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 11/25/24 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.

If/when the block lookup code is changed as you propose then this
restriction (addr < end) can be relaxed.  But it doesn't make sense to
merge the relaxed restriction, without the block lookup changes.

And no, I don't see that as a reason to merge all of the changes at
once.  I think splitting the original large change into many small steps
is the correct approach.  And sometimes that will mean that we check
something in, and then revise it in a later commit.  That's not a
problem with this approach, it's an advantage of this approach.  It
makes it clearer how we got to the final destination.  And each step is
smaller, and easier to review.  This is the preferred approach for GDB
patches.

I feel that, as the author of the original large change, you're looking
ahead and you're frustrated that this code isn't inline with how you
feel the code should finally look.  But just because I hope we can check
this code in first, doesn't mean that I will prevent this code being
changed later on.  As GDB evolves (e.g. if the block lookup code
changes) then I'm happy for this code to evolve with it.

To (I hope) offer you some confidence, I have, on my machine, created a
branch with this patch (without the 'addr == end' change), followed by
the next two patches I plan to post (once this is merged), and then, on
top of that, I have rebased your original series.  This includes all of
your original tests completely unmodified.

I have tested this merge with gcc versions 14.2.0, 13.3.0, 12.2.0,
11.5.0, 10.5.0, 9.5.0, 9.3.1, 8.4.0, 8.1.0, and in all cases, all of
your original tests pass.  I'd rather not post this merged branch just
yet, as I'm worried that this might derail review of this patch even
more, I really don't want to start discussing the next patches before
they are even posted, but if it's the only way to move this patch
forward then I could share the branch.  I do plan to make this unified
branch available when I post the next two patches I'd like to upstream,
as I think it will actually help at that point.

My hope is that you will be willing to accept this change on the
understanding that this code might need to be modified in the future.

For my part I also accept that this code might need to change in the
future.

Thanks,
Andrew

---

### Patch to show 'addr == end' doesn't work (yet) ###

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index c178b13d96d..60daf032036 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -11351,7 +11351,7 @@ dwarf2_addr_in_block_ranges (CORE_ADDR addr, struct block *block)
   /* Check if ADDR is within any of the block's sub-ranges.  */
   for (const blockrange &br : block->ranges ())
     {
-      if (addr >= br.start () && addr < br.end ())
+      if (addr >= br.start () && addr <= br.end ())
 	return true;
     }
 
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp b/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp
index 79b1783b2ec..9e4fb781a8d 100644
--- a/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp
+++ b/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp
@@ -179,8 +179,8 @@ proc run_test { entry_label dwarf_version } {
 	     "    $::foo_5\\.\\.$::foo_6"]
 }
 
-foreach_with_prefix entry_label { foo_3 foo_4 } {
-    foreach_with_prefix dwarf_version { 4 5 } {
+foreach_with_prefix entry_label { foo_2 } {
+    foreach_with_prefix dwarf_version { 4 } {
 	run_test $entry_label $dwarf_version
     }
 }