Fix displaced stepping watchpoint check order

Message ID 20210608154230.354202-1-luis.machado@linaro.org
State New
Headers show
Series
  • Fix displaced stepping watchpoint check order
Related show

Commit Message

Metzger, Markus T via Gdb-patches June 8, 2021, 3:42 p.m.
When checking the stopped data address, I noticed, under some circumstances,
that the instruction at PC wasn't the expected one. This happens because the
displaced stepping machinery restores the buffer before checking if the
instruction executed successfully, which in turn calls the watchpoint check.

I guess this was never noticed because stopped data address checks usually
don't need to fetch the instruction at PC, but AArch64 needs to do it from
now on.

We should check if the instruction executed successfully before we restore the
scratchpad contents.

Regression tested on aarch64-linux/Ubuntu 20.04.

gdb/ChangeLog:

YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

	* displaced-stepping.c (displaced_step_buffers::finish): Move check
	upwards.
---
 gdb/displaced-stepping.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

-- 
2.25.1

Comments

Metzger, Markus T via Gdb-patches June 15, 2021, 2:09 p.m. | #1
On 6/8/21 12:42 PM, Luis Machado wrote:
> When checking the stopped data address, I noticed, under some circumstances,

> that the instruction at PC wasn't the expected one. This happens because the

> displaced stepping machinery restores the buffer before checking if the

> instruction executed successfully, which in turn calls the watchpoint check.

> 

> I guess this was never noticed because stopped data address checks usually

> don't need to fetch the instruction at PC, but AArch64 needs to do it from

> now on.

> 

> We should check if the instruction executed successfully before we restore the

> scratchpad contents.

> 

> Regression tested on aarch64-linux/Ubuntu 20.04.

> 

> gdb/ChangeLog:

> 

> YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

> 

> 	* displaced-stepping.c (displaced_step_buffers::finish): Move check

> 	upwards.

> ---

>   gdb/displaced-stepping.c | 8 +++++---

>   1 file changed, 5 insertions(+), 3 deletions(-)

> 

> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c

> index 59b78c22f6a..06324d523d8 100644

> --- a/gdb/displaced-stepping.c

> +++ b/gdb/displaced-stepping.c

> @@ -227,6 +227,11 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     ULONGEST len = gdbarch_max_insn_length (arch);

>   

> +  /* Check if the execution was successful before restoring the buffer

> +     contents.  */

> +  bool instruction_executed_successfully

> +    = displaced_step_instruction_executed_successfully (arch, sig);

> +

>     /* Restore memory of the buffer.  */

>     write_memory_ptid (thread->ptid, buffer->addr,

>   		     buffer->saved_copy.data (), len);

> @@ -237,9 +242,6 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     regcache *rc = get_thread_regcache (thread);

>   

> -  bool instruction_executed_successfully

> -    = displaced_step_instruction_executed_successfully (arch, sig);

> -

>     if (instruction_executed_successfully)

>       {

>         gdbarch_displaced_step_fixup (arch, copy_insn_closure.get (),

>
Metzger, Markus T via Gdb-patches June 22, 2021, 1:56 a.m. | #2
On 6/8/21 12:42 PM, Luis Machado wrote:
> When checking the stopped data address, I noticed, under some circumstances,

> that the instruction at PC wasn't the expected one. This happens because the

> displaced stepping machinery restores the buffer before checking if the

> instruction executed successfully, which in turn calls the watchpoint check.

> 

> I guess this was never noticed because stopped data address checks usually

> don't need to fetch the instruction at PC, but AArch64 needs to do it from

> now on.

> 

> We should check if the instruction executed successfully before we restore the

> scratchpad contents.

> 

> Regression tested on aarch64-linux/Ubuntu 20.04.

> 

> gdb/ChangeLog:

> 

> YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

> 

> 	* displaced-stepping.c (displaced_step_buffers::finish): Move check

> 	upwards.

