linux-kernel - Re: [PATCH bpf-next 08/11] samples: bpf: fix shifting unsigned long by 32 positions

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Date:   Wed, 27 Apr 2022 11:53:40 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Alexander Lobakin <alobakin@...me>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dmitrii Dolgov <9erthalion6@...il.com>,
        Quentin Monnet <quentin@...valent.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Chenbo Feng <fengc@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Daniel Wagner <daniel.wagner@...-carit.de>,
        Thomas Graf <tgraf@...g.ch>,
        Ong Boon Leong <boon.leong.ong@...el.com>,
        "linux-perf-use." <linux-perf-users@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH bpf-next 08/11] samples: bpf: fix shifting unsigned long
 by 32 positions

On Wed, Apr 27, 2022 at 8:55 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 4/20/22 10:18 AM, Andrii Nakryiko wrote:
> > On Thu, Apr 14, 2022 at 3:46 PM Alexander Lobakin <alobakin@...me> wrote:
> >>
> >> On 32 bit systems, shifting an unsigned long by 32 positions
> >> yields the following warning:
> >>
> >> samples/bpf/tracex2_kern.c:60:23: warning: shift count >= width of type [-Wshift-count-overflow]
> >>          unsigned int hi = v >> 32;
> >>                              ^  ~~
> >>
> >
> > long is always 64-bit in BPF, but I suspect this is due to
> > samples/bpf/Makefile still using this clang + llc combo, where clang
> > is called with native target and llc for -target bpf. Not sure if we
> > are ready to ditch that complicated combination. Yonghong, do we still
> > need that or can we just use -target bpf in samples/bpf?
>
> Current most bpf programs in samples/bpf do not use vmlinux.h and CO-RE.
> They direct use kernel header files. That is why clang C -> IR
> compilation still needs to be native.
>
> We could just use -target bpf for the whole compilation but that needs
> to change the code to use vmlinux.h and CO-RE. There are already a
> couple of sample bpf programs did this.

Right, I guess I'm proposing to switch samples/bpf to vmlinux.h. Only
purely networking BPF apps can get away with not using vmlinux.h
because they might avoid dependency on kernel types. But even then a
lot of modern networking apps seem to be gaining elements of more
generic tracing and would rely on CO-RE for staying "portable" between
kernels. So it might be totally fine to just use CO-RE universally in
samples/bpf?

>
> >
> >
> >> The usual way to avoid this is to shift by 16 two times (see
> >> upper_32_bits() macro in the kernel). Use it across the BPF sample
> >> code as well.
> >>
> >> Fixes: d822a1926849 ("samples/bpf: Add counting example for kfree_skb() function calls and the write() syscall")
> >> Fixes: 0fb1170ee68a ("bpf: BPF based latency tracing")
> >> Fixes: f74599f7c530 ("bpf: Add tests and samples for LWT-BPF")
> >> Signed-off-by: Alexander Lobakin <alobakin@...me>
> >> ---
> >>   samples/bpf/lathist_kern.c      | 2 +-
> >>   samples/bpf/lwt_len_hist_kern.c | 2 +-
> >>   samples/bpf/tracex2_kern.c      | 2 +-
> >>   3 files changed, 3 insertions(+), 3 deletions(-)
> >>
> >
> > [...]