gdb: avoid resolving dynamic properties for non-allocated arrays

In PR gdb/27059 an issue was discovered where GDB would sometimes
trigger undefined behaviour in the form of signed integer overflow.
The problem here is that GDB was reading random garbage from the
inferior memory space, assuming this data was valid, and performing
arithmetic on it.

This bug raises an interesting general problem with GDB's DWARF
expression evaluator, which is this:

We currently assume that the DWARF expressions being evaluated are
well formed, and well behaving.  As an example, this is the expression
that the bug was running into problems on, this was used as the
expression for a DW_AT_byte_stride of a DW_TAG_subrange_type:

	DW_OP_push_object_address;
	DW_OP_plus_uconst: 88;
	DW_OP_deref;
	DW_OP_push_object_address;
	DW_OP_plus_uconst: 32;
	DW_OP_deref;
	DW_OP_mul

Two values are read from the inferior and multiplied together.  GDB
should not assume that any value read from the inferior is in any way
sane, as such the implementation of DW_OP_mul should be guarding
against overflow and doing something semi-sane here.

However, it turns out that the original bug PR gdb/27059, is hitting a
more specific case, which doesn't require changes to the DWARF
expression evaluator, so I'm going to leave the above issue for
another day.

In the test mentioned in the bug GDB is actually trying to resolve the
dynamic type of a Fortran array that is NOT allocated.  A
non-allocated Fortran array is one that does not have any data
allocated for it yet, and even the upper and lower bounds of the array
are not yet known.

It turns out that, at least for gfortran compiled code, the data
fields that describe the byte-stride are not initialised until the
array is allocated.

This leads me to the following conclusion: GDB should not try to
resolve the bounds, or stride information for an array that is not
allocated (or not associated, a similar, but slightly different
Fortran feature).  Instead, each of these properties should be set to
undefined if the array is not allocated (or associated).

That is what this commit does.  There's a new flag that is passed
around during the dynamic array resolution.  When this flag is true
the dynamic properties are resolved using the DWARF expressions as
they currently are, but when this flag is false the expressions are
not evaluated, and instead the properties are set to undefined.

gdb/ChangeLog:

	PR gdb/27059
	* eval.c (evaluate_subexp_for_sizeof): Handle not allocated and
	not associated arrays.
	* f-lang.c (fortran_adjust_dynamic_array_base_address_hack): Don't
	adjust arrays that are not allocated/associated.
	* gdbtypes.c (resolve_dynamic_range): Update header comment.  Add
	new parameter which is used to sometimes set dynamic properties to
	undefined.
	(resolve_dynamic_array_or_string): Update header comment.  Add new
	parameter which is used to guard evaluating dynamic properties.
	Resolve allocated/associated properties first.

gdb/testsuite/ChangeLog:

	PR gdb/27059
	* gdb.dwarf2/dyn-type-unallocated.c: New file.
	* gdb.dwarf2/dyn-type-unallocated.exp: New file.

1 files changed, 4 insertions, 0 deletions

diff --git a/gdb/f-lang.c b/gdb/f-lang.c
index 28a66fd..e06bbb4 100644
--- a/gdb/f-lang.c
+++ b/gdb/f-lang.c
@@ -1391,6 +1391,10 @@ fortran_adjust_dynamic_array_base_address_hack (struct type *type,
 {
   gdb_assert (type->code () == TYPE_CODE_ARRAY);
 
+  /* We can't adjust the base address for arrays that have no content.  */
+  if (type_not_allocated (type) || type_not_associated (type))
+    return address;
+
   int ndimensions = calc_f77_array_dims (type);
   LONGEST total_offset = 0;