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

Tom de Vries
2020-08-28T14:31:00Z

On 8/28/20 3:53 PM, Tom de Vries wrote:
> On 8/28/20 3:32 PM, Pedro Alves wrote:
>> On 8/27/20 2:49 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 don't understand the "not useful" remark. If a user does gets
>> the error, they'll probably do:
>>
>> "b 3",
>>
>> and they'll get a breakpoint at line 5, no?
>>
>> That's very likely what a user would do after the error.
>>
>> IMO GDB should do that for the user.
>>
>> So far I don't agree with your patch.
>>
>
> I see what you mean, but let's try this counter-example:
> ...
> cat -n test.c
> 1 int
> 2 main (void)
> 3 {
> 4 goto L2;
> 5
> 6 L3:
> 7 return 0;
> 8
> 9 L1:
> 10 (void)0;
> 11 return 1;
> 12
> 13 L2:
> 14 goto L3;
> 15 }
> 16
> ...
> compiled like this:
> ...
> $ gcc test.c -g
> ...
>
> With the patch, we're not able to set a breakpoint at L1, and setting
> the breakpoint at the corresponding line, line 9:
> ...
> $ gdb a.out
> Reading symbols from a.out...
> (gdb) b main:L1
> Location main:L1 not available
> (gdb) b 9
> Breakpoint 1 at 0x40049c: file test.c, line 14.
> (gdb)
> ...
> yields a breakpoint at line 14, a piece of code that's not reachable
> from L1.
>
> To me, label L1 and line 14 are unrelated enough to convince me to not
> do this automatically.
>

FWIW, lldb does the same:
...
$ lldb a.out
(lldb) target create "a.out"
Current executable set to 'a.out' (x86_64).
(lldb) b main:L1
Breakpoint 1: no locations (pending).
WARNING: Unable to resolve breakpoint to any actual locations.
(lldb) b 9
Breakpoint 2: where = a.out`main + 5 at test.c:14, address =
0x000000000040049c
(lldb)
...

Thanks,
- Tom