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

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Message-ID: <CAP-5=fV3KaHMR1eFtUh0kY6CmVkZjw8vJ0UNcY3phH=2jsiwyg@mail.gmail.com>
Date: Fri, 25 Jul 2025 17:58:05 -0700
From: Ian Rogers <irogers@...gle.com>
To: Zecheng Li <zecheng@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, "Liang, Kan" <kan.liang@...ux.intel.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Zecheng Li <zli94@...u.edu>, Xu Liu <xliuprof@...gle.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/6] perf dwarf-aux: Use signed comparison in match_var_offset

On Fri, Jul 25, 2025 at 1:28 PM Zecheng Li <zecheng@...gle.com> 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@...gle.com>
> ---
>  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.

Thanks,
Ian

>
>         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
>