libbpf: feature-detect arg:ctx tag support in kernel

Add feature detector of kernel-side arg:ctx (__arg_ctx) tag support. If
this is detected, libbpf will avoid doing any __arg_ctx-related BTF
rewriting and checks in favor of letting kernel handle this completely.

test_global_funcs/ctx_arg_rewrite subtest is adjusted to do the same
feature detection (albeit in much simpler, though round-about and
inefficient, way), and skip the tests. This is done to still be able to
execute this test on older kernels (like in libbpf CI).

Note, BPF token series ([0]) does a major refactor and code moving of
libbpf-internal feature detection "framework", so to avoid unnecessary
conflicts we keep newly added feature detection stand-alone with ad-hoc
result caching. Once things settle, there will be a small follow up to
re-integrate everything back and move code into its final place in
newly-added (by BPF token series) features.c file.

  [0] https://patchwork.kernel.org/project/netdevbpf/list/?series=814209&state=*

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240118033143.3384355-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -6757,6 +6757,69 @@ static int clone_func_btf_info(struct btf *btf, int orig_fn_id, struct bpf_progr
 	return fn_id;
 }
 
+static int probe_kern_arg_ctx_tag(void)
+{
+	/* To minimize merge conflicts with BPF token series that refactors
+	 * feature detection code a lot, we don't integrate
+	 * probe_kern_arg_ctx_tag() into kernel_supports() feature-detection
+	 * framework yet, doing our own caching internally.
+	 * This will be cleaned up a bit later when bpf/bpf-next trees settle.
+	 */
+	static int cached_result = -1;
+	static const char strs[] = "\0a\0b\0arg:ctx\0";
+	const __u32 types[] = {
+		/* [1] INT */
+		BTF_TYPE_INT_ENC(1 /* "a" */, BTF_INT_SIGNED, 0, 32, 4),
+		/* [2] PTR -> VOID */
+		BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_PTR, 0, 0), 0),
+		/* [3] FUNC_PROTO `int(void *a)` */
+		BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_FUNC_PROTO, 0, 1), 1),
+		BTF_PARAM_ENC(1 /* "a" */, 2),
+		/* [4] FUNC 'a' -> FUNC_PROTO (main prog) */
+		BTF_TYPE_ENC(1 /* "a" */, BTF_INFO_ENC(BTF_KIND_FUNC, 0, BTF_FUNC_GLOBAL), 3),
+		/* [5] FUNC_PROTO `int(void *b __arg_ctx)` */
+		BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_FUNC_PROTO, 0, 1), 1),
+		BTF_PARAM_ENC(3 /* "b" */, 2),
+		/* [6] FUNC 'b' -> FUNC_PROTO (subprog) */
+		BTF_TYPE_ENC(3 /* "b" */, BTF_INFO_ENC(BTF_KIND_FUNC, 0, BTF_FUNC_GLOBAL), 5),
+		/* [7] DECL_TAG 'arg:ctx' -> func 'b' arg 'b' */
+		BTF_TYPE_DECL_TAG_ENC(5 /* "arg:ctx" */, 6, 0),
+	};
+	const struct bpf_insn insns[] = {
+		/* main prog */
+		BPF_CALL_REL(+1),
+		BPF_EXIT_INSN(),
+		/* global subprog */
+		BPF_EMIT_CALL(BPF_FUNC_get_func_ip), /* needs PTR_TO_CTX */
+		BPF_EXIT_INSN(),
+	};
+	const struct bpf_func_info_min func_infos[] = {
+		{ 0, 4 }, /* main prog -> FUNC 'a' */
+		{ 2, 6 }, /* subprog -> FUNC 'b' */
+	};
+	LIBBPF_OPTS(bpf_prog_load_opts, opts);
+	int prog_fd, btf_fd, insn_cnt = ARRAY_SIZE(insns);
+
+	if (cached_result >= 0)
+		return cached_result;
+
+	btf_fd = libbpf__load_raw_btf((char *)types, sizeof(types), strs, sizeof(strs));
+	if (btf_fd < 0)
+		return 0;
+
+	opts.prog_btf_fd = btf_fd;
+	opts.func_info = &func_infos;
+	opts.func_info_cnt = ARRAY_SIZE(func_infos);
+	opts.func_info_rec_size = sizeof(func_infos[0]);
+
+	prog_fd = bpf_prog_load(BPF_PROG_TYPE_KPROBE, "det_arg_ctx",
+				"GPL", insns, insn_cnt, &opts);
+	close(btf_fd);
+
+	cached_result = probe_fd(prog_fd);
+	return cached_result;
+}
+
 /* Check if main program or global subprog's function prototype has `arg:ctx`
  * argument tags, and, if necessary, substitute correct type to match what BPF
  * verifier would expect, taking into account specific program type. This
@@ -6780,6 +6843,10 @@ static int bpf_program_fixup_func_info(struct bpf_object *obj, struct bpf_progra
 	if (!obj->btf_ext || !prog->func_info)
 		return 0;
 
+	/* don't do any fix ups if kernel natively supports __arg_ctx */
+	if (probe_kern_arg_ctx_tag() > 0)
+		return 0;
+
 	/* some BPF program types just don't have named context structs, so
 	 * this fallback mechanism doesn't work for them
 	 */

tools/testing/selftests/bpf/prog_tests/test_global_funcs.c

@@ -47,6 +47,19 @@ static void subtest_ctx_arg_rewrite(void)
 	struct btf *btf = NULL;
 	__u32 info_len = sizeof(info);
 	int err, fd, i;
+	struct btf *kern_btf = NULL;
+
+	kern_btf = btf__load_vmlinux_btf();
+	if (!ASSERT_OK_PTR(kern_btf, "kern_btf_load"))
+		return;
+
+	/* simple detection of kernel native arg:ctx tag support */
+	if (btf__find_by_name_kind(kern_btf, "bpf_subprog_arg_info", BTF_KIND_STRUCT) > 0) {
+		test__skip();
+		btf__free(kern_btf);
+		return;
+	}
+	btf__free(kern_btf);
 
 	skel = test_global_func_ctx_args__open();
 	if (!ASSERT_OK_PTR(skel, "skel_open"))