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

[Tengda Wu]

On 2026/9/14 14:45, Namhyung Kim 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.
> 
> 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;
> +
> +	/* 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 (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));
> +
> +	} while (dwarf_siblingof(&die_mem, &die_mem) == 0);
> +
> +	if (tag == DW_TAG_structure_type) {
> +		if (is_flex_array_member(&last_mb))

Not sure if this case can actually happen, but adding a check for whether
last_mb is valid is probably better. (I see Sashiko flagged this too.)

Thanks,
Tengda

> +			return true;
> +
> +		if (die_get_real_type(&last_mb, &die_mem))
> +			return die_has_flex_array_recurse(&die_mem, depth + 1);
> +	}
> +
> +	return false;
> +}
> +
> +/**
> + * die_has_flex_array - Check if the given type has a flex-array at the end
> + * @parent_die: a pointer to type DIE
> + *
> + * This function returns %true iff @parent_die is a struct type and has an
> + * array at the end.  Note that the flex-array has no element, it should have
> + * no size and the parent size doesn't include the flex-array.  So it should
> + * locate at the offset of the parent size.
> + *
> + * For simplicity, it assumes the parent size of aligned with the flex-array.
> + */
> +bool die_has_flex_array(Dwarf_Die *parent_die)
> +{
> +	return die_has_flex_array_recurse(parent_die, 0);
> +}
> diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
> index 161f0bf980b6ee6a..9b662db710220522 100644
> --- a/tools/perf/util/dwarf-aux.h
> +++ b/tools/perf/util/dwarf-aux.h
> @@ -189,4 +189,7 @@ void die_collect_global_vars(Dwarf_Die *cu_die, struct die_var_type **var_types)
>  /* Get the frame base information from CFA */
>  int die_get_cfa(Dwarf *dwarf, u64 pc, int *preg, int *poffset);
>  
> +/* Check whether given type has a flex array */
> +bool die_has_flex_array(Dwarf_Die *parent_die);
> +
>  #endif /* _DWARF_AUX_H */