Re: [PATCH] gdb/reverse: Fix stepping over recursive functions

[Bruno Larsen <blarsen@redhat.com>]

On 7/7/22 15:48, Pedro Alves wrote:
> On 2022-05-25 19:02, Bruno Larsen via Gdb-patches wrote:
>> Currently, when using GDB to do reverse debugging, if we try to use the
>> command "reverse next" to skip a recursive function, instead of skipping
>> all of the recursive calls and stopping in the previous line, we stop at
>> the second to last recursive call, and need to manually step backwards
>> until we leave the first call.  This is well documented in PR gdb/16678.
>>
>> This bug happens because when GDB notices that a reverse step has
>> entered into a function, GDB will add a step_resume_breakpoint at the
>> start of the function, then single step out of the prologue once that
>> breakpoint is hit.  Recursion poses a problem because that breakpoint will
>> be hit many times before GDB should actually stop the inferior.  To fix
>> this issue, when the step_resume_breakpoint is hit (and GDB is executing
>> backwards), we analyze if the caller of the frame is the original frame
>> where we started the "reverse next", and if it is, GDB will stop the
>> inferior, otherwise GDB will just ignore the breakpoint.
>>
>> This commit also changes gdb.reverse/step-reverse.exp to contain a
>> recursive function and attempt to both, skip it altogether, and to skip
>> the second call from inside the first call, as this setup broke a
>> previous version of the patch.
>> ---
>>   gdb/infrun.c                               | 24 +++++++++
>>   gdb/testsuite/gdb.reverse/step-reverse.c   | 16 +++++-
>>   gdb/testsuite/gdb.reverse/step-reverse.exp | 59 ++++++++++++++++++++--
>>   3 files changed, 95 insertions(+), 4 deletions(-)
>>
>> diff --git a/gdb/infrun.c b/gdb/infrun.c
>> index 02c98b50c8c..95fb3227aa3 100644
>> --- a/gdb/infrun.c
>> +++ b/gdb/infrun.c
>> @@ -6761,6 +6761,30 @@ process_event_stop_test (struct execution_control_state *ecs)
>>       case BPSTAT_WHAT_STEP_RESUME:
>>         infrun_debug_printf ("BPSTAT_WHAT_STEP_RESUME");
>>   
>> +      /* If we are executing backwards, we are doing a "next" and the
>> +	 current frame is not the same as where we started, the
>> +	 step_resume_breakpoint we have just hit has been added to the start
>> +	 of a function so we can skip the whle function.  However, if we are
> 
> whle -> whole
> 
>> +	 skipping a recursive call, we only want to act as if we hit the
>> +	 breakpoint only when the caller of the current frame is the original
>> +	 frame we were single stepping from.  */
> 
> This wording confused me, took me a while to understand what is meant.  I suggest:
> 
>        /* If we are executing a reverse next, and going backwards entered
>           a different frame, we've set a step_resume_breakpoint at the start
> 	 of the function so we can skip the whole function.  However, if we are
> 	 skipping a recursive call, we want to ignore the breakpoint hit until
>           the caller of the current frame is the original frame we were nexting.  */
> 
> However, see further below.
> 
>> +      if (execution_direction == EXEC_REVERSE
>> +	  && ecs->event_thread->control.step_over_calls == STEP_OVER_ALL
>> +	  && !frame_id_eq (get_stack_frame_id (get_current_frame ()),
>> +			   ecs->event_thread->control.step_stack_frame_id))
>> +	{
>> +	  infrun_debug_printf ("We are not done with recursing yet");
>> +	  /* If the caller's ID is not the same as the starting frame, we
>> +	     can ignore this breakpoint.  */
>> +	  if (!frame_id_eq (frame_unwind_caller_id (get_current_frame ()),
>> +			    ecs->event_thread->control.step_stack_frame_id))
>> +	    {
>> +	      keep_going (ecs);
>> +	      return;
>> +	    }
>> +	} else
>> +	    infrun_debug_printf ("are we not recursing?");
> 
> This last else is odd -- we will print it if execution_direction != EXEC_REVERSE, for
> example.