> ---

>   gdb/displaced-stepping.c | 8 +++++---

>   1 file changed, 5 insertions(+), 3 deletions(-)

> 

> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c

> index 59b78c22f6a..06324d523d8 100644

> --- a/gdb/displaced-stepping.c

> +++ b/gdb/displaced-stepping.c

> @@ -227,6 +227,11 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     ULONGEST len = gdbarch_max_insn_length (arch);

>   

> +  /* Check if the execution was successful before restoring the buffer

> +     contents.  */

> +  bool instruction_executed_successfully

> +    = displaced_step_instruction_executed_successfully (arch, sig);

> +

>     /* Restore memory of the buffer.  */

>     write_memory_ptid (thread->ptid, buffer->addr,

>   		     buffer->saved_copy.data (), len);

> @@ -237,9 +242,6 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     regcache *rc = get_thread_regcache (thread);

>   

> -  bool instruction_executed_successfully

> -    = displaced_step_instruction_executed_successfully (arch, sig);

> -

>     if (instruction_executed_successfully)

>       {

>         gdbarch_displaced_step_fixup (arch, copy_insn_closure.get (),

>
Metzger, Markus T via Gdb-patches July 1, 2021, 1:53 p.m. | #3
On 6/8/21 12:42 PM, Luis Machado wrote:
> When checking the stopped data address, I noticed, under some circumstances,

> that the instruction at PC wasn't the expected one. This happens because the

> displaced stepping machinery restores the buffer before checking if the

> instruction executed successfully, which in turn calls the watchpoint check.

> 

> I guess this was never noticed because stopped data address checks usually

> don't need to fetch the instruction at PC, but AArch64 needs to do it from

> now on.

> 

> We should check if the instruction executed successfully before we restore the

> scratchpad contents.

> 

> Regression tested on aarch64-linux/Ubuntu 20.04.

> 

> gdb/ChangeLog:

> 

> YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

> 

> 	* displaced-stepping.c (displaced_step_buffers::finish): Move check

> 	upwards.

> ---

>   gdb/displaced-stepping.c | 8 +++++---

>   1 file changed, 5 insertions(+), 3 deletions(-)

> 

> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c

> index 59b78c22f6a..06324d523d8 100644

> --- a/gdb/displaced-stepping.c

> +++ b/gdb/displaced-stepping.c

> @@ -227,6 +227,11 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     ULONGEST len = gdbarch_max_insn_length (arch);

>   

> +  /* Check if the execution was successful before restoring the buffer

> +     contents.  */

> +  bool instruction_executed_successfully

> +    = displaced_step_instruction_executed_successfully (arch, sig);

> +

>     /* Restore memory of the buffer.  */

>     write_memory_ptid (thread->ptid, buffer->addr,

>   		     buffer->saved_copy.data (), len);

> @@ -237,9 +242,6 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     regcache *rc = get_thread_regcache (thread);

>   

> -  bool instruction_executed_successfully

> -    = displaced_step_instruction_executed_successfully (arch, sig);

> -

>     if (instruction_executed_successfully)

>       {

>         gdbarch_displaced_step_fixup (arch, copy_insn_closure.get (),

>
Metzger, Markus T via Gdb-patches July 23, 2021, 1:25 p.m. | #4
On 6/8/21 12:42 PM, Luis Machado wrote:
> When checking the stopped data address, I noticed, under some circumstances,

> that the instruction at PC wasn't the expected one. This happens because the

> displaced stepping machinery restores the buffer before checking if the

> instruction executed successfully, which in turn calls the watchpoint check.

> 

> I guess this was never noticed because stopped data address checks usually

> don't need to fetch the instruction at PC, but AArch64 needs to do it from

> now on.

> 

> We should check if the instruction executed successfully before we restore the

> scratchpad contents.

> 

> Regression tested on aarch64-linux/Ubuntu 20.04.

> 

> gdb/ChangeLog:

> 

> YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

> 

> 	* displaced-stepping.c (displaced_step_buffers::finish): Move check

> 	upwards.

