[PATCH] gdb: Check DW_OP_deref_type size against type

[Jielun Wu <firmiana402@gmail.com>]

DWARF v5 requires the explicit size operand of DW_OP_deref_type to
match the size of the referenced base type.  The evaluator currently
passes both sizes to dwarf_expr_context::deref without validating the
encoding, and the generic dereference helper accepts the mismatch by
zero-extending the bytes read from memory.

Reject a mismatch after resolving the base type.  Add a DWARF assembler
test covering both DW_OP_deref_type and DW_OP_GNU_deref_type.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34277
Signed-off-by: Jielun Wu <firmiana402@gmail.com>
---
 gdb/dwarf2/expr.c                             |  3 +
 .../gdb.dwarf2/dw2-deref-type-size.exp        | 78 +++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-deref-type-size.exp

diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
index 3a6b8f58199..ff23fb3e891 100644
--- a/gdb/dwarf2/expr.c
+++ b/gdb/dwarf2/expr.c
@@ -2027,6 +2027,9 @@ dwarf_expr_context::execute_stack_op (gdb::array_view<const gdb_byte> expr)
 		op_ptr = safe_read_uleb128 (op_ptr, op_end, &uoffset);
 		cu_offset type_die_cu_off = (cu_offset) uoffset;
 		type = get_base_type (type_die_cu_off);
+		if (type->length () != addr_size)
+		  error (_("DW_OP_deref_type has different sizes for type and "
+			   "data"));
 	      }
 	    else
 	      type = address_type;
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-deref-type-size.exp b/gdb/testsuite/gdb.dwarf2/dw2-deref-type-size.exp
new file mode 100644
index 00000000000..52eae9e9eae
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-deref-type-size.exp
@@ -0,0 +1,78 @@
+# Copyright 2026 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/>.
+
+# Test that DW_OP_deref_type rejects an explicit dereference size that
+# differs from the size of the referenced base type.
+
+load_lib dwarf.exp
+
+require dwarf2_support
+
+standard_testfile main.c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+
+Dwarf::assemble $asm_file {
+    cu {version 5} {
+	compile_unit {} {
+	    declare_labels uint64_label
+
+	    uint64_label: base_type {
+		DW_AT_name "uint64_t"
+		DW_AT_encoding @DW_ATE_unsigned
+		DW_AT_byte_size 8 DW_FORM_sdata
+	    }
+
+	    foreach {var op} {
+		bad_deref_type DW_OP_deref_type
+		bad_gnu_deref_type DW_OP_GNU_deref_type
+	    } {
+		DW_TAG_variable {
+		    DW_AT_name $var
+		    DW_AT_type :$uint64_label
+		    DW_AT_external 1 DW_FORM_flag
+		    DW_AT_location {
+			variable _constants
+			variable _cu_label
+
+			DW_OP_addr main_label
+			_op .byte $_constants($op) $op
+			_op .byte 4 "dereference size"
+			_op .uleb128 "$uint64_label - $_cu_label" \
+			    "base type DIE offset"
+			DW_OP_stack_value
+		    } SPECIAL_expr
+		}
+	    }
+	}
+    }
+}
+
+if {[prepare_for_testing "failed to prepare" ${testfile} \
+	 [list $srcfile $asm_file] nodebug]} {
+    return
+}
+
+if {![runto_main]} {
+    return
+}
+
+foreach_with_prefix var {
+    bad_deref_type
+    bad_gnu_deref_type
+} {
+    gdb_test "print $var" \
+	"DW_OP_deref_type has different sizes for type and data"
+}
-- 
2.34.1