bpf: Add per-program recursion prevention mechanism

Since both sleepable and non-sleepable programs execute under migrate_disable
add recursion prevention mechanism to both types of programs when they're
executed via bpf trampoline.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210210033634.62081-5-alexei.starovoitov@gmail.com

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

@@ -2,8 +2,8 @@
 /* Copyright (c) 2019 Facebook */
 #include <test_progs.h>
 
-/* x86-64 fits 55 JITed and 43 interpreted progs into half page */
-#define CNT 40
+/* that's kernel internal BPF_MAX_TRAMP_PROGS define */
+#define CNT 38
 
 void test_fexit_stress(void)
 {

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

@@ -4,7 +4,7 @@
 #include <sys/prctl.h>
 #include <test_progs.h>
 
-#define MAX_TRAMP_PROGS 40
+#define MAX_TRAMP_PROGS 38
 
 struct inst {
 	struct bpf_object *obj;
@@ -52,7 +52,7 @@ void test_trampoline_count(void)
 	struct bpf_link *link;
 	char comm[16] = {};
 
-	/* attach 'allowed' 40 trampoline programs */
+	/* attach 'allowed' trampoline programs */
 	for (i = 0; i < MAX_TRAMP_PROGS; i++) {
 		obj = bpf_object__open_file(object, NULL);
 		if (CHECK(IS_ERR(obj), "obj_open_file", "err %ld\n", PTR_ERR(obj))) {