Re: [PATCH] Clear upper bits during sign extension

Yao Qi
2015-01-08T05:40:48Z

Yao Qi <yao@codesourcery.com> writes:

>> + byte_order = gdbarch_byte_order (get_type_arch (type));
>
> How about getting gdbarch via get_frame_arch (frame)? How about
> removing gdb_sign_extend as it is no longer used?
>
> I'll post a full version on top of yours.

Here is the patch, what do you think?

--
Yao (齐尧)

Subject: [PATCH] always read synthetic pointers as signed integers

I see the error message "access outside bounds of object referenced
via synthetic pointer" in the two fails below of mips gdb testing

print d[-2]^M
access outside bounds of object referenced via synthetic pointer^M
(gdb) FAIL: gdb.dwarf2/implptrconst.exp: print d[-2]
(gdb) print/d p[-1]^M
access outside bounds of object referenced via synthetic pointer^M
(gdb) FAIL: gdb.dwarf2/implptrpiece.exp: print/d p[-1]

in the first test, 'd[-2]' is processed by GDB as '* (&d[-2])'. 'd'
is a synthetic pointer, so its value is zero, the address of 'd[-2]'
is -2. In dwarf2loc.c:indirect_pieced_value,

/* This is an offset requested by GDB, such as value subscripts.
However, due to how synthetic pointers are implemented, this is
always presented to us as a pointer type. This means we have to
sign-extend it manually as appropriate. */
byte_offset = value_as_address (value);
if (TYPE_LENGTH (value_type (value)) < sizeof (LONGEST))
byte_offset = gdb_sign_extend (byte_offset,
8 * TYPE_LENGTH (value_type (value)));
byte_offset += piece->v.ptr.offset;

We know that the value is really an offset instead of address, so the
fix is to extract the value as an (signed) offset.

gdb:

2015-01-08 Pedro Alves <palves@redhat.com>
Yao Qi <yao@codesourcery.com>

* dwarf2loc.c (indirect_pieced_value): Don't call
gdb_sign_extend. Call extract_signed_integer instead.
* utils.c (gdb_sign_extend): Remove.
* utils.h (gdb_sign_extend): Remove declaration.
---
gdb/dwarf2loc.c | 16 +++++++++++-----
gdb/utils.c | 17 -----------------
gdb/utils.h | 5 -----
3 files changed, 11 insertions(+), 27 deletions(-)

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 2bd12d6..bdb2160 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -2012,6 +2012,7 @@ indirect_pieced_value (struct value *value)
int i, bit_offset, bit_length;
struct dwarf_expr_piece *piece = NULL;
LONGEST byte_offset;
+ enum bfd_endian byte_order;

type = check_typedef (value_type (value));
if (TYPE_CODE (type) != TYPE_CODE_PTR)
@@ -2056,11 +2057,16 @@ indirect_pieced_value (struct value *value)
/* This is an offset requested by GDB, such as value subscripts.
However, due to how synthetic pointers are implemented, this is
always presented to us as a pointer type. This means we have to
- sign-extend it manually as appropriate. */
- byte_offset = value_as_address (value);
- if (TYPE_LENGTH (value_type (value)) < sizeof (LONGEST))
- byte_offset = gdb_sign_extend (byte_offset,
- 8 * TYPE_LENGTH (value_type (value)));
+ sign-extend it manually as appropriate. Use raw
+ extract_signed_integer directly rather than value_as_address and
+ sign extend afterwards on architectures that would need it
+ (mostly everywhere except MIPS, which has signed addresses) as
+ the later would go through gdbarch_pointer_to_address and thus
+ return a CORE_ADDR with high bits set on architectures that
+ encode address spaces and other things in CORE_ADDR. */
+ byte_order = gdbarch_byte_order (get_frame_arch (frame));
+ byte_offset = extract_signed_integer (value_contents (value),
+ TYPE_LENGTH (type), byte_order);
byte_offset += piece->v.ptr.offset;

gdb_assert (piece);
diff --git a/gdb/utils.c b/gdb/utils.c
index 084db87..72b1e2a 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -3021,23 +3021,6 @@ align_down (ULONGEST v, int n)
return (v & -n);
}

-/* See utils.h. */
-
-LONGEST
-gdb_sign_extend (LONGEST value, int bit)
-{
- gdb_assert (bit >= 1 && bit <= 8 * sizeof (LONGEST));
-
- if (((value >> (bit - 1)) & 1) != 0)
- {
- LONGEST signbit = ((LONGEST) 1) << (bit - 1);
-
- value = (value ^ signbit) - signbit;
- }
-
- return value;
-}
-
/* Allocation function for the libiberty hash table which uses an
obstack. The obstack is passed as DATA. */

diff --git a/gdb/utils.h b/gdb/utils.h
index 0a73864..3debde7 100644
--- a/gdb/utils.h
+++ b/gdb/utils.h
@@ -340,11 +340,6 @@ extern int myread (int, char *, int);
extern ULONGEST align_up (ULONGEST v, int n);
extern ULONGEST align_down (ULONGEST v, int n);

-/* Sign extend VALUE. BIT is the (1-based) index of the bit in VALUE
- to sign-extend. */
-
-extern LONGEST gdb_sign_extend (LONGEST value, int bit);
-
/* Resource limits used by getrlimit and setrlimit. */

enum resource_limit_kind
--
1.9.3