Re: [PATCH] gdb: Check DW_OP_deref_type size against type

[Keith Seitz <keiths@redhat.com>]

Hi,

On 8/14/26 1:53 AM, Jielun Wu wrote:
> 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.

This looks good to me and is consistent with how DW_OP_const_type
handles the identical scenario. Thank you very much for the test.
That is very appreciated.

Reviewed-By: Keith Seitz <keiths@redhat.com>

Thank you for the patch!

Keith

> 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"
> +}