> ---

>   gdb/displaced-stepping.c | 8 +++++---

>   1 file changed, 5 insertions(+), 3 deletions(-)

> 

> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c

> index 59b78c22f6a..06324d523d8 100644

> --- a/gdb/displaced-stepping.c

> +++ b/gdb/displaced-stepping.c

> @@ -227,6 +227,11 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     ULONGEST len = gdbarch_max_insn_length (arch);

>   

> +  /* Check if the execution was successful before restoring the buffer

> +     contents.  */

> +  bool instruction_executed_successfully

> +    = displaced_step_instruction_executed_successfully (arch, sig);

> +

>     /* Restore memory of the buffer.  */

>     write_memory_ptid (thread->ptid, buffer->addr,

>   		     buffer->saved_copy.data (), len);

> @@ -237,9 +242,6 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>   

>     regcache *rc = get_thread_regcache (thread);

>   

> -  bool instruction_executed_successfully

> -    = displaced_step_instruction_executed_successfully (arch, sig);

> -

>     if (instruction_executed_successfully)

>       {

>         gdbarch_displaced_step_fixup (arch, copy_insn_closure.get (),

>
Metzger, Markus T via Gdb-patches July 29, 2021, 7:36 p.m. | #5
I think this is ok, but in all honestly I don't completely understand
how the interaction between watchpoints and displaced stepping is
expected to work.

Just some nits:

On 2021-06-08 11:42 a.m., Luis Machado via Gdb-patches wrote:
> When checking the stopped data address, I noticed, under some circumstances,

> that the instruction at PC wasn't the expected one. This happens because the

> displaced stepping machinery restores the buffer before checking if the

> instruction executed successfully, which in turn calls the watchpoint check.

> 

> I guess this was never noticed because stopped data address checks usually

> don't need to fetch the instruction at PC, but AArch64 needs to do it from

> now on.


Can you clarify what you mean by "from now on"?  Can you indicate what
change you are referring to?

> 

> We should check if the instruction executed successfully before we restore the

> scratchpad contents.

> 

> Regression tested on aarch64-linux/Ubuntu 20.04.

> 

> gdb/ChangeLog:

> 

> YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

> 

> 	* displaced-stepping.c (displaced_step_buffers::finish): Move check

> 	upwards.

> ---

>  gdb/displaced-stepping.c | 8 +++++---

>  1 file changed, 5 insertions(+), 3 deletions(-)

> 

> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c

> index 59b78c22f6a..06324d523d8 100644

> --- a/gdb/displaced-stepping.c

> +++ b/gdb/displaced-stepping.c

> @@ -227,6 +227,11 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>  

>    ULONGEST len = gdbarch_max_insn_length (arch);

>  

> +  /* Check if the execution was successful before restoring the buffer

> +     contents.  */

> +  bool instruction_executed_successfully

> +    = displaced_step_instruction_executed_successfully (arch, sig);


Maybe extend the comment to say "why".  Right now I think it just states
what is in plain sight when looking at the code, I think it would be
more useful if it said why it's important to do that.

Simon
Metzger, Markus T via Gdb-patches July 29, 2021, 8:17 p.m. | #6
Hi,

On 7/29/21 4:36 PM, Simon Marchi wrote:
> I think this is ok, but in all honestly I don't completely understand

> how the interaction between watchpoints and displaced stepping is

> expected to work.


Let me try to clarify. When we attempt to execute an instruction in the 
scratch space (displaced stepping), we may potentially trigger a 
hardware watchpoint.

For AArch64, hardware watchpoints are non-steppable, so that means GDB 
will need to step over that hardware watchpoint so the instruction's 
execution completes (if there is no hardware watchpoint trigger, the 
instruction gets executed just fine).

Now, if the hardware watchpoint trigger did happen (and GDB detects that 
properly), then displaced_step_instruction_executed_successfully (...) 
will return false.

The above check happens after we restore the displaced stepping buffer 
contents. So the original instruction that caused the hardware 
watchpoint trigger is gone. That is fine if we don't have to look at the 
instruction being stepped-over.

