Re: [PATCH bpf-next v2 1/2] libbpf: implement bpf_usdt_arg_size BPF function

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Feb 24, 2025 at 03:57:55PM -0800, Ihor Solodrai wrote:
> Information about USDT argument size is implicitly stored in
> __bpf_usdt_arg_spec, but currently it's not accessbile to BPF programs
> that use USDT.
> 
> Implement bpf_sdt_arg_size() that returns the size of an USDT argument
> in bytes.
> 
> v1->v2:
>   * do not add __bpf_usdt_arg_spec() helper

Reviewed-by: Jiri Olsa <jolsa@kernel.org>

jirka

> 
> v1: https://lore.kernel.org/bpf/20250220215904.3362709-1-ihor.solodrai@linux.dev/
> 
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> ---
>  tools/lib/bpf/usdt.bpf.h | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
> index b811f754939f..2a7865c8e3fe 100644
> --- a/tools/lib/bpf/usdt.bpf.h
> +++ b/tools/lib/bpf/usdt.bpf.h
> @@ -108,6 +108,38 @@ int bpf_usdt_arg_cnt(struct pt_regs *ctx)
>  	return spec->arg_cnt;
>  }
>  
> +/* Returns the size in bytes of the #*arg_num* (zero-indexed) USDT argument.
> + * Returns negative error if argument is not found or arg_num is invalid.
> + */
> +static __always_inline
> +int bpf_usdt_arg_size(struct pt_regs *ctx, __u64 arg_num)
> +{
> +	struct __bpf_usdt_arg_spec *arg_spec;
> +	struct __bpf_usdt_spec *spec;
> +	int spec_id;
> +
> +	spec_id = __bpf_usdt_spec_id(ctx);
> +	if (spec_id < 0)
> +		return -ESRCH;
> +
> +	spec = bpf_map_lookup_elem(&__bpf_usdt_specs, &spec_id);
> +	if (!spec)
> +		return -ESRCH;
> +
> +	if (arg_num >= BPF_USDT_MAX_ARG_CNT)
> +		return -ENOENT;
> +	barrier_var(arg_num);
> +	if (arg_num >= spec->arg_cnt)
> +		return -ENOENT;
> +
> +	arg_spec = &spec->args[arg_num];
> +
> +	/* arg_spec->arg_bitshift = 64 - arg_sz * 8
> +	 * so: arg_sz = (64 - arg_spec->arg_bitshift) / 8
> +	 */
> +	return (unsigned int)(64 - arg_spec->arg_bitshift) / 8;
> +}
> +
>  /* Fetch USDT argument #*arg_num* (zero-indexed) and put its value into *res.
>   * Returns 0 on success; negative error, otherwise.
>   * On error *res is guaranteed to be set to zero.
> -- 
> 2.48.1
> 
>