Re: [PATCH v2 1/3] perf dwarf-aux: Add die_is_compound_type() to handle C++ class types

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Thu, Oct 01, 2026 at 11:14:21AM -0700, Namhyung Kim wrote:
> On Wed, Sep 30, 2026 at 09:10:51PM +0000, sashiko-bot@kernel.org wrote:
> > Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> > - [High] perf/dwarf-aux: stack overflow in __die_find_member_offset_cb via cyclic inheritance
> > - [Medium] perf/dwarf-aux: flex array detection fails for C++ classes
> > --
> > 
> > commit 3f3b9b56b2e28f1713af3b40ab8946d5bbc8d245
> > Author: Yanbo Zhao <yzhao62@ncsu.edu>
> > 
> > perf dwarf-aux: Add die_is_compound_type() to handle C++ class types
> > 
> > This commit introduces die_is_compound_type() to support C++ classes alongside
> > structs and unions. It also updates offset lookups to descend into base class
> > subobjects using DW_TAG_inheritance entries.
> > 
> > > diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> > > index 54f8b5ec74a2f..eb4b8f3475df1 100644
> > > --- a/tools/perf/util/dwarf-aux.c
> > > +++ b/tools/perf/util/dwarf-aux.c
> > > @@ -61,6 +61,14 @@ const char *cu_get_comp_dir(Dwarf_Die *cu_die)
> > >  	return dwarf_formstring(&attr);
> > >  }
> > >  
> > > +bool die_is_compound_type(Dwarf_Die *type_die)
> > > +{
> > > +	int tag = dwarf_tag(type_die);
> > > +
> > > +	return tag == DW_TAG_structure_type || tag == DW_TAG_union_type ||
> > > +	       tag == DW_TAG_class_type;
> > > +}
> > 
> > [Severity: Medium]
> > Does die_has_flex_array_recurse() also need to be updated to use this new
> > helper?
> > 
> > If we look at die_has_flex_array_recurse() in tools/perf/util/dwarf-aux.c,
> > it currently forces flex array detection to return false for C++ classes
> > because it explicitly ignores DW_TAG_class_type:
> > 
> >     if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type)
> >         return false;
> >     ...
> >     if (tag == DW_TAG_structure_type && found) {
> >         if (is_flex_array_member(&last_mb))
> >             return true;
> > 
> > Could this cause accesses to flex array members inside C++ classes to be
> > wrongly rejected as out-of-bounds?
 
> IIUC C++ officially doesn't support flexible arrays and there are
> better alternatives so I guess it's rare to see them in C++ classes.
> Maybe it's ok to ignore them until we find it's really needed.

ok on both counts, there is another patch with a Sashiko report, can you
guys reply to that one as well?

Thanks,

- Arnaldo