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

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Date:   Fri, 15 Apr 2022 16:54:10 -0700
From:   Song Liu <song@...nel.org>
To:     Alexander Lobakin <alobakin@...me>
Cc:     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>, Yonghong Song <yhs@...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-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 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;
>                             ^  ~~
>
> 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>

Acked-by: Song Liu <songliubraving@...com>
> ---
>  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(-)
>
> diff --git a/samples/bpf/lathist_kern.c b/samples/bpf/lathist_kern.c
> index 4adfcbbe6ef4..9744ed547abe 100644
> --- a/samples/bpf/lathist_kern.c
> +++ b/samples/bpf/lathist_kern.c
> @@ -53,7 +53,7 @@ static unsigned int log2(unsigned int v)
>
>  static unsigned int log2l(unsigned long v)
>  {
> -       unsigned int hi = v >> 32;
> +       unsigned int hi = (v >> 16) >> 16;
>
>         if (hi)
>                 return log2(hi) + 32;
> diff --git a/samples/bpf/lwt_len_hist_kern.c b/samples/bpf/lwt_len_hist_kern.c
> index 1fa14c54963a..bf32fa04c91f 100644
> --- a/samples/bpf/lwt_len_hist_kern.c
> +++ b/samples/bpf/lwt_len_hist_kern.c
> @@ -49,7 +49,7 @@ static unsigned int log2(unsigned int v)
>
>  static unsigned int log2l(unsigned long v)
>  {
> -       unsigned int hi = v >> 32;
> +       unsigned int hi = (v >> 16) >> 16;
>         if (hi)
>                 return log2(hi) + 32;
>         else
> diff --git a/samples/bpf/tracex2_kern.c b/samples/bpf/tracex2_kern.c
> index 5bc696bac27d..6bf22056ff95 100644
> --- a/samples/bpf/tracex2_kern.c
> +++ b/samples/bpf/tracex2_kern.c
> @@ -57,7 +57,7 @@ static unsigned int log2(unsigned int v)
>
>  static unsigned int log2l(unsigned long v)
>  {
> -       unsigned int hi = v >> 32;
> +       unsigned int hi = (v >> 16) >> 16;
>         if (hi)
>                 return log2(hi) + 32;
>         else
> --
> 2.35.2
>
>