Re: [Patch] New gdbarch method "dwarf_cfa_op" and migrate SPARC to it

[Jiong Wang <jiong dot wang at foss dot arm dot com>]

On 21/04/17 15:43, Ivo Raisr wrote:
> On 19.4.2017 12:17, Jiong Wang wrote:
> > Hi,
> >
> > Recently a feature called "return address signing" has been added to 
> > GCC to
> > prevent stack smash stack on AArch64.  For details please refer:
> >
> >   https://gcc.gnu.org/ml/gcc-patches/2017-01/msg00376.html
> >
> > GDB needs to be aware of this feature so it can restore the original 
> > return
> > address which is critical for unwinding.
> >
> > On compiler side, whenever return address, i.e. LR register, is 
> > mangled or
> > restored by hardware instruction, compiler is expected to generate a
> > DW_CFA_GNU_window_save CFA instruction to toggle the LR signing status.
> > DW_CFA_GNU_window_save was introduced by SPARC for their register window
> > feature, here AArch64 want to multiplex this DWARF CFA instruction as
> > it's in vendor space.  So, a new gdbarch method, dwarf_cfa_op, is 
> > introduced.
>
> Are you sure to reuse DW_CFA_GNU_window_save for AArch64 purposes? 
> Because semantics of DW_CFA_GNU_window_save (implied from "window 
> save") mean much more than just fiddling with return address. For 
> example as you learned from dwarf2-frame.c, whole register window 
> needs to be copied.

Hi Ivo,

  Thanks very much for testing this on SPARC platform.

  What's really reused is the DWARF CFA number 0x2d behind 
DW_CFA_GNU_window_save.  It is in vendor extension space ( 
DW_CFA_lo_user.. DW_CFA_hi_user) so the semantics depends on vendor 
interpreation.

Regards,
Jiong.