[ was: Re: [PATCH][gdb/breakpoint] Handle setting breakpoint on label
without address ]

On 8/28/20 12:31 PM, Tom de Vries wrote:
> On 8/27/20 3:49 PM, Tom de Vries wrote:
>> On 8/27/20 2:41 PM, Pedro Alves wrote:
>>> On 8/27/20 12:52 PM, Tom de Vries wrote:
>>>> Hi,
>>>>
>>>> Consider test-case test.c:
>>>> ...
>>>> $ cat test.c
>>>> int main (void) {
>>>>   return 0;
>>>>  L1:
>>>>   (void)0;
>>>> }
>>>> ...
>>>>
>>>> Compiled with debug info:
>>>> ...
>>>> $ gcc test.c -g
>>>> ...
>>>>
>>>> When attempting to set a breakpoint at L1, which is a label without address:
>>>> ...
>>>>  <1><f4>: Abbrev Number: 2 (DW_TAG_subprogram)
>>>>     <f5>   DW_AT_name        : main
>>>>  <2><115>: Abbrev Number: 3 (DW_TAG_label)
>>>>     <116>   DW_AT_name        : L1
>>>>     <119>   DW_AT_decl_file   : 1
>>>>     <11a>   DW_AT_decl_line   : 5
>>>>  <2><11b>: Abbrev Number: 0
>>>
>>> Is this a debug info bug,
>>
>> Strictly speaking, this is a debug info bug.  The standard says that:
>> ...
>> The label entry has a DW_AT_low_pc attribute whose value is the address
>> of the first executable instruction for the location identified by the
>> label in the source program.
>> ...
>>
>> But I interpret the missing DW_AT_low_pc attribute as: there is a label
>> in the source, but the corresponding code has been optimized out.
>>
>>> or is the debug info telling us that the
>>> address of the label is the same as the line number's address?
>>>
>>> How about looking up the line number address instead of throwing
>>> an error?
>>>
>>
>> Well, in this particular case, that wouldn't help.
>>
>> With L1 at line 3:
>> ...
>> $ cat -n test.c
>>      1  int main (void) {
>>      2    return 0;
>>      3   L1:
>>      4    (void)0;
>>      5  }
>>      6
>> ...
>> there's no corresponding address:
>> ...
>> $ readelf -wL a.out
>> CU: test.c:
>> File name                            Line number    Starting address
>> View    Stmt
>> test.c                                         1            0x400497
>>            x
>> test.c                                         2            0x40049b
>>            x
>> test.c                                         5            0x4004a0
>>            x
>> test.c                                         -            0x4004a2
>> ...
>>
>> My suspicion is that this won't be useful in general.
>>
> 
> I've pushed this as attached below, with the test-case updated to work
> around PR26546 - "[pie] Setting breakpoint on missing label sets
> breakpoint at offset 0 in NULL section" (
> https://sourceware.org/bugzilla/show_bug.cgi?id=26546 ).

Which is fixed by the patch below.

Any comments?

Thanks,
- Tom