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

Pedro Alves
2020-08-28T15:24:05Z

On 8/28/20 3:30 PM, Tom de Vries wrote:
> 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)
> ...

Well, I don't think you can claim that because it doesn't look
like lldb understands labels at all. Tweak the testcase to:

18 int
19 main (void)
20 {
21 L1:
22 return 0;
23 }

And then:

(lldb) b main:L1
Breakpoint 1: no locations (pending).
WARNING: Unable to resolve breakpoint to any actual locations.

And you get the same message with any random nonexistent label name:

(lldb) b main:FOOBAR
Breakpoint 2: no locations (pending).
WARNING: Unable to resolve breakpoint to any actual locations.
(lldb)