Re: [PATCH v1 1/6] perf dwarf-aux: Use signed comparison in match_var_offset

[Namhyung Kim]

On Fri, Jul 25, 2025 at 05:58:05PM -0700, Ian Rogers wrote:
> On Fri, Jul 25, 2025 at 1:28 PM Zecheng Li <zecheng@xxxxxxxxxx> wrote:
> >
> > match_var_offset compares address offsets to determine if an access
> > falls within a variable's bounds. The offsets involved for those
> > relative to base registers from DW_OP_breg can be negative.
> >
> > The current implementation uses unsigned types (u64) for these offsets,
> > which rejects almost all negative values.
> >
> > This commit changes the local variables within match_var_offset to
> > signed types (s64) before performing comparisons. This ensures correct
> > behavior when addr_offset_ or addr_type_ are negative.
> >
> > Signed-off-by: Zecheng Li <zecheng@xxxxxxxxxx>
> > ---
> >  tools/perf/util/dwarf-aux.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> > index 559c953ca172..bf906dff9ef0 100644
> > --- a/tools/perf/util/dwarf-aux.c
> > +++ b/tools/perf/util/dwarf-aux.c
> > @@ -1388,10 +1388,12 @@ struct find_var_data {
> >  #define DWARF_OP_DIRECT_REGS  32
> >
> >  static bool match_var_offset(Dwarf_Die *die_mem, struct find_var_data *data,
> > -                            u64 addr_offset, u64 addr_type, bool is_pointer)
> > +                            u64 addr_offset_, u64 addr_type_, bool is_pointer)
> >  {
> >         Dwarf_Die type_die;
> >         Dwarf_Word size;
> > +       s64 addr_offset = (s64)addr_offset_;
> > +       s64 addr_type = (s64)addr_type_;
> 
> Would it be better to make the function take signed types? I'm
> thinking if a 32-bit int is passed, with the signature as-is it is
> unclear if sign-extension will happen.

Hmm.. right.  The addr_offset often from 'int' type so negative value
can have the sign-extension problem.

Zecheng, can you please update the function signature to s64 and check
if the final offset is negative or bigger than the size?

Thanks,
Namhyung

> >
> >         if (addr_offset == addr_type) {
> >                 /* Update offset relative to the start of the variable */
> > @@ -1414,7 +1416,7 @@ static bool match_var_offset(Dwarf_Die *die_mem, struct find_var_data *data,
> >         if (dwarf_aggregate_size(&type_die, &size) < 0)
> >                 return false;
> >
> > -       if (addr_offset >= addr_type + size)
> > +       if (addr_offset_ - addr_type_ >= size)
> >                 return false;
> >
> >         /* Update offset relative to the start of the variable */
> > --
> > 2.50.1.470.g6ba607880d-goog
> >