Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

[Arvind Sankar]

On Thu, Sep 17, 2020 at 10:36:02AM -0500, Madhavan T. Venkataraman wrote:
> 
> 
> On 9/16/20 8:04 PM, Florian Weimer wrote:
> > * madvenka:
> > 
> >> 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).

> 
> >> 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.