write_pieced_value: Fix size capping logic

A field f in a structure composed of DWARF pieces may be located in
multiple pieces, where the first and last of those may contain bits from
other fields as well.  So when writing to f, the beginning of the first
and the end of the last of those pieces may have to be skipped.  But the
logic in write_pieced_value for handling one of those pieces is flawed
when the first and last piece are the same, i.e., f is contained in a
single piece:

  < - - - - - - - - - piece_size - - - - - - - - - ->
  +-------------------------------------------------+
  | skipped_bits |   f_bits   | / / / / / / / / / / |
  +-------------------------------------------------+

The current logic determines the size of the sub-piece to operate on by
limiting the piece size to the bit size of f and then subtracting the
skipped bits:

  min (piece_size, f_bits) - skipped_bits

Instead of:

  min (piece_size - skipped_bits, f_bits)

So the resulting sub-piece size is corrupted, leading to wrong handling of
this piece in write_pieced_value.

Note that the same bug was already found in read_pieced_value and fixed
there (but not in write_pieced_value), see PR 15391.

This patch swaps the calculations, bringing them into the same (correct)
order as in read_pieced_value.

gdb/ChangeLog:

	* dwarf2loc.c (write_pieced_value): Fix order of calculations for
	size capping.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/var-pieces.exp: Add test case for modifying a
	variable at nonzero offset.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index c2bceeb8a77477ba49385d13b3beb122fdf5bb1f..880cd60760adce093eb3446a76a1a3276e2a977a 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+       * dwarf2loc.c (write_pieced_value): Fix order of calculations for
+       size capping.
+
 2017-06-13  Yao Qi  <yao.qi@linaro.org>
 
        * mips-linux-nat.c: Move include features/mips*-linux.c to

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 127167d887bc486274a343d3daaa6e0e91888f34..2a45a79b50967c99351b96b96fb74f1ebd5f471f 100644 (file)
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1964,8 +1964,6 @@ write_pieced_value (struct value *to, struct value *from)
          bits_to_skip -= this_size_bits;
          continue;
        }
-      if (this_size_bits > type_len - offset)
-       this_size_bits = type_len - offset;
       if (bits_to_skip > 0)
        {
          dest_offset_bits = bits_to_skip;
@@ -1978,6 +1976,8 @@ write_pieced_value (struct value *to, struct value *from)
          dest_offset_bits = 0;
          source_offset_bits = offset;
        }
+      if (this_size_bits > type_len - offset)
+       this_size_bits = type_len - offset;
 
       this_size = (this_size_bits + source_offset_bits % 8 + 7) / 8;
       source_offset = source_offset_bits / 8;

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 4993a2db79755998a2d101053f36fd930d733544..688efdf6314ffa1bf23c68672fee3e2c9f6ee5d3 100644 (file)
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+       * gdb.dwarf2/var-pieces.exp: Add test case for modifying a
+       variable at nonzero offset.
+
 2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
        * gdb.dwarf2/var-access.c: New file.

diff --git a/gdb/testsuite/gdb.dwarf2/var-access.exp b/gdb/testsuite/gdb.dwarf2/var-access.exp
index a52327d7dbb3ed5f71abd19a8e3920a21e6c0d5e..bd92a44ef54abf40147d9595df84ee937c44a644 100644 (file)
--- a/gdb/testsuite/gdb.dwarf2/var-access.exp
+++ b/gdb/testsuite/gdb.dwarf2/var-access.exp
@@ -178,6 +178,11 @@ gdb_test "print/d s1" " = \\{a = 63, b = 3, c = 0, d = 1\\}" \
     "verify s1.a"
 gdb_test "print/d a" " = \\{0, 1, 63, 3, 4, 5, 6, 7\\}" \
     "verify s1.a through a"
+gdb_test_no_output "set var s1.b = 42"
+gdb_test "print/d s1" " = \\{a = 63, b = 42, c = 0, d = 1\\}" \
+    "verify s1.b"
+gdb_test "print/d a" " = \\{0, 1, 63, 42, 4, 5, 6, 7\\}" \
+    "verify s1.b through a"
 
 # Byte-aligned register- and memory pieces.
 gdb_test_no_output "set var \$[lindex $regname 0] = 81" \