Re: [PATCH v3 bpf-next 2/3] bpftool: Update func representation to include function signature

[Jiri Olsa <olsajiri@gmail.com>]

On Sat, Sep 26, 2026 at 06:51:12PM +0100, Alan Maguire wrote:

SNIP

> +static void btf_locsec_func_str(const struct btf *btf,
> +				const struct btf_loc *loc, char *str, size_t sz)
> +{
> +	const struct btf_type *func, *func_proto, *loc_proto;
> +	const struct btf_param *params;
> +	const __u32 *loc_params;
> +	const char *name;
> +	__u32 i, vlen;
> +	size_t off = 0;
> +
> +	if (!sz)
> +		return;
> +
> +	str[0] = '\0';
> +	func = btf__type_by_id(btf, loc->func);
> +	if (!func || !btf_is_func(func))
> +		goto invalid;
> +
> +	name = btf_str(btf, func->name_off);
> +	func_proto = btf__type_by_id(btf, func->type);
> +	loc_proto = btf__type_by_id(btf, loc->loc_proto);
> +	if (!func_proto || !btf_is_func_proto(func_proto) ||
> +	    !loc_proto || !btf_is_loc_proto(loc_proto) ||
> +	    btf_vlen(func_proto) != btf_vlen(loc_proto))
> +		goto invalid;
> +
> +	params = (const void *)(func_proto + 1);
> +	loc_params = btf_loc_proto_params(loc_proto);
> +	vlen = btf_vlen(func_proto);
> +	if (!btf_locsec_append(str, sz, &off, name) ||
> +	    !btf_locsec_append(str, sz, &off, "("))
> +		return;
> +	for (i = 0; i < vlen; i++) {
> +		const struct btf_type *param_loc;
> +		char param_str[256] = {};
> +
> +		if (!params[i].type) {
> +			/* Handle varargs func proto, must be last parameter */
> +			if (i != vlen - 1)
> +				goto invalid;
> +			if (!btf_locsec_append(str, sz, &off, i ? ", " : "") ||
> +			    !btf_locsec_append(str, sz, &off, "..."))
> +				return;
> +			break;
> +		} else if (loc_params[i]) {
> +			param_loc = btf__type_by_id(btf, loc_params[i]);
> +			btf_loc_param_str(param_loc, param_str, sizeof(param_str));
> +		} else {
> +			snprintf(param_str, sizeof(param_str), "<unavailable>");

hi,
I'm getting many unavailable arguments, like:

jolsa@krava4:~/kernel/linux-qemu-1$ ./tools/bpf/bpftool/bpftool btf dump file /tmp/vmlinux+inline | grep func | grep '<unavailable>' | head -10
	func='cpumask_test_cpu(cpu [reg0], cpumask [<unavailable>])' func_type_id=171851 loc_proto_type_id=171852 offset=7674
	func='instrument_atomic_check_alignment(v [<unavailable>], size [const 0x8])' func_type_id=171855 loc_proto_type_id=171857 offset=7681
	func='instrument_atomic_read(v [<unavailable>], size [const 0x8])' func_type_id=171858 loc_proto_type_id=171857 offset=7681
	func='variable_test_bit(nr [*(reg0)], addr [<unavailable>])' func_type_id=171860 loc_proto_type_id=171862 offset=7681
	func='arch_test_bit(nr [*(reg0)], addr [<unavailable>])' func_type_id=171864 loc_proto_type_id=171862 offset=7681
	func='_test_bit(nr [*(reg0)], addr [<unavailable>])' func_type_id=171865 loc_proto_type_id=171862 offset=7681
	func='__srcu_read_lock_fast(ssp [<unavailable>])' func_type_id=171867 loc_proto_type_id=171868 offset=7691
	func='srcu_read_lock_fast_notrace(ssp [<unavailable>])' func_type_id=171869 loc_proto_type_id=171868 offset=7691
	func='class_srcu_fast_notrace_constructor(l [<unavailable>])' func_type_id=171871 loc_proto_type_id=171868 offset=7691
	func='__srcu_read_unlock_fast(ssp [<unavailable>], scp [reg3])' func_type_id=171873 loc_proto_type_id=171875 offset=7727
jolsa@krava4:~/kernel/linux-qemu-1$ ./tools/bpf/bpftool/bpftool btf dump file /tmp/vmlinux+inline | grep func | grep '<unavailable>' | wc -l
157950

I found on the pahole site 

  /* A zero id in LOC_PROTO represents unavailable location data. */

so I guess it's just not present in dwarf for some reason

jirka