sched/deadline: Fix reset_on_fork reporting of DL tasks

It is possible for sched_getattr() to incorrectly report the state of
the reset_on_fork flag when called on a deadline task.

Indeed, if the flag was set on a deadline task using sched_setattr()
with flags (SCHED_FLAG_RESET_ON_FORK | SCHED_FLAG_KEEP_PARAMS), then
p->sched_reset_on_fork will be set, but __setscheduler() will bail out
early, which means that the dl_se->flags will not get updated by
__setscheduler_params()->__setparam_dl(). Consequently, if
sched_getattr() is then called on the task, __getparam_dl() will
override kattr.sched_flags with the now out-of-date copy in dl_se->flags
and report the stale value to userspace.

To fix this, make sure to only copy the flags that are relevant to
sched_deadline to and from the dl_se->flags field.

Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210727101103.2729607-2-qperret@google.com

kernel/sched/deadline.c

@@ -2741,7 +2741,7 @@ void __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
 	dl_se->dl_runtime = attr->sched_runtime;
 	dl_se->dl_deadline = attr->sched_deadline;
 	dl_se->dl_period = attr->sched_period ?: dl_se->dl_deadline;
-	dl_se->flags = attr->sched_flags;
+	dl_se->flags = attr->sched_flags & SCHED_DL_FLAGS;
 	dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
 	dl_se->dl_density = to_ratio(dl_se->dl_deadline, dl_se->dl_runtime);
 }
@@ -2754,7 +2754,8 @@ void __getparam_dl(struct task_struct *p, struct sched_attr *attr)
 	attr->sched_runtime = dl_se->dl_runtime;
 	attr->sched_deadline = dl_se->dl_deadline;
 	attr->sched_period = dl_se->dl_period;
-	attr->sched_flags = dl_se->flags;
+	attr->sched_flags &= ~SCHED_DL_FLAGS;
+	attr->sched_flags |= dl_se->flags;
 }
 
 /*
@@ -2851,7 +2852,7 @@ bool dl_param_changed(struct task_struct *p, const struct sched_attr *attr)
 	if (dl_se->dl_runtime != attr->sched_runtime ||
 	    dl_se->dl_deadline != attr->sched_deadline ||
 	    dl_se->dl_period != attr->sched_period ||
-	    dl_se->flags != attr->sched_flags)
+	    dl_se->flags != (attr->sched_flags & SCHED_DL_FLAGS))
 		return true;
 
 	return false;

kernel/sched/sched.h

@@ -227,6 +227,8 @@ static inline void update_avg(u64 *avg, u64 sample)
  */
 #define SCHED_FLAG_SUGOV	0x10000000
 
+#define SCHED_DL_FLAGS (SCHED_FLAG_RECLAIM | SCHED_FLAG_DL_OVERRUN | SCHED_FLAG_SUGOV)
+
 static inline bool dl_entity_is_special(struct sched_dl_entity *dl_se)
 {
 #ifdef CONFIG_CPU_FREQ_GOV_SCHEDUTIL