linux-kernel - Re: [PATCH] bpf: avoid warning for wrong pointer cast

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Message-ID: <20160419100956.GB8836@intel.com>
Date:	Tue, 19 Apr 2016 18:09:56 +0800
From:	Philip Li <philip.li@...el.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	"David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Alexei Starovoitov <ast@...com>,
	Daniel Borkmann <daniel@...earbox.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: avoid warning for wrong pointer cast

On Tue, Apr 19, 2016 at 10:33:34AM +0800, Fengguang Wu wrote:
> Hi Alexei,
> 
> On Sat, Apr 16, 2016 at 05:47:42PM -0700, Alexei Starovoitov wrote:
> > On Sat, Apr 16, 2016 at 10:29:33PM +0200, Arnd Bergmann wrote:
> > > Two new functions in bpf contain a cast from a 'u64' to a
> > > pointer. This works on 64-bit architectures but causes a warning
> > > on all 32-bit architectures:
> > > 
> > > kernel/trace/bpf_trace.c: In function 'bpf_perf_event_output_tp':
> > > kernel/trace/bpf_trace.c:350:13: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
> > >   u64 ctx = *(long *)r1;
> > > 
> > > This changes the cast to first convert the u64 argument into a uintptr_t,
> > > which is guaranteed to be the same size as a pointer.
> > > 
> > > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > > Fixes: 9940d67c93b5 ("bpf: support bpf_get_stackid() and bpf_perf_event_output() in tracepoint programs")
> > 
> > Thanks.
> > Acked-by: Alexei Starovoitov <ast@...nel.org>
> > 
> > I guess I started to rely on 0-day build-bot too much.
> > This patch has been in my tree for 2+ weeks and then in net-next and
> > I didn't receive a single email from build-bot about this warning,
> > though I do receive them for my other work-in-progress stuff. Odd.
> > Fengguang, any idea why build-bot sometimes silent?
> 
> Sorry I went off for some time.. Philip, would you help have a check?
Hi Alexei, i have done some investigation for this. Fengguang, pls correct me if my understanding is wrong.

0day has caught the warning for f1ff54 commit around 2016-3-22, which considers the same warning in future is not new one, thus neglect it.

	rli9@inn /kbuild-tests/build-error$ cat kernel-trace-bpf_trace.c:warning:cast-to-pointer-from-integer-of-different-size
	kernel/trace/bpf_trace.c:345:13: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

	2016-03-22 17:12:38 xian bpf:master:321e70e79c94e5e6394a882d567baac949b74000 i386-randconfig-x009-201612 f1ff543dbdd3eff53c8328cfb582f18e6c3d56ec

And for f1ff54's warning, 0day actually bisects to it, but then it checks the head of bpf/master at that time which is 321e70e, find the error is not existed, so
it ignores the interim warning to not send email report.

2016-03-22_17:15:49 O: R: /kbuild/src/consumer/kernel/trace/bpf_trace.c:345:13: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
2016-03-22_17:15:49 O: grep -h -r . /tmp/kernel/i386-randconfig-x009-201612/gcc-5/f1ff543dbdd3eff53c8328cfb582f18e6c3d56ec/build-error # 0 errors 
2016-03-22_17:15:49 O: grep -h -r . /tmp/kernel/i386-randconfig-x009-201612/gcc-5/f1ff543dbdd3eff53c8328cfb582f18e6c3d56ec/build-error/ # 0 errors 
2016-03-22_17:15:49 O: /c/kernel-tests/list-head bpf:master:321e70e79c94e5e6394a882d567baac949b74000 i386-randconfig-x009-201612 
2016-03-22_17:16:03 O: .
2016-03-22_17:16:03 O: ..
2016-03-22_17:16:03 O: kernel-trace-bpf_trace.c:warning:cast-to-pointer-from-integer-of-different-size
2016-03-22_17:16:03 O: 2016-03-22 17:16:03 don't email interim warnings
2016-03-22_17:16:03 O: 2016-03-22 17:16:03 error no longer exist in head bpf/master 321e70e79c94e5e6394a882d567baac949b74000, check 
/tmp/kernel/i386-randconfig-x009-201612/gcc-5/321e70e79c94e5e6394a882d567baac949b74000, errno 14
2016-03-22_17:16:03 O: 2016-03-22 17:16:03 Don't email interim warnings!


what really happens to 321e70e is it has build error (i rerun it to confirm), which is reported at https://lists.01.org/pipermail/kbuild-all/2016-March/018625.html. So this is
a possible change to be done in 0day to report the first bad commit when HEAD is build error.

2016-04-19_15:00:39 O: 2016-04-19 15:00  Running /kbuild-tests/build-queue/lkp-nex06-consumer/i386-randconfig-x009-201612-gcc-5-321e70e79c94e5e6394a882d567baac949b74000
2016-04-19_15:00:39 O: create new KBUILD_OUTPUT dir /kbuild/obj/consumer/i386-randconfig-x009-201612
2016-04-19_15:00:39 O: git checkout -B build-queue 321e70e79c94e5e6394a882d567baac949b74000
2016-04-19_15:07:08 E: 330 real  4732 user  629 sys  1620.50% cpu       i386-randconfig-x009-201612
2016-04-19_15:07:13 O: status: FAIL: build error
2016-04-19_15:07:13 O: ERROR: "tcp_sendpage" [drivers/staging/lustre/lnet/klnds/socklnd/ksocklnd.ko] undefined!

> 
> Thanks,
> Fengguang