linux-kernel - Re: [PATCH] building libtraceevent with clang

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Date:   Mon, 13 Feb 2017 13:26:22 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] building libtraceevent with clang

Em Mon, Feb 13, 2017 at 01:24:55PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Fri, Feb 10, 2017 at 02:14:08PM -0500, Steven Rostedt escreveu:
> > On Fri, 10 Feb 2017 14:03:17 -0300
> > Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > > +	case OLD_RINGBUF_TYPE_TIME_EXTEND:
> > > +		extend = read_4(kbuf, ptr);
> > > +		extend <<= TS_SHIFT;
> > > +		extend += delta;
> > > +		delta = extend;
> > > +		ptr += 4;
> > > +		/* Fall through */
>  
> > No, actually, the length should be zeroed here.
>  
> > 	length = 0;
>  
> > As the ptr is placed passed the metadata. At the end of this case
> > statement, the ptr will be after the metadata. The length is
> > to represent the length of the data that is part of the event. The
> > index is the ptr - start_data + length. As there's no data to this type
> > of event, it should be zero.
>  
> > When I add length = 0, I get more events from my old 2.6.30 trace.dat
> > file :-/  I'll have to update my tests.
>  
> > Care to send another patch?
> 
> I have this now in my tree, and since the patch is basically yours, I
> just made myself the reporter and you the author, please ack.
> 
> 
> commit 1fb82b6cba967afde11957099a9ef440cf8c2248
> Author: Steven Rostedt <rostedt@...dmis.org>
> Date:   Mon Feb 13 13:18:29 2017 -0300
> 
>     tools lib traceevent: Initialize lenght on OLD_RING_BUFFER_TYPE_TIME_STAMP
>     
>     A undefined value was being used for the OLD_RING_BUFFER_TYPE_TIME_STAMP
>     case entry, as the 'lenght' variable was not being initialized, fix it.
>     
>     Caught by the reporter when building tools/perf/ using clang, which emmitted
>     this warning:
>     
>       kbuffer-parse.c:312:7: warning: variable 'length' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
>               case OLD_RINGBUF_TYPE_TIME_EXTEND:
>                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       kbuffer-parse.c:339:29: note: uninitialized use occurs here
>               kbuf->next = kbuf->index + length;
>                                        ^~~~~~
>       kbuffer-parse.c:297:21: note: initialize the variable 'length' to silence this warning
>               unsigned int length;
>                                  ^
>                                   = 0
>     
>     Reported-by: Arnaldo Carvalho de Melo <acme@...hat.com>
>     Cc: Adrian Hunter <adrian.hunter@...el.com>
>     Cc: David Ahern <dsahern@...il.com>
>     Cc: Jiri Olsa <jolsa@...nel.org>
>     Cc: Namhyung Kim <namhyung@...nel.org>
>     Cc: Wang Nan <wangnan0@...wei.com>
>     Link: http://lkml.kernel.org/r/20170210141408.5eeb6e91@gandalf.local.home
>     Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 
> diff --git a/tools/lib/traceevent/kbuffer-parse.c b/tools/lib/traceevent/kbuffer-parse.c
> index 65984f1c2974..2009cb7d9675 100644
> --- a/tools/lib/traceevent/kbuffer-parse.c
> +++ b/tools/lib/traceevent/kbuffer-parse.c
> @@ -315,6 +315,7 @@ static unsigned int old_update_pointers(struct kbuffer *kbuf)
>  		extend += delta;
>  		delta = extend;
>  		ptr += 4;
> +		lenght = 0;

ouch, 'length' :-)

clang provides a really nice error message:

kbuffer-parse.c:318:3: error: use of undeclared identifier 'lenght'; did you mean 'length'?
                lenght = 0;
                ^~~~~~
                length
kbuffer-parse.c:297:15: note: 'length' declared here
        unsigned int length;
                     ^
1 error generated.

I only had to test compile it :-)

>  		break;
>  
>  	case OLD_RINGBUF_TYPE_TIME_STAMP:
>