linux-kernel - Re: [PATCH bpf 2/2] bpf: Avoid using ARRAY

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Date:   Mon, 17 May 2021 10:54:49 -0700
From:   Song Liu <song@...nel.org>
To:     Florent Revest <revest@...omium.org>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        KP Singh <kpsingh@...nel.org>, jackmanb@...gle.com,
        open list <linux-kernel@...r.kernel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH bpf 2/2] bpf: Avoid using ARRAY_SIZE on an uninitialized pointer

On Mon, May 17, 2021 at 2:29 AM Florent Revest <revest@...omium.org> wrote:
>
> The cppcheck static code analysis reported the following error:
> >> helpers.c:713:43: warning: Uninitialized variable: bufs [uninitvar]
>     if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(bufs->tmp_bufs))) {
>                                              ^
>
> ARRAY_SIZE is a macro that expands to sizeofs, so bufs is not actually
> dereferenced at runtime, and the code is actually safe. But to keep
> things tidy, this patch removes the need for a call to ARRAY_SIZE by
> extracting the size of the array into a macro. Cppcheck should no longer
> be confused and the code ends up being a bit cleaner.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Florent Revest <revest@...omium.org>

Acked-by: Song Liu <song@...nel.org>

> ---
>  kernel/bpf/helpers.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 3a5ab614cbb0..73443498d88f 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -698,8 +698,9 @@ static int bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
>  #define MAX_BPRINTF_BUF_LEN    512
>
>  /* Support executing three nested bprintf helper calls on a given CPU */
> +#define MAX_BPRINTF_NEST_LEVEL 3
>  struct bpf_bprintf_buffers {
> -       char tmp_bufs[3][MAX_BPRINTF_BUF_LEN];
> +       char tmp_bufs[MAX_BPRINTF_NEST_LEVEL][MAX_BPRINTF_BUF_LEN];
>  };
>  static DEFINE_PER_CPU(struct bpf_bprintf_buffers, bpf_bprintf_bufs);
>  static DEFINE_PER_CPU(int, bpf_bprintf_nest_level);
> @@ -711,7 +712,7 @@ static int try_get_fmt_tmp_buf(char **tmp_buf)
>
>         preempt_disable();
>         nest_level = this_cpu_inc_return(bpf_bprintf_nest_level);
> -       if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(bufs->tmp_bufs))) {
> +       if (WARN_ON_ONCE(nest_level > MAX_BPRINTF_NEST_LEVEL)) {
>                 this_cpu_dec(bpf_bprintf_nest_level);
>                 preempt_enable();
>                 return -EBUSY;
> --
> 2.31.1.751.gd2f1c929bd-goog
>