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

From: Fengguang Wu
Date: Tue Jul 12 2016 - 20:02:47 EST

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:

[auto build test WARNING on net-next/master]

config: s390-allyesconfig (attached as .config)
compiler: s390x-linux-gnu-gcc (Debian 5.3.1-8) 5.3.1 20160205
wget -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.