[Ada] Crash when trying to set value of packed array element

Consider the following declaration:

   type Small is new Integer range 0 .. 2 ** 4 - 1;
   type Simple_Array is array (1 .. 4) of Small;
   pragma Pack (Simple_Array);

   SA : Simple_Array := (1, 2, 3, 4);

Trying to change the value of one of the elements in the packed array
causes the debugger to crash:

    (gdb) set sa(3) := 9
    [1]    4880 segmentation fault  gdb -q foo

The circumstances leading to the crash are as follow:

   . ada_evaluate_subexp creates a value corresponding to "sa(3)".

   . ada_evaluate_subexp then tries to assign 9 to this value, and
     for this calls value_assign (via ada_value_assign).

   . Because the array is packed, the destination value is 3 bits long,
     and as a result, value_assign uses the parent to determine that
     element byte address and offset:

      | if (value_bitsize (toval))
      |   {
      |     struct value *parent = value_parent (toval);
      |
      |     changed_addr = value_address (parent) + value_offset (toval);

The destination value (corresponding to "sa(3)") was incorrectly created
by ada-lang.c:ada_value_primitive_packed_val, because the "parent" was
left as NULL. So, when we try to dereference it to get the parent address,
GDB crashed.

The first part of the fix therefore consists in setting that field.
This required the addition of a new "setter" in value.[hc].  It fixes
the crash, but is still not sufficient for the assignment to actually
work.

The second part of the problem came from the fact that value_assign
seems to expect the "child"'s address to be equal to the parent's address,
with the difference being the offset. Unfortunately, this requirement was
not followed by ada_value_primitive_packed_val, so the second part of
the fix consisted in fixing that.

Still, this was not sufficient, because it caused a regression when
trying to perform an aggregate assignment of a packed array of packed
record.  The key element here is the nesting of packed entities.
Looking at the way ada_value_primitive_packed_val creates the value
of each sub-component, one can see that the value's offset is set
to the offset compared to the start of the parent. This was meant to
match what value_primitive_field does as well.

So, with our array of records, if the record offset was 2, and if
the field we're interested in that record is at offset 1, the record
value's offset would be set to 2, and the field value's offset would
be set to 1. But the address for both values would be left to the
array's address. This is where things start breaking down, because
the value_address function for our field value would return the
address of the array + 1, instead of + 3.

This is what causes the final issue, here, because ada-lang.c's
value_assign_to_component needs to compute the offset of the
subcomponent compared to the top-level aggregate's start address
(the array in our case). And it does so by subtracting the array's
address from the sub-component's address.  When you have two levels
of packed components, and the mid-level component is at an offset of
the top-level component, things didn't work, because the component's
address was miscomputed (the parent's offset is missing).

The fix consists is fixing value_address to match the work done by
value_primitive_field (where we ignore the parent's offset).

gdb/ChangeLog:

        * value.h (set_value_parent): Add declaration.
        * value.c (set_value_parent): New function.
        (value_address): If VALUE->PARENT is not NULL, then use it as
        the base address instead of VALUE->LOCATION.address.
        * ada-lang.c (ada_value_primitive_packed_val): Keep V's address
        the same as OBJ's address.  Adjust V's offset accordingly.
        Set V's parent.

gdb/testsuite/ChangeLog:

        * gdb.ada/set_pckd_arr_elt: New testcase.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index b813695c9c25126c7f9533371e9850d6495f9538..706788cf63350fb29d7ce710c9cc7f02c8ecdbab 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2012-03-16  Joel Brobecker  <brobecker@adacore.com>
+
+       * value.h (set_value_parent): Add declaration.
+       * value.c (set_value_parent): New function.
+       (value_address): If VALUE->PARENT is not NULL, then use it as
+       the base address instead of VALUE->LOCATION.address.
+       * ada-lang.c (ada_value_primitive_packed_val): Keep V's address
+       the same as OBJ's address.  Adjust V's offset accordingly.
+       Set V's parent.
+
 2012-03-16  Gary Benson  <gbenson@redhat.com>
 
        PR breakpoints/10738

diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index 040d606165d20645f701d63133f68b8c508ee7e4..78a02617f8c1ae490efccc2f7c3386c17a225d73 100644 (file)
--- a/gdb/ada-lang.c
+++ b/gdb/ada-lang.c
@@ -2297,10 +2297,9 @@ ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr,
     }
   else if (VALUE_LVAL (obj) == lval_memory && value_lazy (obj))
     {
-      v = value_at (type,
-                    value_address (obj) + offset);
+      v = value_at (type, value_address (obj));
       bytes = (unsigned char *) alloca (len);
-      read_memory (value_address (v), bytes, len);
+      read_memory (value_address (v) + offset, bytes, len);
     }
   else
     {
@@ -2310,18 +2309,22 @@ ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr,
 
   if (obj != NULL)
     {
-      CORE_ADDR new_addr;
+      long new_offset = offset;
 
       set_value_component_location (v, obj);
-      new_addr = value_address (obj) + offset;
       set_value_bitpos (v, bit_offset + value_bitpos (obj));
       set_value_bitsize (v, bit_size);
       if (value_bitpos (v) >= HOST_CHAR_BIT)
         {
-         ++new_addr;
+         ++new_offset;
           set_value_bitpos (v, value_bitpos (v) - HOST_CHAR_BIT);
         }
-      set_value_address (v, new_addr);
+      set_value_offset (v, new_offset);
+
+      /* Also set the parent value.  This is needed when trying to
+        assign a new value (in inferior memory).  */
+      set_value_parent (v, obj);
+      value_incref (obj);
     }
   else
     set_value_bitsize (v, bit_size);

