Re: [PATCH 3/4] btf: add dt_btf_func_is_traceable()

On 14 Apr 2026, Kris Van Hees spake thusly:

> Add a function to determine whether a function (given by BTF id) can be
> traced using BPF fentry/fexit probes.  It implements checks inspired by
> btf_distill_func_proto() in kernel/bpf/btf.c.

... looks reasonable.

> Signed-off-by: Kris Van Hees <[email protected]>

Reviewed-by: Nick Alcock <[email protected]>

(though you might possibly want to implement the suggestion below, it's
very much optional.)

> ---
>  libdtrace/dt_btf.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++
>  libdtrace/dt_btf.h |  2 ++
>  2 files changed, 67 insertions(+)
>
> diff --git a/libdtrace/dt_btf.c b/libdtrace/dt_btf.c
> index 56eb96a4e..fbf377b4e 100644
> --- a/libdtrace/dt_btf.c
> +++ b/libdtrace/dt_btf.c
> @@ -1020,3 +1020,68 @@ dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
>  
>  	return 0;
>  }
> +
> +/*
> + * Return 1 if the function referenced by the BTF id can be traced using the
> + * fprobe/fexit facility.  Specifically, that the function is not variadic and
> + * that none of its arguments exceeds the acceptable value size (16 bytes).
> + * (See btf_distill_func_proto() in kernel/bpf/btf.c for reference.)
> + */
> +#define MAX_BPF_FUNC_REG_REGS	5

A shame we need to hardwire this. More of a shame that we need to
hardwire it deep in a .c file where it'll almost certainly be overlooked
when the time comes to change it. Stick it in a suitable header, maybe?

> +int
> +dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
> +{
> +	btf_type_t	*type = dt_btf_real_type_by_id(dtp, btf, id);
> +	int		argc;
> +
> +	/* If no prototype is found, BPF fprobes do not work. */
> +	if (!type || BTF_INFO_KIND(type->info) != BTF_KIND_FUNC_PROTO)
> +		return 0;
> +
> +	/* If the return type is a struct or union, BPF fprobes do not work. */
> +	if (type->type != 0) {
> +		btf_type_t	*rtype = dt_btf_real_type_by_id(dtp, btf, type->type);
> +
> +		if (rtype == NULL)
> +			return 0;
> +
> +		switch (BTF_INFO_KIND(rtype->info)) {
> +		case BTF_KIND_STRUCT:
> +		case BTF_KIND_UNION:
> +			return 0;
> +		default:
> +			/* fall-through */
> +		}
> +	}
> +
> +	/*
> +	 * BPF fprobes do not support functions that are variadic or that have
> +	 * any arguments passed by value that are a struct or union of size
> +	 * greater than 16.
> +	 */
> +	argc = BTF_INFO_VLEN(type->info);
> +	if (argc > 0) {
> +		btf_param_t	*args = (btf_param_t *)(type + 1);
> +		int		i;
> +
> +		if (args[argc - 1].type == 0)		/* variadic */
> +			return 0;
> +
> +		for (i = 0; i < argc; i++) {
> +			type = dt_btf_real_type_by_id(dtp, btf, args[i].type);
> +			if (type == NULL)
> +				return 0;
> +
> +			switch (BTF_INFO_KIND(type->info)) {
> +			case BTF_KIND_STRUCT:
> +			case BTF_KIND_UNION:
> +				if (type->size > 16)	/* value size > 16 */
> +					return 0;
> +			default:
> +				/* fall-through */
> +			}
> +		}
> +	}
> +
> +	return 1;
> +}
> diff --git a/libdtrace/dt_btf.h b/libdtrace/dt_btf.h
> index d956ad940..2c921a033 100644
> --- a/libdtrace/dt_btf.h
> +++ b/libdtrace/dt_btf.h
> @@ -28,6 +28,8 @@ extern int dt_btf_func_argc(dtrace_hdl_t *dtp, const dt_btf_t *btf,
>  			    uint32_t id);
>  extern int dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf,
>  			       uint32_t id);
> +extern int dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf,
> +				    uint32_t id);
>  extern int dt_btf_get_module_ids(dtrace_hdl_t *);
>  extern int dt_btf_module_fd(const dt_module_t *);

-- 
NULL && (void)