Re: [PATCH bpf-next 3/5] libbpf: Initialize the bpf_seq_printf parameters array field by field

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Mar 10, 2021 at 2:02 PM Florent Revest <revest@chromium.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.
>
> This is a workaround, ideally the rodata relocation should be supported
> by libbpf but this would require a disproportionate amount of work given
> the actual usecases. (it is very unlikely that one uses a const array of
> relocated addresses)
>
> Signed-off-by: Florent Revest <revest@chromium.org>
> ---
>  tools/lib/bpf/bpf_tracing.h | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/bpf_tracing.h b/tools/lib/bpf/bpf_tracing.h
> index f9ef37707888..f6a2deb3cd5b 100644
> --- a/tools/lib/bpf/bpf_tracing.h
> +++ b/tools/lib/bpf/bpf_tracing.h
> @@ -413,6 +413,34 @@ typeof(name(0)) name(struct pt_regs *ctx)                              \
>  }                                                                          \
>  static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
>
> +#define ___bpf_build_param0(narg, x)
> +#define ___bpf_build_param1(narg, x) ___param[narg - 1] = x
> +#define ___bpf_build_param2(narg, x, args...) ___param[narg - 2] = x; \
> +                                             ___bpf_build_param1(narg, args)
> +#define ___bpf_build_param3(narg, x, args...) ___param[narg - 3] = x; \
> +                                             ___bpf_build_param2(narg, args)
> +#define ___bpf_build_param4(narg, x, args...) ___param[narg - 4] = x; \
> +                                             ___bpf_build_param3(narg, args)
> +#define ___bpf_build_param5(narg, x, args...) ___param[narg - 5] = x; \
> +                                             ___bpf_build_param4(narg, args)
> +#define ___bpf_build_param6(narg, x, args...) ___param[narg - 6] = x; \
> +                                             ___bpf_build_param5(narg, args)
> +#define ___bpf_build_param7(narg, x, args...) ___param[narg - 7] = x; \
> +                                             ___bpf_build_param6(narg, args)
> +#define ___bpf_build_param8(narg, x, args...) ___param[narg - 8] = x; \
> +                                             ___bpf_build_param7(narg, args)
> +#define ___bpf_build_param9(narg, x, args...) ___param[narg - 9] = x; \
> +                                             ___bpf_build_param8(narg, args)
> +#define ___bpf_build_param10(narg, x, args...) ___param[narg - 10] = x; \
> +                                              ___bpf_build_param9(narg, args)
> +#define ___bpf_build_param11(narg, x, args...) ___param[narg - 11] = x; \
> +                                              ___bpf_build_param10(narg, args)
> +#define ___bpf_build_param12(narg, x, args...) ___param[narg - 12] = x; \
> +                                              ___bpf_build_param11(narg, args)

took me some time to get why the [narg - 12] :) it makes sense, but
then I started wondering why not

#define ___bpf_build_param12(narg, x, args...)
___bpf_build_param11(narg, args); ___param[11] = x

? seems more straightforward, no?

also please keep all of them on single line. And to make lines
shorter, let's call it ___bpf_fillX? I also don't like hard-coded
___param, which is both inflexible and is obscure at the point of use
of this macro. So let's pass it as the first argument?

> +#define ___bpf_build_param(args...) \
> +       unsigned long long ___param[___bpf_narg(args)];                 \
> +       ___bpf_apply(___bpf_build_param, ___bpf_narg(args))(___bpf_narg(args), args)
> +

And here I'd pass array as a parameter and let caller define it, so
macro is literally just filling the array elements, not defining the
array itself and what's the type of elements

>  /*
>   * BPF_SEQ_PRINTF to wrap bpf_seq_printf to-be-printed values
>   * in a structure.
> @@ -422,7 +450,7 @@ static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
>                 _Pragma("GCC diagnostic push")                              \
>                 _Pragma("GCC diagnostic ignored \"-Wint-conversion\"")      \
>                 static const char ___fmt[] = fmt;                           \
> -               unsigned long long ___param[] = { args };                   \
> +               ___bpf_build_param(args);                                   \
>                 _Pragma("GCC diagnostic pop")                               \
>                 int ___ret = bpf_seq_printf(seq, ___fmt, sizeof(___fmt),    \
>                                             ___param, sizeof(___param));    \

here you are violating separation of variables and code,
___bpf_build_param is defining a variable, then has code statements,
then you are declaring ___ret after the code. So please split ___ret
definition,

> --
> 2.30.1.766.gb4fecdf3b7-goog
>