> 

> Just some nits:

> 

> On 2021-06-08 11:42 a.m., Luis Machado via Gdb-patches wrote:

>> When checking the stopped data address, I noticed, under some circumstances,

>> that the instruction at PC wasn't the expected one. This happens because the

>> displaced stepping machinery restores the buffer before checking if the

>> instruction executed successfully, which in turn calls the watchpoint check.

>>

>> I guess this was never noticed because stopped data address checks usually

>> don't need to fetch the instruction at PC, but AArch64 needs to do it from

>> now on.

> 

> Can you clarify what you mean by "from now on"?  Can you indicate what

> change you are referring to?

> 


 From the following change 
(https://sourceware.org/pipermail/gdb-patches/2021-July/181095.html) 
onwards, we need to look at the load/store instruction to figure out the 
memory access size so we can reliably tell if a hardware watchpoint has 
triggered. This is due to how AArch64's spec defines how to provide a 
stopped data address, and the valid ranges.

With the old code, if we try to fetch the instruction at PC, we will get 
a bogus value that is not the real instruction that caused the hardware 
watchpoint trigger. Hence why the patch moves the call to 
displaced_step_instruction_executed_successfully (...) up and before we 
restore the displaced stepping buffer.

If a hardware watchpoint trigger takes place and GDB doesn't recognize 
it, then displaced_step_instruction_executed_successfully (...) will 
return true and GDB will move on and will attempt to execute the same 
instruction again, only to be halted due to the same hardware watchpoint 
trigger that it can't detect. So GDB gets into an infinite loop.

More generally, if we ever fail to acknowledge a hardware watchpoint 
trigger on an architecture with non-steppable watchpoints and displaced 
stepping support, we will run into this infinite loop (as far as I can 
tell).

Does that make sense?

>>

>> We should check if the instruction executed successfully before we restore the

>> scratchpad contents.

>>

>> Regression tested on aarch64-linux/Ubuntu 20.04.

>>

>> gdb/ChangeLog:

>>

>> YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

>>

>> 	* displaced-stepping.c (displaced_step_buffers::finish): Move check

>> 	upwards.

>> ---

>>   gdb/displaced-stepping.c | 8 +++++---

>>   1 file changed, 5 insertions(+), 3 deletions(-)

>>

>> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c

>> index 59b78c22f6a..06324d523d8 100644

>> --- a/gdb/displaced-stepping.c

>> +++ b/gdb/displaced-stepping.c

>> @@ -227,6 +227,11 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,

>>   

>>     ULONGEST len = gdbarch_max_insn_length (arch);

>>   

>> +  /* Check if the execution was successful before restoring the buffer

>> +     contents.  */

>> +  bool instruction_executed_successfully

>> +    = displaced_step_instruction_executed_successfully (arch, sig);

> 

> Maybe extend the comment to say "why".  Right now I think it just states

> what is in plain sight when looking at the code, I think it would be

> more useful if it said why it's important to do that.


I can expand it to make it more clear.

> 

> Simon

>
Metzger, Markus T via Gdb-patches July 30, 2021, 12:59 a.m. | #7
On 2021-07-29 4:17 p.m., Luis Machado wrote:
> Hi,

> 

> On 7/29/21 4:36 PM, Simon Marchi wrote:

>> I think this is ok, but in all honestly I don't completely understand

>> how the interaction between watchpoints and displaced stepping is

>> expected to work.

> 

> Let me try to clarify. When we attempt to execute an instruction in the scratch space (displaced stepping), we may potentially trigger a hardware watchpoint.

> 

> For AArch64, hardware watchpoints are non-steppable, so that means GDB will need to step over that hardware watchpoint so the instruction's execution completes (if there is no hardware watchpoint trigger, the instruction gets executed just fine).


What does "need to step over that hardware watchpoint" means,
concretely?  After the watchpoint has triggered, are the side effects of
the instruction committed to memory and registers?  Or are we in a state
as if the instruction didn't complete?  How is that step over done?

