linux-kernel - Re: [PATCH bpf-next v2 4/6] libbpf: Initialize the bpf_seq

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Date:   Fri, 26 Mar 2021 16:01:02 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
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>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 4/6] libbpf: Initialize the bpf_seq_printf
 parameters array field by field

On Tue, Mar 23, 2021 at 7:23 PM Florent Revest <revest@...omium.org> wrote:
>
> When initializing the __param array with a one liner, if all args are
> const, the initial array value will be placed in the rodata section but
> because libbpf does not support relocation in the rodata section, any
> pointer in this array will stay NULL.
>
> Fixes: c09add2fbc5a ("tools/libbpf: Add bpf_iter support")
> Signed-off-by: Florent Revest <revest@...omium.org>
> ---
>  tools/lib/bpf/bpf_tracing.h | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_tracing.h b/tools/lib/bpf/bpf_tracing.h
> index f9ef37707888..d9a4c3f77ff4 100644
> --- a/tools/lib/bpf/bpf_tracing.h
> +++ b/tools/lib/bpf/bpf_tracing.h
> @@ -413,6 +413,22 @@ typeof(name(0)) name(struct pt_regs *ctx)                              \
>  }                                                                          \
>  static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
>
> +#define ___bpf_fill0(arr, p, x)

can you please double-check that no-argument BPF_SEQ_PRINTF won't
generate a warning about spurious ';'? Maybe it's better to have zero
case as `do {} while(0);` ?

> +#define ___bpf_fill1(arr, p, x) arr[p] = x
> +#define ___bpf_fill2(arr, p, x, args...) arr[p] = x; ___bpf_fill1(arr, p + 1, args)
> +#define ___bpf_fill3(arr, p, x, args...) arr[p] = x; ___bpf_fill2(arr, p + 1, args)
> +#define ___bpf_fill4(arr, p, x, args...) arr[p] = x; ___bpf_fill3(arr, p + 1, args)
> +#define ___bpf_fill5(arr, p, x, args...) arr[p] = x; ___bpf_fill4(arr, p + 1, args)
> +#define ___bpf_fill6(arr, p, x, args...) arr[p] = x; ___bpf_fill5(arr, p + 1, args)
> +#define ___bpf_fill7(arr, p, x, args...) arr[p] = x; ___bpf_fill6(arr, p + 1, args)
> +#define ___bpf_fill8(arr, p, x, args...) arr[p] = x; ___bpf_fill7(arr, p + 1, args)
> +#define ___bpf_fill9(arr, p, x, args...) arr[p] = x; ___bpf_fill8(arr, p + 1, args)
> +#define ___bpf_fill10(arr, p, x, args...) arr[p] = x; ___bpf_fill9(arr, p + 1, args)
> +#define ___bpf_fill11(arr, p, x, args...) arr[p] = x; ___bpf_fill10(arr, p + 1, args)
> +#define ___bpf_fill12(arr, p, x, args...) arr[p] = x; ___bpf_fill11(arr, p + 1, args)
> +#define ___bpf_fill(arr, args...) \
> +       ___bpf_apply(___bpf_fill, ___bpf_narg(args))(arr, 0, args)

cool. this is regular enough to easily comprehend :)

> +
>  /*
>   * BPF_SEQ_PRINTF to wrap bpf_seq_printf to-be-printed values
>   * in a structure.
> @@ -421,12 +437,14 @@ static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
>         ({                                                                  \
>                 _Pragma("GCC diagnostic push")                              \
>                 _Pragma("GCC diagnostic ignored \"-Wint-conversion\"")      \
> +               unsigned long long ___param[___bpf_narg(args)];             \
>                 static const char ___fmt[] = fmt;                           \
> -               unsigned long long ___param[] = { args };                   \
> +               int __ret;                                                  \
> +               ___bpf_fill(___param, args);                                \
>                 _Pragma("GCC diagnostic pop")                               \

Let's clean this up a little bit;
1. static const char ___fmt should be the very first
2. _Pragma scope should be minimal necessary, which includes only
___bpf_fill, right?
3. Empty line after int __ret; and let's keep three underscores for consistency.


> -               int ___ret = bpf_seq_printf(seq, ___fmt, sizeof(___fmt),    \
> -                                           ___param, sizeof(___param));    \
> -               ___ret;                                                     \
> +               __ret = bpf_seq_printf(seq, ___fmt, sizeof(___fmt),         \
> +                                      ___param, sizeof(___param));         \
> +               __ret;                                                      \

but actually you don't need __ret at all, just bpf_seq_printf() here, right?


>         })
>
>  #endif
> --
> 2.31.0.291.g576ba9dcdaf-goog
>