Re: [PATCH] gdb: Don't skip prologue for explicit line breakpoints in assembler

[Pedro Alves <palves at redhat dot com>]

On 6/22/19 12:05 PM, Andrew Burgess wrote:
> * Pedro Alves <palves@redhat.com> [2019-06-21 14:43:26 +0100]:
> 
>> On 6/21/19 12:23 AM, Andrew Burgess wrote:
>>
>>> I spent some more time trying to find a path that would call both
>>> 'decode_digits_list_mode' and then 'skip_prologue_sal', but I still
>>> can't find one.
>>
>> But won't that change affect any code path that ends up in
>> skip_prologue_sal with explicit_line set?
> 
>     [ Disclaimer: In the below I'll take about 'in current testing we
>          never do X'.  I understand that this doesn't mean GDB will
>          _never_ do X as our testing doesn't guarantee to hit every
>          possible code path, it's more an invitation for people to
>          suggest how me might create a test that does do X. ]
> 
> Indeed.  My claim is that in the current testing we never get to
> skip_prologue_sal with explicit_line set.  My patch means we do now
> enter skip_prologue_sal with explicit_line set, and I find that the
> existing check that uses explicit_line means GDB doesn't behave as I'd
> like.
> 
> Given that in HEAD explicit_line only seems to be set when decoding a
> line spec in 'list_mode', my current belief is that explicit_line is
> never set in a condition where the flag will then be checked.  In
> other words, I think the explicit_line is currently useless.

Since the flag is checked in breakpoint.c, it likely had a use
for breakpoint mode too, though as you say, it's probably not used today.

Greping for explicit_line finds this case:


  set_breakpoint_location_function (loc,
				    sal->explicit_pc || sal->explicit_line);

in 

  add_location_to_breakpoint

I wonder whether that explicit_line use makes sense here.

set_breakpoint_location_function says:

 /* Initialize loc->function_name.  EXPLICIT_LOC says no indirect function
    resolutions should be made as the user specified the location explicitly
    enough.  */

 static void
 set_breakpoint_location_function (struct bp_location *loc, int explicit_loc)
 {


I'm not immediately seeing how to set a breakpoint at an ifunc symbol
by line number such that you end up in set_breakpoint_location_function
with loc->msymbol != NULL.

We should probably delete that sal->explicit_line reference.

> 
>>
>> E.g.:
>>
>> /* Helper function for break_command_1 and disassemble_command.  */
>>
>> void
>> resolve_sal_pc (struct symtab_and_line *sal)
>> {
>>   CORE_ADDR pc;
>>
>>   if (sal->pc == 0 && sal->symtab != NULL)
>>     {
>>       if (!find_line_pc (sal->symtab, sal->line, &pc))
>> 	error (_("No line %d in file \"%s\"."),
>> 	       sal->line, symtab_to_filename_for_display (sal->symtab));
>>       sal->pc = pc;
>>
>>       /* If this SAL corresponds to a breakpoint inserted using a line
>>          number, then skip the function prologue if necessary.  */
>>       if (sal->explicit_line)
>> 	skip_prologue_sal (sal);
>>     }
>>
>> Is that path unreachable today?
> 
> In current testing we enter this code block once, by accident.
> 
> The test 'gdb.dwarf2/dw2-objfile-overlap.exp' enters the block because
> we load a symbol file at address 0.  The check '(sal->pc == 0 &&
> sal->symtab != NULL)' is trying to find SALs where the 'pc' has not
> been set, in our case the 'pc' has been set; to zero.  When we then
> call 'find_line_pc' and then 'sal->pc = pc', we reset the 'pc' to zero
> again.
> 
> In this one case the explicit_line flag is false, so skip_prologue_sal
> is never called.
> 
> As an aside how would you feel about a patch that made the 'pc' field
> of symtab_and_line private, and updated all users to use getter/setter
> methods?  

Sounds fine to me in principle.

> I already did this in order to add a 'is_pc_initialised?'
> type method to symtab_and_line.  When I add this and change the above
> code to say this:
> 
>   void
>   resolve_sal_pc (struct symtab_and_line *sal)
>   {
>     CORE_ADDR pc;
> 
>     if (sal->symtab != NULL && !sal->pc_p ())
>       {
>         // ... etc ...
> 
> then we no longer enter this block at all during the current testing.

So does this mean that linespec nowadays always fills in the PC then?

If that's the case, when do we need that is_pc_initialized method?

I wonder what else is not reachable around here, laying around
waiting to be garbage collected...

>>> Looking back at how the explicit_line flag was originally used when
>>> it was added in commit ed0616c6b78a0966, things have changed quite a
>>> bit in the 10+ years since.  There were some tests added along with
>>> the explicit_line flag (gdb.cp/mb-*.exp) and these all pass both in
>>> master and in my patched branch.
>>>
>>> My current thinking is that the explicit_line flag was no longer doing
>>> anything useful in master, but if someone disagrees I'd love to
>>> understand more about this.
>>
>> I seem to recall that GDB didn't use to update a breakpoint's line
>> number to advance to the next line number that includes some actual
>> compiled code.  Like if you set a breakpoint at line 10 below:
>>
>>  10    // just a comment
>>  11    i++;
>>
>> you end up with a breakpoint at line 11.  Maybe it's old code
>> related to that.
> 
> I wonder if what you meant to say here is the breakpoint is placed at
> the address of line 11, but is recorded as being at line 10.  This
> actually would line up with what the explicit line flag was doing if
> the explicit line flag was being set.

Yes, exactly.

> 
> The problem seems to be that when the explicit_line flag was first
> added there was just function for decoding linespec line numbers
> 'decode_all_digits'.  At some point in time this split into
> decode_digits_ordinary and decode_digits_list_mode, when this happened
> the explicit_line flag was only ever being set in one path.
> 
> I suspect that if the behaviour you discussed above ever existed, then
> it was before the split in how digits were decoded.
> 
> I'm running out of time to investigate this today, but when I get some
> more time I'll dig a little more on this line of enquiry to see if I
> can confirm or deny the above theory.

Did you check whether we're already setting explicit_line when
parsing "b -line N", i.e., when using the explicit locations syntax?

> 
>>
>> Maybe I'm misremembering.
>>
>> In any case, if you change this, then you also should change
>> the function's entry comment:
>>
>>  /* Adjust SAL to the first instruction past the function prologue.
>>     If the PC was explicitly specified, the SAL is not changed.
>>     If the line number was explicitly specified, at most the SAL's PC
>>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>     is updated.  If SAL is already past the prologue, then do nothing.  */
>>     ^^^^^^^^^^
> 
> Would this be OK?  (I'm not pushing anything until the above questions
> are resolved):

Sure.

> 
> diff --git a/gdb/symtab.c b/gdb/symtab.c
> index c10e6b3e358..6e7e32fb4d8 100644
> --- a/gdb/symtab.c
> +++ b/gdb/symtab.c
> @@ -3673,8 +3673,10 @@ skip_prologue_using_lineinfo (CORE_ADDR func_addr, struct symtab *symtab)
>  
>  /* Adjust SAL to the first instruction past the function prologue.
>     If the PC was explicitly specified, the SAL is not changed.
> -   If the line number was explicitly specified, at most the SAL's PC
> -   is updated.  If SAL is already past the prologue, then do nothing.  */
> +   If the line number was explicitly specified then the SAL can still be
> +   updated, unless the language for SAL is assembler, in which case the SAL
> +   will be left unchanged.
> +   If SAL is already past the prologue, then do nothing.  */
>  
>  void
>  skip_prologue_sal (struct symtab_and_line *sal)
Thanks,
Pedro Alves