Fix mapping between sched_attr and SchedPolicy

kernel/src/syscall/sched_getattr.rs

@@ -69,16 +69,26 @@ impl TryFrom<SchedPolicy> for LinuxSchedAttr {
                 ..Default::default()
             },
 
+            // The SCHED_IDLE policy is mapped to the highest nice value of
+            // `SchedPolicy::Fair` instead of `SchedPolicy::Idle`. Tasks of the
+            // latter policy are invisible to the user API.
+            SchedPolicy::Fair(Nice::MAX) => LinuxSchedAttr {
+                sched_policy: SCHED_IDLE,
+                ..Default::default()
+            },
+
             SchedPolicy::Fair(nice) => LinuxSchedAttr {
                 sched_policy: SCHED_NORMAL,
                 sched_nice: nice.value().get().into(),
                 ..Default::default()
             },
 
-            SchedPolicy::Idle => LinuxSchedAttr {
-                sched_policy: SCHED_IDLE,
-                ..Default::default()
-            },
+            SchedPolicy::Idle => {
+                return Err(Error::with_message(
+                    Errno::EACCES,
+                    "attr for idle tasks are not accessible",
+                ))
+            }
         })
     }
 }
@@ -112,7 +122,10 @@ impl TryFrom<LinuxSchedAttr> for SchedPolicy {
                     .map_err(|msg| Error::with_message(Errno::EINVAL, msg))?,
             )),
 
-            SCHED_IDLE => SchedPolicy::Idle,
+            // The SCHED_IDLE policy is mapped to the highest nice value of
+            // `SchedPolicy::Fair` instead of `SchedPolicy::Idle`. Tasks of the
+            // latter policy are invisible to the user API.
+            SCHED_IDLE => SchedPolicy::Fair(Nice::MAX),
 
             _ => {
                 return Err(Error::with_message(

test/apps/sched/sched_attr_idle.c

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MPL-2.0
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <pthread.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#define SCHED_IDLE 5
+
+void *test(void *__arg)
+{
+	struct sched_param param = { .sched_priority = 0 };
+	assert(sched_setscheduler(0, SCHED_IDLE, &param) == 0);
+	sleep(1);
+	return NULL;
+}
+
+int main()
+{
+	pthread_t thread;
+	assert(pthread_create(&thread, NULL, test, NULL) == 0);
+	test(NULL);
+
+	pthread_join(thread, NULL);
+	printf("Test completed\n");
+
+	return 0;
+}

test/apps/scripts/process.sh

@@ -39,6 +39,7 @@ pthread/pthread_test
 pty/open_pty
 pty/pty_blocking
 sched/sched_attr
+sched/sched_attr_idle
 shm/posix_shm
 signal_c/parent_death_signal
 signal_c/signal_test