selftests/bpf: validate async callback return value check correctness

Adjust timer/timer_ret_1 test to validate more carefully verifier logic
of enforcing async callback return value. This test will pass only if
return result is marked precise and read.

Acked-by: Eduard Zingerman <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/progs/timer_failure.c

···
 	__type(value, struct elem);
 } timer_map SEC(".maps");
 
-static int timer_cb_ret1(void *map, int *key, struct bpf_timer *timer)
+__naked __noinline __used
+static unsigned long timer_cb_ret_bad()
 {
-	if (bpf_get_smp_processor_id() % 2)
-		return 1;
-	else
-		return 0;
+	asm volatile (
+		"call %[bpf_get_prandom_u32];"
+		"if r0 s> 1000 goto 1f;"
+		"r0 = 0;"
+	"1:"
+		"goto +0;" /* checkpoint */
+		/* async callback is expected to return 0, so branch above
+		 * skipping r0 = 0; should lead to a failure, but if exit
+		 * instruction doesn't enforce r0's precision, this callback
+		 * will be successfully verified
+		 */
+		"exit;"
+		:
+		: __imm(bpf_get_prandom_u32)
+		: __clobber_common
+	);
 }
 
 SEC("fentry/bpf_fentry_test1")
-__failure __msg("should have been in [0, 0]")
-int BPF_PROG2(test_ret_1, int, a)
+__log_level(2)
+__flag(BPF_F_TEST_STATE_FREQ)
+__failure
+/* check that fallthrough code path marks r0 as precise */
+__msg("mark_precise: frame0: regs=r0 stack= before 22: (b7) r0 = 0")
+/* check that branch code path marks r0 as precise */
+__msg("mark_precise: frame0: regs=r0 stack= before 24: (85) call bpf_get_prandom_u32#7")
+__msg("should have been in [0, 0]")
+long BPF_PROG2(test_bad_ret, int, a)
 {
 	int key = 0;
 	struct bpf_timer *timer;
···
 	timer = bpf_map_lookup_elem(&timer_map, &key);
 	if (timer) {
 		bpf_timer_init(timer, &timer_map, CLOCK_BOOTTIME);
-		bpf_timer_set_callback(timer, timer_cb_ret1);
+		bpf_timer_set_callback(timer, timer_cb_ret_bad);
 		bpf_timer_start(timer, 1000, 0);
 	}