Re: [PATCH net-next 3/3] bpf: avoid stack copy and use skb ctx for event output

[Fengguang Wu]

Hi Daniel,

On Wed, Jul 13, 2016 at 01:45:47AM +0200, Daniel Borkmann wrote:
> On 07/13/2016 01:25 AM, kbuild test robot wrote:
> > Hi,
> >
> > [auto build test WARNING on net-next/master]
> >
> > url:    https://github.com/0day-ci/linux/commits/Daniel-Borkmann/BPF-event-output-helper-improvements/20160713-065944
> > config: s390-allyesconfig (attached as .config)
> > compiler: s390x-linux-gnu-gcc (Debian 5.3.1-8) 5.3.1 20160205
> > reproduce:
> >          wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
> >          chmod +x ~/bin/make.cross
> >          # save the attached .config to linux build tree
> >          make.cross ARCH=s390
> >
> > All warnings (new ones prefixed by >>):
> >
> >     kernel/trace/bpf_trace.c: In function 'bpf_perf_event_output':
> >     kernel/trace/bpf_trace.c:284:1: warning: 'bpf_perf_event_output' uses dynamic stack allocation
> >      }
> >      ^
> >     kernel/trace/bpf_trace.c: In function 'bpf_event_output':
> > > > kernel/trace/bpf_trace.c:319:1: warning: 'bpf_event_output' uses dynamic stack allocation
> >      }
> >      ^
>
> Hmm, searching a bit on lkml, it seems these warnings on s390 are actually mostly
> harmless I believe [1][2] ... looks like they are there to find structs sitting
> on stack, for example, at least that's also what the currently existing one in the
> above line (bpf_trace.c +284) appears to be about.

Yes it does look so. All such warnings happen only in s390:

% g -h -o '[^ ]*config' *dynamic-stack* | sort | uniq -c | sort -nr
   118 s390-allyesconfig
    80 s390-allmodconfig

Let's ignore all of them on s390.

Thanks,
Fengguang

>   [1] http://lkml.iu.edu/hypermail/linux/kernel/1601.2/04074.html
>   [2] https://lkml.org/lkml/2013/6/25/42