Re: [PATCH] gdb: handle DW_AT_entry_pc pointing at an empty sub-range

[Andrew Burgess <aburgess@redhat.com>]

Bernd Edlinger <bernd.edlinger@hotmail.de> writes:

> On 11/26/24 18:48, Andrew Burgess wrote:
>> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>> 
>>> On 11/25/24 15:30, Andrew Burgess wrote:
>>>> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>>>>
>>>>> Okay, I just wanted to point out that in my opinion the debug info which
>>>>> points at the end of a sub-range is not incorrect, just maybe on a border
>>>>> line, where the dwarf spec is unclear.  So you should not say:
>>>>> "after all, the DWARF spec is clear that such a range covers no code."
>>>>>
>>>>> But there are obviously not only cases where the entry_pc points at
>>>>> an empty sub-range, but also in very rare cases the entry_pc points at
>>>>> the end of a non-empty sub-range.
>>>>> So could you please change the check in dwarf2_addr_in_block_ranges
>>>>> from addr >= start && addr < end to addr >= start && addr <= end.
>>>>
>>>> Could you expand on why you believe that the DWARF spec is unclear in
>>>> this regard.  I came to my conclusion based on this text within the
>>>> DWARF-5 specification, section 2.17.3 Non-Contiguous Address Ranges:
>>>>
>>>>   Bounded range. This kind of entry defines an address range that is
>>>>   included in the range list. The starting address is the lowest address
>>>>   of the address range. The ending address is the address of the first
>>>>   location past the highest address of the address range
>>>>
>>>> This seems pretty clear (to me) that the end address is not part of the
>>>> region covered by a range.
>>>>
>>>
>>> Yes, but on the other hand, when we look at line table entries, each has a
>>> PC and a VIEW number, and even the DW_AT_entry_pc has a DW_AT_GNU_entry_view,
>>> just the range list does not have a view number, and that is inconsistent
>>> with the concept of location views.
>>>
>>> Consider as a simple example an inline function:
>>>
>>> int f(int x)
>>> {
>>>  x++;
>>>  return x;
>>> }
>>>
>>> it will most likely just be compiled into one "inc eax" or similar,
>>> and of course you may want to set a break point on the return statement,
>>> to inspect 'x' after the increment, but that will be on 'pc == end' !
>>>
>>> But if the location view number would not be missing from the rnglist
>>> it would be obvious whether the corresponding view number is still within
>>> subroutine and not outside.  So in my opinion it is a defect in the
>>> specification that it does not reflect this use case.
>> 
>> You make an interesting argument that the specification is deficient.
>> But I'm not sure how this helps with this discussion.  I would like to
>> avoid derailing this conversation with discussion of missing DWARF
>> features.
>> 
>>>
>>>> Additionally, if we start to accept 'addr == end' then this is going to
>>>> cause problems elsewhere.  GDB will place a b/p at the 'end' address,
>>>> but when GDB then performs block lookup, GDB will not return the block
>>>> we expect, and so GDB will not report the inferior as having stopped in
>>>> the scope that the user expects.
>>>>
>>>
>>> No, because this is exactly what the core of my patch does, admittedly
>>> I also modified the block lookup code a bit, to handle that case.
>>> So I strongly disagree here: we have to accept 'addr == end' and other
>>> corner cases, otherwise my patch won't work in the end, regardless of in
>>> how many small bug-fixes it can be split up, because it depends exactly
>>> on not ignoring any information while parsing the debug info.
>> 
>> But accepting 'addr == end' only works if you also change the block
>> lookup mechanism, which isn't part of this patch.  This patch is based
>> on the state of block lookup as it exists today.
>> 
>> I've included a patch below which applies on top of this patch (i.e. the
>> one this thread is about), it changes the check to accept 'addr == end'
>> as you suggest.  It also updates the test so that an inline function
>> (bar) has DW_AT_entry_pc point at the 'end' address of a non-empty
>> sub-range.
>> 
>> Here's a GDB session with that patch applied:
>> 
>>   (gdb) b bar
>>   Breakpoint 1 at 0x401137
>>   (gdb) r
>>   Starting program: /tmp/gdb/testsuite/outputs/gdb.dwarf2/dw2-entry-pc-in-empty-range/dw2-entry-pc-in-empty-range-4 
>>   
>>   Breakpoint 1, 0x0000000000401137 in foo ()
>>   (gdb) maintenance info blocks 
>>   Blocks at 0x401137:
>>     from objfile: [(objfile *) 0x3b11720] /tmp/gdb/testsuite/outputs/gdb.dwarf2/dw2-entry-pc-in-empty-range/dw2-entry-pc-in-empty-range-4
>>   
>>   [(block *) 0x357e680] 0x401106..0x401185
>>     entry pc: 0x401106
>>     is global block
>>     symbol count: 1
>>     is contiguous
>>   [(block *) 0x357e630] 0x401106..0x401185
>>     entry pc: 0x401106
>>     is static block
>>     is contiguous
>>   [(block *) 0x357e5e0] 0x401106..0x401185
>>     entry pc: 0x401106
>>     function: foo
>>     is contiguous
>>   (gdb) 
>> 
>> As you can see, GDB stops at an address which doesn't then resolve to
>> the bar block.
>> 
>
> Ah, okay, thanks for the test case I looked into it and tried it out with
> a gdb built with my patch.
>
> A breakpoint an an empty subrange works as expected, but indeed a breakpoint
> at the end of a non-empty subrange does behave as you described.
> But with real code examples a break point at the end of a non-empty subrange
> does work, and that is because my "heuristic" which does the magic, uses
> weak line-table entries near the end of a subrange to determine what to do
> here.  So because the test case does not have a line table at all, the
> test case is not realistic in that aspect, and does not tell you what happens
> here in a real world example.
>
> So a patch that allows for start <= addr && addr <= end should be applied,
> probably with a line table that looks more like a real line table especially
> at the end of a subrange, then the test should probably work out of the box.
> But it is okay to do that after the rest of the series is applied.

Could you include the details for what you'd like the line table to look
like please, I guess from the test program you were trying.  I'd like to
try and get the updated test right first time!  Ideally I guess I'd need
the `maint info blocks` output showing the inline function block, and
the `maint info line-table` output for the line table that covers the
inline and its containing function.

I'll get starting using my own demo programs, but I'd really like to
meet your expectations on this.

Thanks,
Andrew