linux-kernel - Re: [PATCH v1 3/6] perf dwarf-aux: Better type matching for stack variables

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aIVxK-xXxmHXzLTV@google.com>
Date: Sat, 26 Jul 2025 17:22:03 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Zecheng Li <zecheng@...gle.com>, 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>,
	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 3/6] perf dwarf-aux: Better type matching for stack
 variables

On Fri, Jul 25, 2025 at 06:17:33PM -0700, Ian Rogers wrote:
> On Fri, Jul 25, 2025 at 1:28 PM Zecheng Li <zecheng@...gle.com> wrote:
> >
> > Utilizes the previous is_breg_access_indirect function to determine if
> > the stack location stores the variable itself or the struct it points
> > to.
> >
> > If the DWARF expression shows DW_OP_stack_value without DW_OP_deref, it
> > indicates the variable value is the reg + offset itself, and the stack
> > location it points to is the dereferenced type.
> >
> > Signed-off-by: Zecheng Li <zecheng@...gle.com>
> > ---
> >  tools/perf/util/dwarf-aux.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> > index 814c96ea509f..4039dbd2b8c0 100644
> > --- a/tools/perf/util/dwarf-aux.c
> > +++ b/tools/perf/util/dwarf-aux.c
> > @@ -1635,6 +1635,14 @@ static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
> >         if (die_get_real_type(die_mem, &type_die) == NULL)
> >                 return DIE_FIND_CB_SIBLING;
> >
> > +       if ((ops->atom == DW_OP_fbreg || ops->atom == DW_OP_breg7) &&
> 
> A comment saying the significance of DW_OP_fbreg and DW_OP_breg7 would
> be useful, for example, why not DW_OP_breg6? Isn't breg7 going to be
> x86 specific?

Good point!  Right, it's x86 specific and we cannot hard-code.  It needs
a way to indicate this register is a stack pointer on the target arch.
Please see the instruction tracking code which checks state->stack_reg.
 
On x86_64, DWARF reg 7 is RSP.  If RBP is used as a frame pointer, it's
probably using DW_OP_fbreg expression.

> 
> > +           dwarf_tag(&type_die) == DW_TAG_pointer_type &&
> > +           is_breg_access_indirect(ops, nops)) {

Maybe it needs to check the register inside is_breg_access_indirect().

Thanks,
Namhyung


> > +               /* Get the target type of the pointer */
> > +               if (die_get_real_type(&type_die, &type_die) == NULL)
> > +                       return DIE_FIND_CB_SIBLING;
> > +       }
> > +
> >         vt = malloc(sizeof(*vt));
> >         if (vt == NULL)
> >                 return DIE_FIND_CB_END;
> > --
> > 2.50.1.470.g6ba607880d-goog
> >