Re: [2/5][AArch64] Generate dwarf information for -msign-return-address

[Jiong Wang]

On 13/01/17 16:09, Richard Earnshaw (lists) wrote:

On 06/01/17 11:47, Jiong Wang wrote:


This patch is an update on DWARF generation for return address signing.

According to new proposal, we simply needs to generate REG_CFA_WINDOW_SAVE
annotation.

gcc/

2017-01-06  Jiong Wang  

 * config/aarch64/aarch64.c (aarch64_expand_prologue): Generate
dwarf
 annotation (REG_CFA_WINDOW_SAVE) for return address signing.
 (aarch64_expand_epilogue): Likewise.



I don't think we should be overloading REG_CFA_WINDOW_SAVE internally in
the compiler -- it's one thing to do it in the dwarf output tables, but
quite another to be doing it elsewhere in the compiler.

Instead we should create a new reg note kind and use that, but in the
final dwarf output then emit the overloaded opcode.


I can see the reason for doing this is if you want to seperate the interpretion
of GCC CFA reg-note and the final DWARF CFA operation.  My understanding is all
reg notes defined in gcc/reg-note.def should have general meaning, even the
CFA_WINDOW_SAVE.  For those which are architecture specific we might need a
mechanism to define them in backend only.
   
For general reg-notes in gcc/reg-note.def, they are not always have the

corresponding standard DWARF CFA operation, for example CFA_WINDOW_SAVE,
therefore if we want to achieve what you described, I think we also need to
define a new target hook which maps a GCC CFA reg-note into architecture DWARF
CFA operation.

Regards,
Jiong

Re: [2/5][AArch64] Generate dwarf information for -msign-return-address

[Richard Earnshaw (lists)]

On 06/01/17 11:47, Jiong Wang wrote:
> On 11/11/16 18:22, Jiong Wang wrote:
>> This patch generate DWARF description for pointer authentication. 
>> DWARF value
>> expression is used to describe the authentication action.
>>
>> Please see the cover letter and AArch64 DWARF specification for the
>> semantics
>> of AArch64 DWARF operations.
>>
>> When authentication key index is A key, we use compact DWARF
>> description which
>> can largely save DWARF frame size, otherwise we fallback to general
>> operator.
>>
>>
>>
>> Example
>> ===
>>
>>  int
>>  cal (int a, int b, int c)
>>  {
>>return a + dec (b) + c;
>>  }
>>
>> Compact DWARF description
>> (-march=armv8.3-a -msign-return-address)
>> ===
>>
>>DW_CFA_advance_loc: 4 to 0004
>>DW_CFA_val_expression: r30 (x30) (DW_OP_AARCH64_paciasp)
>>DW_CFA_advance_loc: 4 to 0008
>>DW_CFA_val_expression: r30 (x30) (DW_OP_AARCH64_paciasp_deref: -24)
>>
>> General DWARF description
>> ===
>> (-march=armv8.3-a -msign-return-address -mpauth-key=b_key)
>>
>>DW_CFA_advance_loc: 4 to 0004
>>DW_CFA_val_expression: r30 (x30) (DW_OP_breg30 (x30): 0;
>> DW_OP_AARCH64_pauth: 18)
>>DW_CFA_advance_loc: 4 to 0008
>>DW_CFA_val_expression: r30 (x30) (DW_OP_dup; DW_OP_const1s: -24;
>> DW_OP_plus; DW_OP_deref; DW_OP_AARCH64_pauth: 18)
>>
>>  From Linux kernel testing, -msign-return-address will introduce +24%
>> .debug_frame size increase when signing all functions and using compact
>> description, and about +45% .debug_frame size increase if using general
>> description.
>>
>>
>> gcc/
>> 2016-11-11  Jiong Wang
>>
>>  * config/aarch64/aarch64.h (aarch64_pauth_action_type): New
>> enum.
>>  * config/aarch64/aarch64.c
>> (aarch64_attach_ra_auth_dwarf_note): New function.
>>  (aarch64_attach_ra_auth_dwarf_general): New function.
>>  (aarch64_attach_ra_auth_dwarf_shortcut): New function.
>>  (aarch64_save_callee_saves): Generate dwarf information if LR
>> is signed.
>>  (aarch64_expand_prologue): Likewise.
>>  (aarch64_expand_epilogue): Likewise.
> 
> This patch is an update on DWARF generation for return address signing.
> 
> According to new proposal, we simply needs to generate REG_CFA_WINDOW_SAVE
> annotation.
> 
> gcc/
> 
> 2017-01-06  Jiong Wang  
> 
> * config/aarch64/aarch64.c (aarch64_expand_prologue): Generate
> dwarf
> annotation (REG_CFA_WINDOW_SAVE) for return address signing.
> (aarch64_expand_epilogue): Likewise.
> 
> 

I don't think we should be overloading REG_CFA_WINDOW_SAVE internally in
the compiler -- it's one thing to do it in the dwarf output tables, but
quite another to be doing it elsewhere in the compiler.

Instead we should create a new reg note kind and use that, but in the
final dwarf output then emit the overloaded opcode.

R.

