a feature called "Closures".
Closures use trampolines to jump to ABI handlers that handle calling
conventions and call a target function. libffi is used by a lot
of different applications. To name a few:

	- Python
	- Java
	- Javascript
	- Ruby FFI
	- Lisp
	- Objective C

libffi does not actually need this.  It currently collocates
trampolines and the data they need on the same page, but that's
actually unecessary.  It's possible to avoid doing this just by
changing libffi, without any kernel changes.

I think this has already been done for the iOS port.

The trampoline table that has been implemented for the iOS port (MACH)
is based on PC-relative data referencing. That is, the code and data
are placed in adjacent pages so that the code can access the data using
an address relative to the current PC.

This is an ISA feature that is not supported on all architectures.

Now, if it is a performance feature, we can include some architectures
and exclude others. But this is a security feature. IMO, we cannot
exclude any architecture even if it is a legacy one as long as Linux
is running on the architecture. So, we need a solution that does
not assume any specific ISA feature.

Which ISA does not support PIC objects? You mentioned i386 below, but
i386 does support them, it just needs to copy the PC into a GPR first
(see below).

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The code for trampoline X in the trampoline table is:

	load	&code_table[X], code_reg
	load	(code_reg), code_reg
	load	&data_table[X], data_reg
	load	(data_reg), data_reg
	jump	code_reg

The addresses &code_table[X] and &data_table[X] are baked into the
trampoline code. So, PC-relative data references are not needed. The user
can modify code_table[X] and data_table[X] dynamically.

You can put this code into the libffi shared object and map it from
there, just like the rest of the libffi code.  To get more
trampolines, you can map the page containing the trampolines multiple
times, each instance preceded by a separate data page with the control
information.

If you put the code in the libffi shared object, how do you pass data to
the code at runtime? If the code we are talking about is a function, then
there is an ABI defined way to pass data to the function. But if the
code we are talking about is some arbitrary code such as a trampoline,
there is no ABI defined way to pass data to it except in a couple of
platforms such as HP PA-RISC that have support for function descriptors
in the ABI itself.

As mentioned before, if the ISA supports PC-relative data references
(e.g., X86 64-bit platforms support RIP-relative data references)
then we can pass data to that code by placing the code and data in
adjacent pages. So, you can implement the trampoline table for X64.
i386 does not support it.

i386 just needs a tiny bit of code to copy the PC into a GPR first, i.e.
the trampoline would be:

	call	1f
1:	pop	%data_reg
	movl	(code_table + X - 1b)(%data_reg), %code_reg
	movl	(data_table + X - 1b)(%data_reg), %data_reg
	jmp	*(%code_reg)

I do not understand the point about passing data at runtime. This
trampoline is to achieve exactly that, no? 

Thanks.

For libffi, I think the proposed standard trampoline won't actually
work, because not all ABIs have two scratch registers available to use
as code_reg and data_reg. Eg i386 fastcall only has one, and register
has zero scratch registers. I believe 32-bit ARM only has one scratch
register as well.

For i386 you'd need something that saves a register on the stack first,
maybe like the below with a 16-byte trampoline and a 16-byte context
structure that has the address of the code to jump to in the first
dword:

	.balign 4096
	trampoline_page:

	.rept	4096/16-1
	0:	endbr32
		push	%eax
		call	__x86.get_pc_thunk.ax
	1:	jmp	trampoline
	.balign 16
	.endr

	.org trampoline_page + 4096 - 16
	__x86.get_pc_thunk.ax:
		movl	(%esp), %eax
		ret
	trampoline:
		subl	$(1b-0b), %eax
		jmp	*(table-trampoline_page)(%eax)

	.org trampoline_page + 4096
	table:

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