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Sender: Arvind Sankar <niveditas98@gmail.com>
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Wed, 23 Sep 2020 15:51:47 -0400
To:     "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
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Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor
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On Wed, Sep 23, 2020 at 02:17:30PM -0500, Madhavan T. Venkataraman wrote:
> 
> 
> On 9/23/20 4:11 AM, Arvind Sankar wrote:
> > 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.
> 
> The trampoline is invoked as a function call in the libffi case. Any
> caller saved register can be used as code_reg, can it not? And the
> scratch register is needed only to jump to the code. After that, it
> can be reused for any other purpose.
> 
> However, for ARM, you are quite correct. There is only one scratch
> register. This means that I have to provide two types of trampolines:
> 
> 	- If an architecture has enough scratch registers, use the currently
> 	  defined trampoline.
> 
> 	- If the architecture has only one scratch register, but has PC-relative
> 	  data references, then embed the code address at the bottom of the
> 	  trampoline and access it using PC-relative addressing.
> 
> Thanks for pointing this out.
> 
> Madhavan

libffi is trying to provide closures with non-standard ABIs as well: the
actual user function is standard ABI, but the closure can be called with
a different ABI. If the closure was created with FFI_REGISTER abi, there
are no registers available for the trampoline to use: EAX, EDX and ECX
contain the first three arguments of the function, and every other
register is callee-save.

I provided a sample of the kind of trampoline that would be needed in
this case -- it's position-independent and doesn't clobber any registers
at all, and you get 255 trampolines per page. If I take another 16-byte
slot out of the page for the end trampoline that does the actual work,
I'm sure I could even come up with one that can just call a normal C
function, only the return might need special handling depending on the
return type.

And again, do you actually have any example of an architecture that
cannot run position-independent code? PC-relative addressing is an
implementation detail: the fact that it's available for x86_64 but not
for i386 just makes position-independent code more cumbersome on i386,
but it doesn't make it impossible. For the tiny trampolines here, it
makes almost no difference.