gdb: fix value_subscript when array upper bound is not known

Since commit 7c6f27129631 ("gdb: make get_discrete_bounds check for
non-constant range bounds"), subscripting  flexible array member fails:

    struct no_size
    {
      int n;
      int items[];
    };

    (gdb) p *ns
    $1 = {n = 3, items = 0x5555555592a4}
    (gdb) p ns->items[0]
    Cannot access memory at address 0xfffe555b733a0164
    (gdb) p *((int *) 0x5555555592a4)
    $2 = 101  <--- we would expect that
    (gdb) p &ns->items[0]
    $3 = (int *) 0xfffe5559ee829a24  <--- wrong address

Since the flexible array member (items) has an unspecified size, the array type
created for it in the DWARF doesn't have dimensions (this is with gcc 9.3.0,
Ubuntu 20.04):

    0x000000a4:   DW_TAG_array_type
                    DW_AT_type [DW_FORM_ref4]       (0x00000038 "int")
                    DW_AT_sibling [DW_FORM_ref4]    (0x000000b3)

    0x000000ad:     DW_TAG_subrange_type
                      DW_AT_type [DW_FORM_ref4]     (0x00000031 "long unsigned int")

This causes GDB to create a range type (TYPE_CODE_RANGE) with a defined
constant low bound (dynamic_prop with kind PROP_CONST) and an undefined
high bound (dynamic_prop with kind PROP_UNDEFINED).

value_subscript gets both bounds of that range using
get_discrete_bounds.  Before commit 7c6f27129631, get_discrete_bounds
didn't check the kind of the dynamic_props and would just blindly read
them as if they were PROP_CONST.  It would return 0 for the high bound,
because we zero-initialize the range_bounds structure.  And it didn't
really matter in this case, because the returned high bound wasn't used
in the end.

Commit 7c6f27129631 changed get_discrete_bounds to return a failure if
either the low or high bound is not a constant, to make sure we don't
read a dynamic prop that isn't a PROP_CONST as a PROP_CONST.  This
change made get_discrete_bounds start to return a failure for that
range, and as a result would not set *lowp and *highp.  And since
value_subscript doesn't check get_discrete_bounds' return value, it just
carries on an uses an uninitialized value for the low bound.  If
value_subscript did check the return value of get_discrete_bounds, we
would get an error message instead of a bogus value.  But it would still
be a bug, as we wouldn't be able to print the flexible array member's
elements.

Looking at value_subscript, we see that the low bound is always needed,
but the high bound is only needed if !c_style.  So, change
value_subscript to use get_discrete_low_bound and
get_discrete_high_bound separately.  This fixes the case described
above, where the low bound is known but the high bound isn't (and is not
needed).  This restores the original behavior without accessing a
dynamic_prop in a wrong way.

A test is added.  In addition to the case described above, a case with
an array member of size 0 is added, which is a GNU C extension that
existed before flexible array members were introduced.  That case
currently fails when compiled with gcc <= 8.  gcc <= 8 produces DWARF
similar to the one shown above, while gcc 9 adds a DW_AT_count of 0 in
there, which makes the high bound known.  A case where an array member
of size 0 is the only member of the struct is also added, as that was
how PR 28675 was originally reported, and it's an interesting corner
case that I think could trigger other funny bugs.

Question about the implementation: in value_subscript, I made it such
that if the low or high bound is unknown, we fall back to zero.  That
effectively makes it the same as it was before 7c6f27129631.  But should
we instead error() out?

gdb/ChangeLog:

	PR 26875, PR 26901
	* gdbtypes.c (get_discrete_low_bound): Make non-static.
	(get_discrete_high_bound): Make non-static.
	* gdbtypes.h (get_discrete_low_bound): New declaration.
	(get_discrete_high_bound): New declaration.
	* valarith.c (value_subscript): Only fetch high bound if
	necessary.

gdb/testsuite/ChangeLog:

	PR 26875, PR 26901
	* gdb.base/flexible-array-member.c: New test.
	* gdb.base/flexible-array-member.exp: New test.

Change-Id: I832056f80e6c56f621f398b4780d55a3a1e299d7

1 files changed, 15 insertions, 7 deletions

diff --git a/gdb/valarith.c b/gdb/valarith.c
index 077bcb4..37988f1 100644
--- a/gdb/valarith.c
+++ b/gdb/valarith.c
@@ -150,25 +150,33 @@ value_subscript (struct value *array, LONGEST index)
       || tarray->code () == TYPE_CODE_STRING)
     {
       struct type *range_type = tarray->index_type ();
-      LONGEST lowerbound, upperbound;
+      gdb::optional<LONGEST> lowerbound = get_discrete_low_bound (range_type);
+      if (!lowerbound.has_value ())
+	lowerbound = 0;
 
-      get_discrete_bounds (range_type, &lowerbound, &upperbound);
       if (VALUE_LVAL (array) != lval_memory)
-	return value_subscripted_rvalue (array, index, lowerbound);
+	return value_subscripted_rvalue (array, index, *lowerbound);
 
       if (!c_style)
 	{
-	  if (index >= lowerbound && index <= upperbound)
-	    return value_subscripted_rvalue (array, index, lowerbound);
+	  gdb::optional<LONGEST> upperbound
+	    = get_discrete_high_bound (range_type);
+
+	  if (!upperbound.has_value ())
+	    upperbound = 0;
+
+	  if (index >= *lowerbound && index <= *upperbound)
+	    return value_subscripted_rvalue (array, index, *lowerbound);
+
 	  /* Emit warning unless we have an array of unknown size.
 	     An array of unknown size has lowerbound 0 and upperbound -1.  */
-	  if (upperbound > -1)
+	  if (*upperbound > -1)
 	    warning (_("array or string index out of range"));
 	  /* fall doing C stuff */
 	  c_style = true;
 	}
 
-      index -= lowerbound;
+      index -= *lowerbound;
       array = value_coerce_array (array);
     }