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

[Song Liu]

On Thu, Apr 14, 2022 at 3:46 PM Alexander Lobakin <alobakin@xxxxx> 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@xxxxx>

Acked-by: Song Liu <songliubraving@xxxxxx>
> ---
>  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
>
>