Oops, this was for my own debugging, wasn't meant to be sent

> 
> I also don't understand why do we need all these different frame checks as above.  Breakpoints
> can have a frame associated, so that when a breakpoint hits, the breakpoint.c stuff checks
> whether the hit happened in the right frame, and if not, the breakpoint is ignored / set to
> be auto-stepped-over, and BPSTAT_WHAT_STEP_RESUME isn't reached.
> 
> The actual problem seems to me that we create the step-resume breakpoint in
> question without giving it a frame.  If I do that instead of your fix, like so:
> 
> diff --git c/gdb/infrun.c w/gdb/infrun.c
> index 02c98b50c8c..1360b8da70e 100644
> --- c/gdb/infrun.c
> +++ w/gdb/infrun.c
> @@ -7118,8 +7118,8 @@ process_event_stop_test (struct execution_control_state *ecs)
>                    symtab_and_line sr_sal;
>                    sr_sal.pc = ecs->stop_func_start;
>                    sr_sal.pspace = get_frame_program_space (frame);
> -                 insert_step_resume_breakpoint_at_sal (gdbarch,
> -                                                       sr_sal, null_frame_id);
> +                 insert_step_resume_breakpoint_at_sal
> +                   (gdbarch, sr_sal, get_stack_frame_id (frame));
>                  }
>              }
>            else
> 
> and then run your updated testcase, it passes.  I also see no regressions in gdb.reverse/ tests.

Wait... GDB can _do_ that? Well, this makes the solution much easier! Sorry for making you
re-do my solution. This looks like a much better solution

> 
> WDYT?  Are you seeing something that would break with this approach?
> 
>> +
>>         delete_step_resume_breakpoint (ecs->event_thread);
>>         if (ecs->event_thread->control.proceed_to_finish
>>   	  && execution_direction == EXEC_REVERSE)
>> diff --git a/gdb/testsuite/gdb.reverse/step-reverse.c b/gdb/testsuite/gdb.reverse/step-reverse.c
>> index aea2a98541d..a390ac2580c 100644
>> --- a/gdb/testsuite/gdb.reverse/step-reverse.c
>> +++ b/gdb/testsuite/gdb.reverse/step-reverse.c
>> @@ -26,6 +26,17 @@ int callee() {		/* ENTER CALLEE */
>>     return myglob++;	/* ARRIVED IN CALLEE */
>>   }			/* RETURN FROM CALLEE */
>>   
>> +/* We need to make this function take more than a single instruction
>> +   to run, otherwise it could hide PR gdb/16678, as reverse execution can
>> +   step over a single-instruction function.  */
>> +int recursive_callee (int val){
>> +    if (val == 0) return 0;
>> +    val /= 2;
>> +    if (val>1)
>> +	val++;
>> +    return recursive_callee (val);	/* RECURSIVE CALL */
>> +} /* EXIT RECURSIVE FUNCTION */
>> +
>>   /* A structure which, we hope, will need to be passed using memcpy.  */
>>   struct rhomboidal {
>>     int rather_large[100];
>> @@ -51,6 +62,9 @@ int main () {
>>      y = y + 4;
>>      z = z + 5;	/* STEP TEST 2 */
>>   
>> +   /* Test that next goes over recursive calls too */
>> +   recursive_callee (32); /* NEXT OVER THIS RECURSION */
>> +
>>      /* Test that "next" goes over a call */
>>      callee();	/* NEXT OVER THIS CALL */
>>   
>> @@ -60,7 +74,7 @@ int main () {
>>      /* Test "stepi" */
>>      a[5] = a[3] - a[4]; /* FINISH TEST */
>>      callee();	/* STEPI TEST */
>> -
>> +
>>      /* Test "nexti" */
>>      callee();	/* NEXTI TEST */
>>   
>> diff --git a/gdb/testsuite/gdb.reverse/step-reverse.exp b/gdb/testsuite/gdb.reverse/step-reverse.exp
>> index 997b62604d5..4f56b4785ca 100644
>> --- a/gdb/testsuite/gdb.reverse/step-reverse.exp
>> +++ b/gdb/testsuite/gdb.reverse/step-reverse.exp
>> @@ -47,9 +47,11 @@ gdb_test "step" ".*STEP TEST 1.*" "step test 1"
>>   gdb_test "next 2" ".*NEXT TEST 2.*" "next test 2"
>>   gdb_test "step 3" ".*STEP TEST 2.*" "step test 2"
>>   
>> +# next through a recursive function call
>> +gdb_test "next 2" "NEXT OVER THIS CALL.*" "next over recursion"
>> +
>>   # step over call
>>   
>> -gdb_test "step" ".*NEXT OVER THIS CALL.*" "step up to call"
>>   gdb_test "next" ".*STEP INTO THIS CALL.*" "next over call"
>>   
>>   # step into call
>> @@ -118,7 +120,7 @@ gdb_test_multiple "stepi" "$test_message" {
>>   
>>   set test_message "stepi back from function call"
>>   gdb_test_multiple "stepi" "$test_message" {
>> -    -re "NEXTI TEST.*$gdb_prompt $" {
>> +    -re -wrap "NEXTI TEST.*" {
>>   	pass "$test_message"
>>       }
>>       -re "ARRIVED IN CALLEE.*$gdb_prompt $" {
>> @@ -143,7 +145,6 @@ gdb_test_multiple "stepi" "$test_message" {
>>   ###
>>   
>>   # Set reverse execution direction
>> -
>>   gdb_test_no_output "set exec-dir reverse" "set reverse execution"
>>   
>>   # stepi backward thru return and into a function
>> @@ -247,6 +248,58 @@ gdb_test_multiple "step" "$test_message" {
>>   
>>   gdb_test "next" ".*NEXT OVER THIS CALL.*" "reverse next over call"
>>   
>> +# Dont reverse the execution direction yet, as we will need another
>> +# forward step after this test
> 
> - Dont -> Don't
> 
> - Add missing period.
> 
> 
>> +
>> +set step_out 0
>> +gdb_test_multiple "next" "reverse next over recursion" {
>> +    -re -wrap ".*NEXT OVER THIS RECURSION.*" {
>> +	pass "reverse next over recursion"
>> +    }
>> +    -re -wrap ".*RECURSIVE CALL.*" {
>> +	fail "reverse next over recursion"
>> +	set step_out 1
>> +    }
>> +}
> 
> Please use $gdb_test_name.
> 
>> +if { "$step_out" == 1 } {
>> +    gdb_test_multiple "next" "stepping out of recursion" {
> 
> "stepping out" -> "step out"
> 
>> +	-re -wrap "NEXT OVER THIS RECURSION.*" {
>> +	    set step_out 0
>> +	}
>> +	-re -wrap ".*" {
>> +	    send_gdb "reverse-next\n"
>> +	    exp_continue
>> +	}
>> +    }
>> +}
> 
> No pass call?  I guess I see -- this whole $step_out thing is to kind of
> handle the previous test's failure?  Why do we have this at all?

