Re: [RFA 1/2] mips: Switch inferior function calls to ON_STACK method.

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> Date: Thu, 3 May 2012 22:08:58 +0100
> From: "Maciej W. Rozycki" <macro@codesourcery.com>
> 
> Joel,
> 
> > This patch switches the mips code to use the ON_STACK method
> > for function calls instead of AT_SYMBOL, which we want to remove.
> 
>  Thanks for this work -- can you give me a reference to some background 
> information as to why exactly we want to remove the AT_SYMBOL method?

The AT_SYMBOL method relies on a magic symbol being present in the
binarie that's being debugged.  There is no guarantee that that magic
symbol is actually present in your binary.

> > Another little detail on the implementation of mips_push_dummy_code.
> > It starts by aligning the stack.  AFAIK, the stack is supposed to
> > always be aligned to at least 4 bytes (4 bytes for mips32, 8 bytes
> > for mips64). So, the initial alignment shouldn't be necessary, since
> > that's good enough aligment for our breakpoint instruction.  But
> > in the end, I chose to keep it, JIC. We could possibly change the
> > code to align to 4 instead of 16 like mips_frame_align does, if
> > we want to.
> 
>  For the record: the respective ABIs mandate that the stack is aligned to 
> 8 bytes for 32-bit targets and to 16 bytes for 64-bit targets.  However 
> the user may have fiddled with SP, so I think it's better to stay safe 
> and therefore I agree it's better if we prealign the stack and avoid 
> crashing the debuggee in this context.

Like I wrote elsewhere, the generic code that calls push_dummy_code()
already alignes the stack, so it isn't necessary to do it again here.

> >    /* MIPS version of CALL_DUMMY.  */
> >  
> > -  /* NOTE: cagney/2003-08-05: Eventually call dummy location will be
> > -     replaced by a command, and all targets will default to on stack
> > -     (regardless of the stack's execute status).  */
> > -  set_gdbarch_call_dummy_location (gdbarch, AT_SYMBOL);
> > +  set_gdbarch_call_dummy_location (gdbarch, ON_STACK);
> > +  set_gdbarch_push_dummy_code (gdbarch, mips_push_dummy_code);
> >    set_gdbarch_frame_align (gdbarch, mips_frame_align);
> >  
> >    set_gdbarch_convert_register_p (gdbarch, mips_convert_register_p);
> 
>  So what if the stack pages are indeed not executable (their page entries 
> have the XI aka Execute Inhibit bit set)?

The resulting SIGSEGV will be recognized by GDB and handled
appropriately; see infrun.c:handle_inferior_event().