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Date:   Wed, 9 Feb 2022 19:06:20 -0800
From:   Dan Li <ashimida@...ux.alibaba.com>
To:     gcc-patches@....gnu.org, richard.earnshaw@....com,
        marcus.shawcroft@....com, kyrylo.tkachov@....com, hp@....gnu.org,
        ndesaulniers@...gle.com, nsz@....gnu.org, pageexec@...il.com,
        qinzhao@....gnu.org, linux-hardening@...r.kernel.org,
        richard.sandiford@....com
Subject: Re: [PATCH] [PATCH,v4,1/1,AARCH64][PR102768] aarch64: Add compiler
 support for Shadow Call Stack



On 2/9/22 08:08, Richard Sandiford wrote:
> Dan Li <ashimida@...ux.alibaba.com> writes:
>> +
>> +  /* When shadow call stack is enabled, the scs_pop in the epilogue will
>> +     restore x30, and we don't need to pop x30 again in the traditional
>> +     way.  Pop candidates record the registers that need to be popped
>> +     eventually.  */
>> +  if (frame.is_scs_enabled)
>> +    {
>> +      if (frame.wb_push_candidate2 == R30_REGNUM)
>> +	frame.wb_pop_candidate2 = INVALID_REGNUM;
>> +      else if (frame.wb_push_candidate1 == R30_REGNUM)
>> +	frame.wb_pop_candidate1 = INVALID_REGNUM;
> 
> Although it makes no difference to the behaviour, I think it would be
> clearer to use pop rather than push in the checks here.
> 

Got it.
>> @@ -7885,8 +7914,8 @@ aarch64_save_callee_saves (poly_int64 start_offset,
>>         bool frame_related_p = aarch64_emit_cfi_for_reg_p (regno);
>>   
>>         if (skip_wb
>> -	  && (regno == cfun->machine->frame.wb_candidate1
>> -	      || regno == cfun->machine->frame.wb_candidate2))
>> +	  && (regno == cfun->machine->frame.wb_push_candidate1
>> +	      || regno == cfun->machine->frame.wb_push_candidate2))
>>   	continue;
>>   
>>         if (cfun->machine->reg_is_wrapped_separately[regno])
>> @@ -7996,8 +8025,8 @@ aarch64_restore_callee_saves (poly_int64 start_offset, unsigned start,
>>         rtx reg, mem;
>>   
>>         if (skip_wb
>> -	  && (regno == cfun->machine->frame.wb_candidate1
>> -	      || regno == cfun->machine->frame.wb_candidate2))
>> +	  && (regno == cfun->machine->frame.wb_push_candidate1
>> +	      || regno == cfun->machine->frame.wb_push_candidate2))
> 
> Shouldn't this be using pop rather than push?
> 

There might be a little difference:

- Using push candidates means that a register to be ignored in pop
candidates will not be emitted again during the "restore" (pop_candidates
should always be a subset of push_candidates, since popping a register
without a push might not make sense).

- Using pop candidates means that a registers to be ignored in pop
candidates will be re-emitted during the "restore". For example,
if we specify to ignore the x20 register in pop:

--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -7502,6 +7502,8 @@ aarch64_layout_frame (void)
         frame.wb_pop_candidate1 = INVALID_REGNUM;
      }
  
+  if (frame.wb_pop_candidate2 == R20_REGNUM)
+       frame.wb_pop_candidate2 = INVALID_REGNUM;
    /* If candidate2 is INVALID_REGNUM, we need to adjust max_push_offset to
       256 to ensure that the offset meets the requirements of emit_move_insn.
       Similarly, if candidate1 is INVALID_REGNUM, we need to set

With the test case:

int main(void)
{
         __asm__ ("":::"x19", "x20");
         return 0;
}

When we use "pop_candidate[12]", one more insn is emitted:

0000000000400604 <main>:
    400604:       a9bf53f3        stp     x19, x20, [sp, #-16]!
    400608:       52800000        mov     w0, #0x0
+  40060c:       f94007f4        ldr     x20, [sp, #8]
    400610:       f84107f3        ldr     x19, [sp], #16
    400614:       d65f03c0        ret

But in the case of ignoring a specific register (like scs ignores x30),
there is no difference between the two (because we always need
to explicitly specify which registers to ignore in the parameter of
aarch64_restore_callee_saves).

If pop looks better here, I'd like to change it to pop in the
next version :).