This is used to re-sync GDB, in case the previous test failed. I didn't want
to add a pass because I wanted the expected passes to decrease if we got the
failure.

I can add the pass if you think it is important, but I will definitely document
why this is here.

> 
>> +
>> +# Step forward over recursion again so we can test stepping over calls
>> +# inside the recursion itself.
>> +gdb_test_no_output "set exec-dir forward" "forward again to test recursion"
>> +gdb_test "next" "NEXT OVER THIS CALL.*" "reverse next over recursion again"
> 
> This "next" wasn't a reverse next.
> 
>> +gdb_test_no_output "set exec-dir reverse" "reverse again to test recursion"
>> +
>> +gdb_test "step" ".*EXIT RECURSIVE FUNCTION.*" "enter recursive function"
>> +set step_pass 1
>> +gdb_test_multiple "next" "step over recursion inside the recursion" {
>> +    -re -wrap ".*EXIT RECURSIVE FUNCTION.*" {
>> +	set step_pass 0
>> +	send_gdb "next\n"
>> +	exp_continue
>> +    }
>> +    -re -wrap ".*NEXT OVER THIS RECURSION.*" {
>> +	if {$step_pass} {
>> +	    pass "step over recursion inside the recursion"
>> +	} else {
>> +	    fail "step over recursion inside the recursion"
>> +	}
> 
>          gdb_assert $step_pass $gdb_test_name
> 
>> +    }
>> +    -re -wrap ".*" {
>> +	send_gdb "next\n"
>> +	exp_continue
>> +    }
>> +}
>> +
>>   # step/next backward with count
>>   
> 
> 
>>   gdb_test "step 3" ".*REVERSE STEP TEST 1.*" "reverse step test 1"
> 
> I actually have to leave now, and haven't had a chance to finish looking at the testcase
> in much detail.  But figured it's best to press send anyhow.  So...  <send>.

It really was! I'll take a second look over my changes and send a v2 soon, fixing everything
I haven't responded to.


Cheers!
Bruno Larsen
> 
> Pedro Alves
>