[PATCHv2] Fix an undefined behavior in record_line

Bernd Edlinger bernd.edlinger@hotmail.de
Tue Mar 24 10:20:25 GMT 2020



On 3/24/20 10:10 AM, Andrew Burgess wrote:
> * Bernd Edlinger <bernd.edlinger@hotmail.de> [2020-03-23 22:25:42 +0100]:
> 
>> On 3/22/20 4:25 AM, Bernd Edlinger wrote:
>>> On 3/13/20 12:55 PM, Bernd Edlinger wrote:
>>>> Additionally do not completely remove symbols
>>>> at the same PC than the end marker, instead
>>>> make them non-is-stmt breakpoints.
>>>>
>>>> Also fix the condition when the line table need to be resized,
>>>> that was wasting one element.
> 
> I suspect this commit message has evolved overtime - having the first
> word be "additionally" seems a little strange.
> 

I'll re-think the commit message, thanks.

>>>>
>>>> 2020-03-10  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>>>> 	* buildsym.c (record_line): Fix ub and preserve lines at eof.
> 
> Typo: ub -> up
> 
>>>> ---
>>>>  gdb/buildsym.c | 28 +++++++++++-----------------
>>>>  1 file changed, 11 insertions(+), 17 deletions(-)
>>>>
>>>> diff --git a/gdb/buildsym.c b/gdb/buildsym.c
>>>> index 7155db3..960a36c 100644
>>>> --- a/gdb/buildsym.c
>>>> +++ b/gdb/buildsym.c
>>>> @@ -695,7 +695,7 @@ struct blockvector *
>>>>  	}
>>>>      }
>>>>  
>>>> -  if (subfile->line_vector->nitems + 1 >= subfile->line_vector_length)
>>>> +  if (subfile->line_vector->nitems >= subfile->line_vector_length)
>>>>      {
>>>>        subfile->line_vector_length *= 2;
>>>>        subfile->line_vector = (struct linetable *)
>>>> @@ -705,27 +705,21 @@ struct blockvector *
>>>>  		      * sizeof (struct linetable_entry))));
>>>>      }
> 
> This part seems separate to what comes below I think.  This should be
> a separate commit.
> 

Okay, good point.  That should be easy.

>>>>  
>>>> -  /* Normally, we treat lines as unsorted.  But the end of sequence
>>>> -     marker is special.  We sort line markers at the same PC by line
>>>> -     number, so end of sequence markers (which have line == 0) appear
>>>> -     first.  This is right if the marker ends the previous function,
>>>> -     and there is no padding before the next function.  But it is
>>>> -     wrong if the previous line was empty and we are now marking a
>>>> -     switch to a different subfile.  We must leave the end of sequence
>>>> -     marker at the end of this group of lines, not sort the empty line
>>>> -     to after the marker.  The easiest way to accomplish this is to
>>>> -     delete any empty lines from our table, if they are followed by
>>>> -     end of sequence markers.  All we lose is the ability to set
>>>> -     breakpoints at some lines which contain no instructions
>>>> -     anyway.  */
>>>> +  /* The end of sequence marker is special.  We need to reset the
>>>> +     is_stmt flag on previous lines at the same PC, otherwise these
>>>> +     lines may cause problems.  All we lose is the ability to set
>>>> +     breakpoints at some lines which contain no instructions
>>>> -     anyway.  */
> 
> You need to expand on what "problems" means here.  Someone coming back
> to this code in the future will have no idea why we're making this
> change, and with no tests for this commit they can't even try to
> figure out the "problems" by looking at a test.
> 

I will try to explain that better, yes.

>>>>    if (line == 0 && subfile->line_vector->nitems > 0)
>>>>      {
>>>> -      e = subfile->line_vector->item + subfile->line_vector->nitems - 1;
>>>> -      while (subfile->line_vector->nitems > 0 && e->pc == pc)
>>>> +      e = subfile->line_vector->item + subfile->line_vector->nitems;
>>>> +      do
>>>>  	{
>>>>  	  e--;
>>>> -	  subfile->line_vector->nitems--;
>>>> +	  if (e->pc != pc || e->line == 0)
>>>> +	    break;
>>>> +	  e->is_stmt = 0;
>>>>  	}
>>>> +      while (e > subfile->line_vector->item);
>>>>      }
>>>>  
>>>>    e = subfile->line_vector->item + subfile->line_vectoms++;
>>>>
>>
>> Andrew, this is the place where currently the is-stmt entries
>> are deleted.  With your is-stmt patch this code is executed in more
>> cases than before.  Therefore I would suggest to convert them
>> to !is_stmt lines for now, but maybe in the long run add a new flag
>> that allows them to be used in the file:line case, but make these
>> lines behave differently when stepping, I am only trying to fix
>> the case where you step out of the subroutine.
> 
> I'm super uncomfortable with any code that changes is-stmt to
> !is-stmt, as I worry about what we might be giving up.  You say "All
> we lose is the ability to set breakpoints at some lines which contain
> no instructions anyway.", but I'll need to work through some examples
> to see what this actually means in practice before I can be happy with
> this change.
> 

There is no pressure from my side to do anything about it.
I am just saying is-stmt -> !is-stmt is better than removing
is-stmt lines that are at the same PC by chance.

I will come up with an updated patch, eventually, but will need
to spend more time on the openssl project now, to meet the schedule for the
next release.


Bernd.


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