>>

>> Just some nits:

>>

>> On 2021-06-08 11:42 a.m., Luis Machado via Gdb-patches wrote:

>>> When checking the stopped data address, I noticed, under some circumstances,

>>> that the instruction at PC wasn't the expected one. This happens because the

>>> displaced stepping machinery restores the buffer before checking if the

>>> instruction executed successfully, which in turn calls the watchpoint check.

>>>

>>> I guess this was never noticed because stopped data address checks usually

>>> don't need to fetch the instruction at PC, but AArch64 needs to do it from

>>> now on.

>>

>> Can you clarify what you mean by "from now on"?  Can you indicate what

>> change you are referring to?

>>

> 

> From the following change (https://sourceware.org/pipermail/gdb-patches/2021-July/181095.html) onwards, we need to look at the load/store instruction to figure out the memory access size so we can reliably tell if a hardware watchpoint has triggered. This is due to how AArch64's spec defines how to provide a stopped data address, and the valid ranges.


Ok, but that patch you linked isn't merged yet?  So it sounds strange to
say "from now on", it sounds like there's a dependency between the two
patches.  Let's say the current patch is merged before the other one,
maybe it should say "but AArch64 will need to do it it an upcoming
patch", and then you can given the link.

> With the old code, if we try to fetch the instruction at PC, we will get a bogus value that is not the real instruction that caused the hardware watchpoint trigger. Hence why the patch moves the call to displaced_step_instruction_executed_successfully (...) up and before we restore the displaced stepping buffer.

> 

> If a hardware watchpoint trigger takes place and GDB doesn't recognize it, then displaced_step_instruction_executed_successfully (...) will return true and GDB will move on and will attempt to execute the same instruction again, only to be halted due to the same hardware watchpoint trigger that it can't detect. So GDB gets into an infinite loop.

> 

> More generally, if we ever fail to acknowledge a hardware watchpoint trigger on an architecture with non-steppable watchpoints and displaced stepping support, we will run into this infinite loop (as far as I can tell).

> 

> Does that make sense?


Yes, this help.  Please feel free to include in the commit message any
additional detail that you gave here, since it might help somebody else
in the future.

The code change is fine, so once you send a revised version it should go
fairly quickly.  Sorry for the delay.

Thanks!

Simon
Metzger, Markus T via Gdb-patches July 30, 2021, 1:32 a.m. | #8
On 7/29/21 9:59 PM, Simon Marchi wrote:
> 

> 

> On 2021-07-29 4:17 p.m., Luis Machado wrote:

>> Hi,

>>

>> On 7/29/21 4:36 PM, Simon Marchi wrote:

>>> I think this is ok, but in all honestly I don't completely understand

>>> how the interaction between watchpoints and displaced stepping is

>>> expected to work.

>>

>> Let me try to clarify. When we attempt to execute an instruction in the scratch space (displaced stepping), we may potentially trigger a hardware watchpoint.

>>

>> For AArch64, hardware watchpoints are non-steppable, so that means GDB will need to step over that hardware watchpoint so the instruction's execution completes (if there is no hardware watchpoint trigger, the instruction gets executed just fine).

> 

> What does "need to step over that hardware watchpoint" means,

> concretely?  After the watchpoint has triggered, are the side effects of

> the instruction committed to memory and registers?  Or are we in a state

> as if the instruction didn't complete?  How is that step over done?


No side effects of the instruction are committed in this case. Memory 
and registers will have their old values as if the instruction didn't 
execute.

 From reading the code, most architectures have non-steppable hardware 
watchpoints. GDB just disables the hardware watchpoints, single-steps 
past that instruction and then enables the hardware watchpoints again.

GDB just disables all hardware watchpoints for the sake of simplicity. 
You can see this logic in infrun.c:handle_signal_stop, around this comment:

/* If necessary, step over this watchpoint.  We'll be back to display
    it in a moment.  */
if (stopped_by_watchpoint
     && (target_have_steppable_watchpoint ()
         || gdbarch_have_nonsteppable_watchpoint (gdbarch)))

It is actually best to disable all hardware watchpoints. If we end up 
disabling just one hardware watchpoint, and then we happen to have 
another hardware watchpoint that is active and also getting triggered, 
we might be stuck in an endless loop as well.

