Fix dwarf2read.c:dwarf2_find_containing_comp_unit's binary search

Fix dwarf2read.c:dwarf2_find_containing_comp_unit's binary search

First of all, I would like to express my gratitude to Keith Seitz, Jan
Kratochvil and Tom Tromey, who were really kind and helped a lot with
this bug.  The patch itself was authored by Jan.

This all began with:

  https://bugzilla.redhat.com/show_bug.cgi?id=1639242
  py-bt is broken, results in exception

In summary, the error reported by the bug above is:

  $ gdb -args python3
  GNU gdb (GDB) Fedora 8.1.1-3.fc28
  (...)
  Reading symbols from python3...Reading symbols from /usr/lib/debug/usr/bin/python3.6-3.6.6-1.fc28.x86_64.debug...done.
  done.
  Dwarf Error: could not find partial DIE containing offset 0x316 [in module /usr/lib/debug/usr/bin/python3.6-3.6.6-1.fc28.x86_64.debug]

After a long investigation, and after thinking that the problem might
actually be on DWZ's side, we were able to determine that there's
something wrong going on when
dwarf2read.c:dwarf2_find_containing_comp_unit performs a binary search
over all of the CUs belonging to an objfile in order to find the CU
which contains a DIE at an specific offset.  The current algorithm is:

  static struct dwarf2_per_cu_data *
  dwarf2_find_containing_comp_unit (sect_offset sect_off,
				    unsigned int offset_in_dwz,
				    struct dwarf2_per_objfile *dwarf2_per_objfile)
  {
    struct dwarf2_per_cu_data *this_cu;
    int low, high;
    const sect_offset *cu_off;

    low = 0;
    high = dwarf2_per_objfile->all_comp_units.size () - 1;
    while (high > low)
      {
	struct dwarf2_per_cu_data *mid_cu;
	int mid = low + (high - low) / 2;

	mid_cu = dwarf2_per_objfile->all_comp_units[mid];
	cu_off = &mid_cu->sect_off;
	if (mid_cu->is_dwz > offset_in_dwz
	    || (mid_cu->is_dwz == offset_in_dwz && *cu_off >= sect_off))
	  high = mid;
	else
	  low = mid + 1;
      }

For the sake of this example, let's consider that "sect_off =
0x7d".

There are a few important things going on here.  First,
"dwarf2_per_objfile->all_comp_units ()" will be sorted first by
whether the CU is a DWZ CU, and then by cu->sect_off.  In this
specific bug, "offset_in_dwz" is false, which means that, for the most
part of the loop, we're going to do "high = mid" (i.e, we'll work with
the lower part of the vector).

In our particular case, when we reach the part where "mid_cu->is_dwz
== offset_in_dwz" (i.e, both are false), we end up with "high = 2" and
"mid = 1".  I.e., there are only 2 elements in the vector who are not
DWZ.  The vector looks like this:

  #0: cu->sect_off = 0;   length = 114;  is_dwz = false  <-- low
  #1: cu->sect_off = 114; length = 7796; is_dwz = false  <-- mid
  #2: cu->sect_off = 0;   length = 28;   is_dwz = true   <-- high
  ...

The CU we want is #1, which is exactly where "mid" is.  Also, #1 is
not DWZ, which is also exactly what we want.  So we perform the second
comparison:

  (mid_cu->is_dwz == offset_in_dwz && *cu_off >= sect_off)
                                      ^^^^^^^^^^^^^^^^^^^

Because "*cu_off = 114" and "sect_off = 0x7d", this evaluates to
false, so we end up with "low = mid + 1 = 2", which actually gives us
the wrong CU (i.e., a CU that is DWZ).  Next in the code, GDB does:

    gdb_assert (low == high);
    this_cu = dwarf2_per_objfile->all_comp_units[low];
    cu_off = &this_cu->sect_off;
    if (this_cu->is_dwz != offset_in_dwz || *cu_off > sect_off)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      {
	if (low == 0 || this_cu->is_dwz != offset_in_dwz)
	  error (_("Dwarf Error: could not find partial DIE containing "
		 "offset %s [in module %s]"),
		 sect_offset_str (sect_off),
		 bfd_get_filename (dwarf2_per_objfile->objfile->obfd));
	...

Triggering the error we saw in the original bug report.

It's important to notice that we see the error message because the
selected CU is a DWZ one, but we're looking for a non-DWZ CU here.
However, even when the selected CU is *not* a DWZ (and we don't see
any error message), we still end up with the wrong CU.  For example,
suppose that the vector had:

  #0: cu->sect_off = 0;    length = 114;  is_dwz = false
  #1: cu->sect_off = 114;  length = 7796; is_dwz = false
  #2: cu->sect_off = 7910; length = 28;   is_dwz = false
  ...

I.e., #2's "is_dwz" is false instead of true.  In this case, we still
want #1, because that's where the DIE is located.  After the loop ends
up in #2, we have "is_dwz" as false, which is what we wanted, so we
compare offsets.  In this case, "7910 >= 0x7d", so we set "mid = high
= 2".  Next iteration, we have "mid = 0 + (2 - 0) / 2 = 1", and thus
we examining #1.  "is_dwz" is still false, but "114 >= 0x7d" also
evaluates to false, so "low = mid + 1 = 2", which makes the loop stop.
Therefore, we end up choosing #2 as our CU, even though #1 is the
right one.

The problem here is happening because we're comparing "sect_off"
directly against "*cu_off", while we should actually be comparing
against "*cu_off + mid_cu->length" (i.e., the end offset):

  ...
  || (mid_cu->is_dwz == offset_in_dwz
      && *cu_off + mid_cu->length >= sect_off))
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ...

And this is what the patch does.  The idea is that if GDB is searching
for an offset that falls above the *end* of the CU being
analyzed (i.e., "mid"), then the next iteration should try a
higher-offset CU next.  The previous algorithm was using
the *beginning* of the CU.

Unfortunately, I could not devise a testcase for this problem, so I am
proposing a fix with this huge explanation attached to it in the hope
that it is sufficient.  After talking a bit to Keith (our testcase
guru), it seems that one would have to create an objfile with both DWZ
and non-DWZ sections, which may prove very hard to do, I think.

I ran this patch on our BuildBot, and no regressions were detected.

gdb/ChangeLog:
2018-11-30  Jan Kratochvil  <jan.kratochvil@redhat.com>
	    Keith Seitz  <keiths@redhat.com>
	    Tom Tromey  <tom@tromey.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	https://bugzilla.redhat.com/show_bug.cgi?id=1613614
	* dwarf2read.c (dwarf2_find_containing_comp_unit): Add
	'mid_cu->length' to '*cu_off' when checking if 'sect_off' is
	inside the CU.