Re: [PATCH v2 2/4] perf dwarf-aux: Add die_has_flex_array() helper

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On Sun, 13 Sep 2026 23:45:33 -0700
Namhyung Kim <namhyung@xxxxxxxxxx> wrote:

> The die_has_flex_array() returns true when the given type is a compound
> type and contains an array at the end.  To prevent an infinite recursion
> add a depth field to the internal function.

Hi, thanks for this nice extension!
BTW, I have a comment on this implementation.

> 
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/util/dwarf-aux.c | 77 +++++++++++++++++++++++++++++++++++++
>  tools/perf/util/dwarf-aux.h |  3 ++
>  2 files changed, 80 insertions(+)
> 
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index d7160f87ac7d7ab3..465824e6513eb0ac 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -2180,3 +2180,80 @@ Dwarf_Die *die_deref_ptr_type(Dwarf_Die *ptr_die, int offset,
>  
>  	return die_get_member_type(&type_die, offset, die_mem);
>  }
> +
> +static bool is_flex_array_member(Dwarf_Die *mb_die)
> +{
> +	Dwarf_Die type_die;
> +	Dwarf_Word size;
> +
> +	/* get the type of the member */
> +	if (die_get_real_type(mb_die, &type_die) == NULL)
> +		return false;
> +
> +	if (dwarf_tag(&type_die) != DW_TAG_array_type)
> +		return false;
> +
> +	return dwarf_aggregate_size(&type_die, &size) < 0 || size == 0;
> +}
> +
> +#define MAX_FLEX_ARRAY_RECURSION  256  /* arbitrary */
> +
> +static bool die_has_flex_array_recurse(Dwarf_Die *parent_die, int depth)
> +{
> +	Dwarf_Die die_mem, last_mb;
> +	int tag = dwarf_tag(parent_die);
> +
> +	if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type)
> +		return false;

What happen if the parent_die is "typedef struct {..." ?
I think you need to use die_get_real_type() here. ;)

> +
> +	/* prevent infinite recursion */
> +	if (depth > MAX_FLEX_ARRAY_RECURSION)
> +		return false;
> +
> +	if (dwarf_child(parent_die, &die_mem))
> +		return false;
> +
> +	do {
> +		if (dwarf_tag(&die_mem) != DW_TAG_member)
> +			continue;

If the member is a "const" member, you may have to use
die_get_real_type() to get the actual type.
(in this case, you need a cursor DIE for dwarf_siblingof()) 

> +
> +		if (tag == DW_TAG_union_type) {
> +			if (is_flex_array_member(&die_mem))
> +				return true;
> +
> +			if (die_get_real_type(&die_mem, &last_mb) &&
> +			    die_has_flex_array_recurse(&last_mb, depth + 1))
> +				return true;
> +		}
> +
> +		if (tag == DW_TAG_structure_type)
> +			memcpy(&last_mb, &die_mem, sizeof(last_mb));

To find the last member, I think you'd better check the
DW_AT_data_member_location and DW_AT_decl_line to ensure the
DIE is the last member.

Thank you,


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>