On Wed, Mar 10, 2021 at 2:02 PM Florent Revest <revest@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
> 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
>
