Re: [PATCH] gdb: Change "list ." command's error when no debuginfo is available

[Andrew Burgess <aburgess@redhat.com>]

Guinevere Larsen <blarsen@redhat.com> writes:

> On 06/03/2024 14:18, Andrew Burgess wrote:
>> Guinevere Larsen <blarsen@redhat.com> writes:
>>
>>> From: Simon Marchi <simark@simark.ca>
>>>
>>> Currently, when a user tries to list the current location, there are 2
>>> different error messages that can happen, either:
>>>
>>>      (gdb) list .
>>>      No symbol table is loaded.  Use the "file" command.
>>> or
>>>      (gdb) list .
>>>      No debug information available to print source lines.
>>>
>>> The difference here is if gdb can find any symtabs at all or not, which
>>> is not a difference a user should concern themselves with.
>> Why not?
>>
>> Maybe "No symbol table is loaded" isn't the clearest, maybe "No debug
>> information is loaded" might be better, but the first message tells me
>> that there is no debug information, maybe GDB failed to find it, or
>> maybe I need to provide it.  The second tells me that GDB did find debug
>> information that covers this address, but it lacks the required detail
>> to help, maybe I forgot -g when compiling?
>
> The situation where I see the second message pops up is when we're, for 
> example, in the main function and there are symbols for libc. I may be 
> misunderstanding something, but I don't think that debug information 
> covers the address the inferior is in.
>
> The situation that we were specifically thinking about was that
>
>>
>> I'm not 100% against this change, I just don't find the motivation super
>> compelling.
>>
>>>                                                              This commit
>>> changes it so that the error is always:
>>>
>>>      (gdb) list .
>>>      No debug information available to print source lines at current PC (0x55555555511d).
>>> or
>>>      (gdb) list .
>>>      No debug information available to print source lines at default location.
>>>
>>> The difference now is if the inferior has started already, which is
>>> controlled by the user and may be useful.
>>>
>>> Unfortunately, it isn't as easy to differentiate if the symtab found for
>>> other list parameters is correct, so other invocations, such as "list +"
>>> still retain their original error message.
>>>
>>> Co-Authored-By: Simon Marchi <simark@simark.ca>
>>> ---
>>>   gdb/cli/cli-cmds.c | 48 ++++++++++++++++++++++++++++++++--------------
>> For sure this patch should have an associated test change.
> That's a good point... Do you know how I'd be able to get a test running 
> with no symbols whatsoever, even if the system that is doing the testing 
> has libc symbols?

Could you compile a test binary statically, and then strip all debug
information from it?  Would that do what you need?

Thanks,
Andrew



>
> -- 
> Cheers,
> Guinevere Larsen
> She/Her/Hers
>
>>
>> Thanks,
>> Andrew
>>
>>>   1 file changed, 34 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
>>> index df11f956245..31f0b6d9907 100644
>>> --- a/gdb/cli/cli-cmds.c
>>> +++ b/gdb/cli/cli-cmds.c
>>> @@ -1236,37 +1236,39 @@ list_command (const char *arg, int from_tty)
>>>     /* Pull in the current default source line if necessary.  */
>>>     if (arg == NULL || ((arg[0] == '+' || arg[0] == '-' || arg[0] == '.') && arg[1] == '\0'))
>>>       {
>>> -      set_default_source_symtab_and_line ();
>>> -      symtab_and_line cursal = get_current_source_symtab_and_line ();
>>> -
>>>         /* If this is the first "list" since we've set the current
>>>   	 source line, center the listing around that line.  */
>>>         if (get_first_line_listed () == 0 && (arg == nullptr || arg[0] != '.'))
>>>   	{
>>> -	  list_around_line (arg, cursal);
>>> +	  set_default_source_symtab_and_line ();
>>> +	  list_around_line (arg, get_current_source_symtab_and_line ());
>>>   	}
>>>   
>>>         /* "l" and "l +" lists the next few lines, unless we're listing past
>>>   	 the end of the file.  */
>>>         else if (arg == nullptr || arg[0] == '+')
>>>   	{
>>> +	  set_default_source_symtab_and_line ();
>>> +	  const symtab_and_line cursal = get_current_source_symtab_and_line ();
>>>   	  if (last_symtab_line (cursal.symtab) >= cursal.line)
>>>   	    print_source_lines (cursal.symtab,
>>>   				source_lines_range (cursal.line), 0);
>>>   	  else
>>> -	    {
>>> -	      error (_("End of the file was already reached, use \"list .\" to"
>>> -		       " list the current location again"));
>>> -	    }
>>> +	    error (_("End of the file was already reached, use \"list .\" to"
>>> +		     " list the current location again"));
>>>   	}
>>>   
>>>         /* "l -" lists previous ten lines, the ones before the ten just
>>>   	 listed.  */
>>>         else if (arg[0] == '-')
>>>   	{
>>> +	  set_default_source_symtab_and_line ();
>>> +	  const symtab_and_line cursal = get_current_source_symtab_and_line ();
>>> +
>>>   	  if (get_first_line_listed () == 1)
>>>   	    error (_("Already at the start of %s."),
>>>   		   symtab_to_filename_for_display (cursal.symtab));
>>> +
>>>   	  source_lines_range range (get_first_line_listed (),
>>>   				    source_lines_range::BACKWARD);
>>>   	  print_source_lines (cursal.symtab, range, 0);
>>> @@ -1275,25 +1277,43 @@ list_command (const char *arg, int from_tty)
>>>         /* "list ." lists the default location again.  */
>>>         else if (arg[0] == '.')
>>>   	{
>>> +	  std::optional<const symtab_and_line> cursal;
>>>   	  if (target_has_stack ())
>>>   	    {
>>>   	      /* Find the current line by getting the PC of the currently
>>>   		 selected frame, and finding the line associated to it.  */
>>>   	      frame_info_ptr frame = get_selected_frame (nullptr);
>>>   	      CORE_ADDR curr_pc = get_frame_pc (frame);
>>> -	      cursal = find_pc_line (curr_pc, 0);
>>> +	      cursal.emplace (find_pc_line (curr_pc, 0));
>>> +
>>> +	      if (cursal->symtab == nullptr)
>>> +		error
>>> +		  (_("No debug information available to print source lines at "
>>> +		     "current PC (%s)."), paddress (get_frame_arch (frame),
>>> +						    curr_pc));
>>>   	    }
>>>   	  else
>>>   	    {
>>>   	      /* The inferior is not running, so reset the current source
>>>   		 location to the default (usually the main function).  */
>>>   	      clear_current_source_symtab_and_line ();
>>> -	      set_default_source_symtab_and_line ();
>>> -	      cursal = get_current_source_symtab_and_line ();
>>> +	      try
>>> +		{
>>> +		  set_default_source_symtab_and_line ();
>>> +		}
>>> +	      catch (const gdb_exception &e)
>>> +		{
>>> +		  error (_("No debug information available to print source "
>>> +			   "lines at default location"));
>>> +		}
>>> +	      cursal.emplace (get_current_source_symtab_and_line ());
>>> +
>>> +	      // Not sure if always true, just guessing.
>>> +	      gdb_assert (cursal->symtab != nullptr);
>>>   	    }
>>> -	  if (cursal.symtab == nullptr)
>>> -	    error (_("No debug information available to print source lines."));
>>> -	  list_around_line (arg, cursal);
>>> +
>>> +	  list_around_line (arg, *cursal);
>>> +
>>>   	  /* Set the repeat args so just pressing "enter" after using "list ."
>>>   	     will print the following lines instead of the same lines again. */
>>>   	  if (from_tty)
>>> -- 
>>> 2.43.0