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index ee59720698f191478c9b533a0d3013052a923eb6..f4a75ddb28105cff0b72bdf6fb5e9209598aeec3 100644 (file)
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2012-03-16  Joel Brobecker  <brobecker@adacore.com>
+
+       * gdb.ada/set_pckd_arr_elt: New testcase.
+
 2012-03-16  Gary Benson  <gbenson@redhat.com>
 
        PR breakpoints/10738

diff --git a/gdb/testsuite/gdb.ada/set_pckd_arr_elt.exp b/gdb/testsuite/gdb.ada/set_pckd_arr_elt.exp
new file mode 100644 (file)
index 0000000..7f6f1d3
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/set_pckd_arr_elt.exp
@@ -0,0 +1,47 @@
+# Copyright 2012 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib "ada.exp"
+
+if { [skip_ada_tests] } { return -1 }
+
+set testdir "set_pckd_arr_elt"
+set testfile "${testdir}/foo"
+set srcfile ${srcdir}/${subdir}/${testfile}.adb
+set binfile ${objdir}/${subdir}/${testfile}
+
+file mkdir ${objdir}/${subdir}/${testdir}
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable {debug}] != ""} {
+    return -1
+}
+
+clean_restart ${testfile}
+
+set bp_location [gdb_get_line_number "STOP" ${testdir}/foo.adb]
+runto "foo.adb:$bp_location"
+
+gdb_test "print sa(3) := 9" " = 9"
+
+# To verify that the assignment was made correctly, we use the fact
+# that the program passes this very same element as an argument to
+# one of the functions.  So we insert a breakpoint on that function,
+# and verify that the argument value is correct.
+
+gdb_breakpoint "update_small"
+
+gdb_test "continue" \
+        "Breakpoint .*, pck\\.update_small \\(s=9\\) at .*pck.adb:.*" \
+        "continue to update_small"
+

diff --git a/gdb/testsuite/gdb.ada/set_pckd_arr_elt/foo.adb b/gdb/testsuite/gdb.ada/set_pckd_arr_elt/foo.adb
new file mode 100644 (file)
index 0000000..7f4f45d
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/set_pckd_arr_elt/foo.adb
@@ -0,0 +1,22 @@
+--  Copyright 2012 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+with Pck; use Pck;
+
+procedure Foo is
+   SA : Simple_Array := (1, 2, 3, 4);
+begin
+   Update_Small (SA (3));  -- STOP
+end Foo;

diff --git a/gdb/testsuite/gdb.ada/set_pckd_arr_elt/pck.adb b/gdb/testsuite/gdb.ada/set_pckd_arr_elt/pck.adb
new file mode 100644 (file)
index 0000000..a2bec81
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/set_pckd_arr_elt/pck.adb
@@ -0,0 +1,21 @@
+--  Copyright 2012 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package body Pck is
+   procedure Update_Small (S : in out Small) is
+   begin
+      null;
+   end Update_Small;
+end Pck;

diff --git a/gdb/testsuite/gdb.ada/set_pckd_arr_elt/pck.ads b/gdb/testsuite/gdb.ada/set_pckd_arr_elt/pck.ads
new file mode 100644 (file)
index 0000000..6be95e2
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/set_pckd_arr_elt/pck.ads
@@ -0,0 +1,22 @@
+--  Copyright 2012 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package Pck is
+   type Small is new Integer range 0 .. 2 ** 6 - 1;
+   type Simple_Array is array (1 .. 4) of Small;
+   pragma Pack (Simple_Array);
+
+   procedure Update_Small (S : in out Small);
+end Pck;

diff --git a/gdb/value.c b/gdb/value.c
index e8eb33f3400f7155931c3aeb01202d5dc449e908..c23803a33497d42ef0995a8bc0e339cdad771633 100644 (file)
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -806,6 +806,14 @@ value_parent (struct value *value)
   return value->parent;
 }
 
+/* See value.h.  */
+
+void
+set_value_parent (struct value *value, struct value *parent)
+{
+  value->parent = parent;
+}
+
 gdb_byte *
 value_contents_raw (struct value *value)
 {
@@ -1101,7 +1109,10 @@ value_address (const struct value *value)
   if (value->lval == lval_internalvar
       || value->lval == lval_internalvar_component)
     return 0;
-  return value->location.address + value->offset;
+  if (value->parent != NULL)
+    return value_address (value->parent) + value->offset;
+  else
+    return value->location.address + value->offset;
 }
 
 CORE_ADDR

diff --git a/gdb/value.h b/gdb/value.h
index 3ce0f88138317546636c3058511334595268172c..d501b52e5d1b23986e61fbc62a49a1ac852f350f 100644 (file)
--- a/gdb/value.h
+++ b/gdb/value.h
@@ -74,6 +74,7 @@ extern void set_value_bitpos (struct value *, int bit);
    bitfields.  */
 
 struct value *value_parent (struct value *);
+extern void set_value_parent (struct value *value, struct value *parent);
 
 /* Describes offset of a value within lval of a structure in bytes.
    If lval == lval_memory, this is an offset to the address.  If lval