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

[Jiong Wang <jiong.wang@foss.arm.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.