>> +  /* When shadow call stack is enabled, the scs_pop in the epilogue will
>> +     restore x30, we don't need to restore x30 again in the traditional
>> +     way.  */
>> +  if (cfun->machine->frame.is_scs_enabled)
>> +    aarch64_restore_callee_saves (callee_offset - sve_callee_adjust,
>> +				  R0_REGNUM, R29_REGNUM,
>> +				  callee_adjust != 0, &cfi_ops);
>> +  else
>> +    aarch64_restore_callee_saves (callee_offset - sve_callee_adjust,
>> +				  R0_REGNUM, R30_REGNUM,
>> +				  callee_adjust != 0, &cfi_ops);
>> +
> 
> Very minor, but I think it would be better to have:
> 
>    unsigned int last_gpr = (cfun->machine->frame.is_scs_enabled
> 			   ? R29_REGNUM : R30_REGNUM);
> 
> so that we don't need to repeat the other arguments.  There's then
> less risk of the two versions getting out of sync.
> 

Got it.

>>   
>>     if (need_barrier_p)
>>       emit_insn (gen_stack_tie (stack_pointer_rtx, stack_pointer_rtx));
>> @@ -9066,6 +9109,17 @@ aarch64_expand_epilogue (bool for_sibcall)
>>         RTX_FRAME_RELATED_P (insn) = 1;
>>       }
>>   
>> +  /* Pop return address from shadow call stack.  */
>> +  if (cfun->machine->frame.is_scs_enabled)
>> +    {
>> +      machine_mode mode = aarch64_reg_save_mode (R30_REGNUM);
>> +      rtx reg = gen_rtx_REG (mode, R30_REGNUM);
>> +
>> +      insn = emit_insn (gen_scs_pop ());
>> +      add_reg_note (insn, REG_CFA_RESTORE, reg);
>> +      RTX_FRAME_RELATED_P (insn) = 1;
>> +    }
>> +
>>     /* We prefer to emit the combined return/authenticate instruction RETAA,
>>        however there are three cases in which we must instead emit an explicit
>>        authentication instruction.
>> @@ -16492,6 +16546,10 @@ aarch64_override_options_internal (struct gcc_options *opts)
>>         aarch64_stack_protector_guard_offset = offs;
>>       }
>>   
>> +  if ((flag_sanitize & SANITIZE_SHADOW_CALL_STACK)
>> +      && !fixed_regs[R18_REGNUM])
>> +    error ("%<-fsanitize=shadow-call-stack%> requires %<-ffixed-x18%>");
>> +
>>     initialize_aarch64_code_model (opts);
>>     initialize_aarch64_tls_size (opts);
>>   
>> @@ -26505,6 +26563,9 @@ aarch64_libgcc_floating_mode_supported_p
>>   #undef TARGET_ASM_FUNCTION_EPILOGUE
>>   #define TARGET_ASM_FUNCTION_EPILOGUE aarch64_sls_emit_blr_function_thunks
>>   
>> +#undef TARGET_HAVE_SHADOW_CALL_STACK
>> +#define TARGET_HAVE_SHADOW_CALL_STACK true
>> +
>>   struct gcc_target targetm = TARGET_INITIALIZER;
>>   
>>   #include "gt-aarch64.h"
>> diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
>> index 2792bb29adb..b5efe083f30 100644
>> --- a/gcc/config/aarch64/aarch64.h
>> +++ b/gcc/config/aarch64/aarch64.h
>> @@ -906,9 +906,21 @@ struct GTY (()) aarch64_frame
>>   	 Indicated by CALLEE_ADJUST == 0 && EMIT_FRAME_CHAIN.
>>   
>>        These fields indicate which registers we've decided to handle using
>> -     (1) or (2), or INVALID_REGNUM if none.  */
>> -  unsigned wb_candidate1;
>> -  unsigned wb_candidate2;
>> +     (1) or (2), or INVALID_REGNUM if none.
>> +
>> +     In some cases we don't always need to pop all registers in the push
>> +     candidates, pop candidates record which registers need to be popped
>> +     eventually.  The initial value of a pop candidate is copied from its
>> +     corresponding push candidate.
>> +
>> +     Currently, the pop candidates are only used for shadow call stack.
> 
> Maybe s/the/different/, since the variables themselves are used
> regardless of -fsanitize.
> 

Got it.

Thanks,
Dan

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