Re: [PATCH 09/18] tracing: Add indexing of arguments for function based events

Steven Rostedt
2018-02-09T00:19:10Z

On Fri, 9 Feb 2018 08:56:15 +0900
Namhyung Kim <namhyung@kernel.org> wrote:

> On Thu, Feb 08, 2018 at 10:43:43AM -0500, Steven Rostedt wrote:
> > On Thu, 8 Feb 2018 19:59:24 +0900
> > Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > > > @@ -347,6 +361,8 @@ static long long get_arg(struct func_arg *arg, unsigned long val)
> > > > char buf[8];
> > > > int ret;
> > > >
> > > > + val += arg->index;
> > > > +
> > > > if (!arg->indirect)
> > > > return val;
> > >
> > > So this also works without the indirect, and just add the immediate to
> > > the value.
> >
> > Not sure what you are asking here. The immediate adds to the current
> > value, where as the indirect will then look what's at that location.
>
> I expected that the immediate offset is only meaningful with the
> indirect (dereference) as the doc says just about it. So I asked it
> was intentional or not.
>

Yes it is intentional, but rather useless without an indirect. I mean,
you could just add to the value if you want :-)

The reason it doesn't need the indirect is because there's some types
(arrays and strings) that don't need the indirect. For example, with
the net_device with the perm_addr at 558 bytes away:

echo 'ip_rcv(NULL, x8[6] perm_addr+558)' > function_events

produces:

<idle>-0 [003] ..s3 1809.074329: __netif_receive_skb_core->ip_rcv(perm_addr=b4,b5,2f,ce,18,65)

-- Steve