selftests/bpf: Query BPF_MAX_TRAMP_LINKS using BTF

Do not hard-code the value, since for s390x it will be smaller than
for x86.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-4-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -2,14 +2,19 @@
 /* Copyright (c) 2019 Facebook */
 #include <test_progs.h>
 
-/* that's kernel internal BPF_MAX_TRAMP_PROGS define */
-#define CNT 38
-
 void serial_test_fexit_stress(void)
 {
-	int fexit_fd[CNT] = {};
+	int bpf_max_tramp_links, err, i;
-	int link_fd[CNT] = {};
+	int *fd, *fexit_fd, *link_fd;
-	int err, i;
+
+	bpf_max_tramp_links = get_bpf_max_tramp_links();
+	if (!ASSERT_GE(bpf_max_tramp_links, 1, "bpf_max_tramp_links"))
+		return;
+	fd = calloc(bpf_max_tramp_links * 2, sizeof(*fd));
+	if (!ASSERT_OK_PTR(fd, "fd"))
+		return;
+	fexit_fd = fd;
+	link_fd = fd + bpf_max_tramp_links;
 
 	const struct bpf_insn trace_program[] = {
 		BPF_MOV64_IMM(BPF_REG_0, 0),
@@ -28,7 +33,7 @@ void serial_test_fexit_stress(void)
 		goto out;
 	trace_opts.attach_btf_id = err;
 
-	for (i = 0; i < CNT; i++) {
+	for (i = 0; i < bpf_max_tramp_links; i++) {
 		fexit_fd[i] = bpf_prog_load(BPF_PROG_TYPE_TRACING, NULL, "GPL",
 					    trace_program,
 					    sizeof(trace_program) / sizeof(struct bpf_insn),
@@ -44,10 +49,11 @@ void serial_test_fexit_stress(void)
 	ASSERT_OK(err, "bpf_prog_test_run_opts");
 
 out:
-	for (i = 0; i < CNT; i++) {
+	for (i = 0; i < bpf_max_tramp_links; i++) {
 		if (link_fd[i])
 			close(link_fd[i]);
 		if (fexit_fd[i])
 			close(fexit_fd[i]);
 	}
+	free(fd);
 }

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

@@ -2,8 +2,6 @@
 #define _GNU_SOURCE
 #include <test_progs.h>
 
-#define MAX_TRAMP_PROGS 38
-
 struct inst {
 	struct bpf_object *obj;
 	struct bpf_link   *link;
@@ -37,14 +35,21 @@ void serial_test_trampoline_count(void)
 {
 	char *file = "test_trampoline_count.bpf.o";
 	char *const progs[] = { "fentry_test", "fmod_ret_test", "fexit_test" };
-	struct inst inst[MAX_TRAMP_PROGS + 1] = {};
+	int bpf_max_tramp_links, err, i, prog_fd;
 	struct bpf_program *prog;
 	struct bpf_link *link;
-	int prog_fd, err, i;
+	struct inst *inst;
 	LIBBPF_OPTS(bpf_test_run_opts, opts);
 
+	bpf_max_tramp_links = get_bpf_max_tramp_links();
+	if (!ASSERT_GE(bpf_max_tramp_links, 1, "bpf_max_tramp_links"))
+		return;
+	inst = calloc(bpf_max_tramp_links + 1, sizeof(*inst));
+	if (!ASSERT_OK_PTR(inst, "inst"))
+		return;
+
 	/* attach 'allowed' trampoline programs */
-	for (i = 0; i < MAX_TRAMP_PROGS; i++) {
+	for (i = 0; i < bpf_max_tramp_links; i++) {
 		prog = load_prog(file, progs[i % ARRAY_SIZE(progs)], &inst[i]);
 		if (!prog)
 			goto cleanup;
@@ -91,4 +96,5 @@ cleanup:
 		bpf_link__destroy(inst[i].link);
 		bpf_object__close(inst[i].obj);
 	}
+	free(inst);
 }

tools/testing/selftests/bpf/test_progs.c

@@ -17,6 +17,7 @@
 #include <sys/select.h>
 #include <sys/socket.h>
 #include <sys/un.h>
+#include <bpf/btf.h>
 
 static bool verbose(void)
 {
@@ -967,6 +968,43 @@ int write_sysctl(const char *sysctl, const char *value)
 	return 0;
 }
 
+int get_bpf_max_tramp_links_from(struct btf *btf)
+{
+	const struct btf_enum *e;
+	const struct btf_type *t;
+	__u32 i, type_cnt;
+	const char *name;
+	__u16 j, vlen;
+
+	for (i = 1, type_cnt = btf__type_cnt(btf); i < type_cnt; i++) {
+		t = btf__type_by_id(btf, i);
+		if (!t || !btf_is_enum(t) || t->name_off)
+			continue;
+		e = btf_enum(t);
+		for (j = 0, vlen = btf_vlen(t); j < vlen; j++, e++) {
+			name = btf__str_by_offset(btf, e->name_off);
+			if (name && !strcmp(name, "BPF_MAX_TRAMP_LINKS"))
+				return e->val;
+		}
+	}
+
+	return -1;
+}
+
+int get_bpf_max_tramp_links(void)
+{
+	struct btf *vmlinux_btf;
+	int ret;
+
+	vmlinux_btf = btf__load_vmlinux_btf();
+	if (!ASSERT_OK_PTR(vmlinux_btf, "vmlinux btf"))
+		return -1;
+	ret = get_bpf_max_tramp_links_from(vmlinux_btf);
+	btf__free(vmlinux_btf);
+
+	return ret;
+}
+
 #define MAX_BACKTRACE_SZ 128
 void crash_handler(int signum)
 {

tools/testing/selftests/bpf/test_progs.h

@@ -394,6 +394,8 @@ int kern_sync_rcu(void);
 int trigger_module_test_read(int read_sz);
 int trigger_module_test_write(int write_sz);
 int write_sysctl(const char *sysctl, const char *value);
+int get_bpf_max_tramp_links_from(struct btf *btf);
+int get_bpf_max_tramp_links(void);
 
 #ifdef __x86_64__
 #define SYS_NANOSLEEP_KPROBE_NAME "__x64_sys_nanosleep"