> 

>>>

>>> Just some nits:

>>>

>>> On 2021-06-08 11:42 a.m., Luis Machado via Gdb-patches wrote:

>>>> When checking the stopped data address, I noticed, under some circumstances,

>>>> that the instruction at PC wasn't the expected one. This happens because the

>>>> displaced stepping machinery restores the buffer before checking if the

>>>> instruction executed successfully, which in turn calls the watchpoint check.

>>>>

>>>> I guess this was never noticed because stopped data address checks usually

>>>> don't need to fetch the instruction at PC, but AArch64 needs to do it from

>>>> now on.

>>>

>>> Can you clarify what you mean by "from now on"?  Can you indicate what

>>> change you are referring to?

>>>

>>

>>  From the following change (https://sourceware.org/pipermail/gdb-patches/2021-July/181095.html) onwards, we need to look at the load/store instruction to figure out the memory access size so we can reliably tell if a hardware watchpoint has triggered. This is due to how AArch64's spec defines how to provide a stopped data address, and the valid ranges.

> 

> Ok, but that patch you linked isn't merged yet?  So it sounds strange to

> say "from now on", it sounds like there's a dependency between the two


Absolutely. There is a dependency. My plan is to merge this fix first, 
and then merge the AArch64 hardware watchpoint detection fixes. I just 
didn't group those together, but that's the right order. I'll make sure 
to point out the dependency in the other patch.

> patches.  Let's say the current patch is merged before the other one,

> maybe it should say "but AArch64 will need to do it it an upcoming

> patch", and then you can given the link.

> 

>> With the old code, if we try to fetch the instruction at PC, we will get a bogus value that is not the real instruction that caused the hardware watchpoint trigger. Hence why the patch moves the call to displaced_step_instruction_executed_successfully (...) up and before we restore the displaced stepping buffer.

>>

>> If a hardware watchpoint trigger takes place and GDB doesn't recognize it, then displaced_step_instruction_executed_successfully (...) will return true and GDB will move on and will attempt to execute the same instruction again, only to be halted due to the same hardware watchpoint trigger that it can't detect. So GDB gets into an infinite loop.

>>

>> More generally, if we ever fail to acknowledge a hardware watchpoint trigger on an architecture with non-steppable watchpoints and displaced stepping support, we will run into this infinite loop (as far as I can tell).

>>

>> Does that make sense?

> 

> Yes, this help.  Please feel free to include in the commit message any

> additional detail that you gave here, since it might help somebody else

> in the future.


I'll make the commit message more detailed.

> 

> The code change is fine, so once you send a revised version it should go

> fairly quickly.  Sorry for the delay.


No worries. Thanks for the feedback!

> 

> Thanks!

> 

> Simon

>

Patch

diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c
index 59b78c22f6a..06324d523d8 100644
--- a/gdb/displaced-stepping.c
+++ b/gdb/displaced-stepping.c
@@ -227,6 +227,11 @@  displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,
 
   ULONGEST len = gdbarch_max_insn_length (arch);
 
+  /* Check if the execution was successful before restoring the buffer
+     contents.  */
+  bool instruction_executed_successfully
+    = displaced_step_instruction_executed_successfully (arch, sig);
+
   /* Restore memory of the buffer.  */
   write_memory_ptid (thread->ptid, buffer->addr,
 		     buffer->saved_copy.data (), len);
@@ -237,9 +242,6 @@  displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,
 
   regcache *rc = get_thread_regcache (thread);
 
-  bool instruction_executed_successfully
-    = displaced_step_instruction_executed_successfully (arch, sig);
-
   if (instruction_executed_successfully)
     {
       gdbarch_displaced_step_fixup (arch, copy_insn_closure.get (),