dummy frame on stack and SPARC

dummy frame on stack and SPARC

[Andrew Cagney]

(thinking out loud)

The sparc architecture, when calling a struct return function has a 
sequence like:

	call struct_return_function, return address = pc + 8
	nop
	.word sizeof returned object
return_address:
	... next instruction ....

That is, the return address is several instructions beyond the call 
instruction and, more importantly, one of those "instructions" is an a 
word to inform the callee of additional ABI information (from memory, 
how much space the return structure needs).

When correctly implemeting sparc dummy calls, this behavior needs to be 
implemented.  Hence the call needs 2 words (the struct return size and 
the breakpoint)instead of just one (the breakpoint) and the code dummy 
no longer fits in _start.  Conseqently, the SPARC, by default, pushes 
the code dummy on the stack.

Problem is, this assumes that the target has an executable stack 
(security?).  As a consequence, the SPARC either:

- writes a two word code dummy onto the stack and correctly handls 
struct return
- writes a one word code dummy into _start and fails when given a struct 
return function

Instead of doing this, would it be possible to always write the code 
dummy onto the stack and, when the stack isn't executable, recognize the 
  sigseg (?) as an attempt to return to the call dummy?

The only thing I'm not sure about is exactly when a SPARC will recognize 
the segmentation violation and abort.  It, thanks to the delayed branch 
code, could be a cycle too early :-(

Andrew