linux-kernel - Re: [PATCH v2 01/10] perf dwarf-aux: Use signed variable types in match_var

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Message-ID: <aK_8kt5Yf9MDoPdu@google.com>
Date: Wed, 27 Aug 2025 23:52:02 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Zecheng Li <zecheng@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Xu Liu <xliuprof@...gle.com>, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/10] perf dwarf-aux: Use signed variable types in
 match_var_offset

On Mon, Aug 25, 2025 at 07:54:03PM +0000, Zecheng Li 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.

Right, I thought it cannot get negative offsets except for stack access
(e.g. fbreg).  But it turns out that container_of() trick can generate
them with optimizing compilers.

> 
> Change the signature of match_var_offset to use signed types (s64). This
> ensures correct behavior when addr_offset or addr_type are negative.
> 
> Signed-off-by: Zecheng Li <zecheng@...gle.com>

I've confirmed it produced slightly better results on my test sets.

Reviewed-by: Namhyung Kim <namhyung@...nel.org>

Thanks,
Namhyung

> ---
>  tools/perf/util/dwarf-aux.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index 559c953ca172..920054425578 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -1388,18 +1388,19 @@ 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)
> +			     s64 addr_offset, s64 addr_type, bool is_pointer)
>  {
>  	Dwarf_Die type_die;
>  	Dwarf_Word size;
> +	s64 offset = addr_offset - addr_type;
>  
> -	if (addr_offset == addr_type) {
> +	if (offset == 0) {
>  		/* Update offset relative to the start of the variable */
>  		data->offset = 0;
>  		return true;
>  	}
>  
> -	if (addr_offset < addr_type)
> +	if (offset < 0)
>  		return false;
>  
>  	if (die_get_real_type(die_mem, &type_die) == NULL)
> @@ -1414,11 +1415,11 @@ 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 ((u64)offset >= size)
>  		return false;
>  
>  	/* Update offset relative to the start of the variable */
> -	data->offset = addr_offset - addr_type;
> +	data->offset = offset;
>  	return true;
>  }
>  
> -- 
> 2.51.0.261.g7ce5a0a67e-goog
>