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From:   Song Liu <song@kernel.org>
Date:   Fri, 15 Apr 2022 16:54:10 -0700
Message-ID: <CAPhsuW5KnLUfN2Vom1c2_+ZSeqfvnYpULhEVujeUBa_6OBQseg@mail.gmail.com>
Subject: Re: [PATCH bpf-next 08/11] samples: bpf: fix shifting unsigned long
 by 32 positions
To:     Alexander Lobakin <alobakin@pm.me>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Dmitrii Dolgov <9erthalion6@gmail.com>,
        Quentin Monnet <quentin@isovalent.com>,
        Tiezhu Yang <yangtiezhu@loongson.cn>,
        Kumar Kartikeya Dwivedi <memxor@gmail.com>,
        Chenbo Feng <fengc@google.com>,
        Willem de Bruijn <willemb@google.com>,
        Daniel Wagner <daniel.wagner@bmw-carit.de>,
        Thomas Graf <tgraf@suug.ch>,
        Ong Boon Leong <boon.leong.ong@intel.com>,
        linux-perf-users@vger.kernel.org,
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On Thu, Apr 14, 2022 at 3:46 PM Alexander Lobakin <alobakin@pm.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@pm.me>

Acked-by: Song Liu <songliubraving@fb.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
>
>