libbpf: integrate __arg_ctx feature detector into kernel_supports()

Now that feature detection code is in bpf-next tree, integrate __arg_ctx
kernel-side support into kernel_supports() framework.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240125205510.3642094-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/features.c

@@ -407,6 +407,61 @@ static int probe_kern_btf_enum64(int token_fd)
 					     strs, sizeof(strs), token_fd));
 }
 
+static int probe_kern_arg_ctx_tag(int token_fd)
+{
+	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,
+		.token_fd = token_fd,
+		.prog_flags = token_fd ? BPF_F_TOKEN_FD : 0,
+	);
+	int prog_fd, btf_fd, insn_cnt = ARRAY_SIZE(insns);
+
+	btf_fd = libbpf__load_raw_btf((char *)types, sizeof(types), strs, sizeof(strs), token_fd);
+	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);
+
+	return probe_fd(prog_fd);
+}
+
 typedef int (*feature_probe_fn)(int /* token_fd */);
 
 static struct kern_feature_cache feature_cache;
@@ -476,6 +531,9 @@ static struct kern_feature_desc {
 	[FEAT_UPROBE_MULTI_LINK] = {
 		"BPF multi-uprobe link support", probe_uprobe_multi_link,
 	},
+	[FEAT_ARG_CTX_TAG] = {
+		"kernel-side __arg_ctx tag", probe_kern_arg_ctx_tag,
+	},
 };
 
 bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)

tools/lib/bpf/libbpf.c

@@ -6462,69 +6462,6 @@ 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), 0);
-	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
@@ -6549,7 +6486,7 @@ static int bpf_program_fixup_func_info(struct bpf_object *obj, struct bpf_progra
 		return 0;
 
 	/* don't do any fix ups if kernel natively supports __arg_ctx */
-	if (probe_kern_arg_ctx_tag() > 0)
+	if (kernel_supports(obj, FEAT_ARG_CTX_TAG))
 		return 0;
 
 	/* some BPF program types just don't have named context structs, so

tools/lib/bpf/libbpf_internal.h

@@ -372,6 +372,8 @@ enum kern_feature_id {
 	FEAT_SYSCALL_WRAPPER,
 	/* BPF multi-uprobe link support */
 	FEAT_UPROBE_MULTI_LINK,
+	/* Kernel supports arg:ctx tag (__arg_ctx) for global subprogs natively */
+	FEAT_ARG_CTX_TAG,
 	__FEAT_CNT,
 };