> 
> 2.patch
> 
> 
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index 
> 002895a167ce0deb45a5c1726527651af18bb4df..20ed79e5690f45ec121ef516245c686cc0cc82b5
>  100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -3553,7 +3553,11 @@ aarch64_expand_prologue (void)
>  
>/* Sign return address for functions.  */
>if (aarch64_return_address_signing_enabled ())
> -emit_insn (gen_pacisp ());
> +{
> +  insn = emit_insn (gen_pacisp ());
> +  add_reg_note (insn, REG_CFA_WINDOW_SAVE, const0_rtx);
> +  RTX_FRAME_RELATED_P (insn) = 1;
> +}
>  
>if (flag_stack_usage_info)
>  current_function_static_stack_size = frame_size;
> @@ -3698,7 +3702,11 @@ aarch64_expand_epilogue (bool for_sibcall)
>   want to use the CFA of the function which calls eh_return.  */
>if (aarch64_return_address_signing_enabled ()
>&& (for_sibcall || !TARGET_ARMV8_3 || crtl->calls_eh_return))
> -emit_insn (gen_autisp ());
> +{
> +  insn = emit_insn (gen_autisp ());
> +  add_reg_note (insn, REG_CFA_WINDOW_SAVE, const0_rtx);
> +  RTX_FRAME_RELATED_P (insn) = 1;
> +}
>  
>/* Stack adjustment for exception handler.  */
>if (crtl->calls_eh_return)
> 

[2/5][AArch64] Generate dwarf information for -msign-return-address

[Jiong Wang]

On 11/11/16 18:22, Jiong Wang wrote:

This patch generate DWARF description for pointer authentication.  DWARF value
expression is used to describe the authentication action.

Please see the cover letter and AArch64 DWARF specification for the semantics
of AArch64 DWARF operations.

When authentication key index is A key, we use compact DWARF description which
can largely save DWARF frame size, otherwise we fallback to general operator.



Example
===

 int
 cal (int a, int b, int c)
 {
   return a + dec (b) + c;
 }

Compact DWARF description
(-march=armv8.3-a -msign-return-address)
===

   DW_CFA_advance_loc: 4 to 0004
   DW_CFA_val_expression: r30 (x30) (DW_OP_AARCH64_paciasp)
   DW_CFA_advance_loc: 4 to 0008
   DW_CFA_val_expression: r30 (x30) (DW_OP_AARCH64_paciasp_deref: -24)

General DWARF description
===
(-march=armv8.3-a -msign-return-address -mpauth-key=b_key)

   DW_CFA_advance_loc: 4 to 0004
   DW_CFA_val_expression: r30 (x30) (DW_OP_breg30 (x30): 0; 
DW_OP_AARCH64_pauth: 18)
   DW_CFA_advance_loc: 4 to 0008
   DW_CFA_val_expression: r30 (x30) (DW_OP_dup; DW_OP_const1s: -24; DW_OP_plus; 
DW_OP_deref; DW_OP_AARCH64_pauth: 18)

 From Linux kernel testing, -msign-return-address will introduce +24%
.debug_frame size increase when signing all functions and using compact
description, and about +45% .debug_frame size increase if using general
description.


gcc/
2016-11-11  Jiong Wang

 * config/aarch64/aarch64.h (aarch64_pauth_action_type): New enum.
 * config/aarch64/aarch64.c (aarch64_attach_ra_auth_dwarf_note): New 
function.
 (aarch64_attach_ra_auth_dwarf_general): New function.
 (aarch64_attach_ra_auth_dwarf_shortcut): New function.
 (aarch64_save_callee_saves): Generate dwarf information if LR is 
signed.
 (aarch64_expand_prologue): Likewise.
 (aarch64_expand_epilogue): Likewise.


This patch is an update on DWARF generation for return address signing.

According to new proposal, we simply needs to generate REG_CFA_WINDOW_SAVE
annotation.

gcc/

2017-01-06  Jiong Wang  

* config/aarch64/aarch64.c (aarch64_expand_prologue): Generate dwarf
annotation (REG_CFA_WINDOW_SAVE) for return address signing.
(aarch64_expand_epilogue): Likewise.


diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 002895a167ce0deb45a5c1726527651af18bb4df..20ed79e5690f45ec121ef516245c686cc0cc82b5 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -3553,7 +3553,11 @@ aarch64_expand_prologue (void)
 
   /* Sign return address for functions.  */
   if (aarch64_return_address_signing_enabled ())
-emit_insn (gen_pacisp ());
+{
+  insn = emit_insn (gen_pacisp ());
+  add_reg_note (insn, REG_CFA_WINDOW_SAVE, const0_rtx);
+  RTX_FRAME_RELATED_P (insn) = 1;
+}
 
   if (flag_stack_usage_info)
 current_function_static_stack_size = frame_size;
@@ -3698,7 +3702,11 @@ aarch64_expand_epilogue (bool for_sibcall)
  want to use the CFA of the function which calls eh_return.  */
   if (aarch64_return_address_signing_enabled ()
   && (for_sibcall || !TARGET_ARMV8_3 || crtl->calls_eh_return))
-emit_insn (gen_autisp ());
+{
+  insn = emit_insn (gen_autisp ());
+  add_reg_note (insn, REG_CFA_WINDOW_SAVE, const0_rtx);
+  RTX_FRAME_RELATED_P (insn) = 1;
+}
 
   /* Stack adjustment for exception handler.  */
   if (crtl->calls_eh_return)