[PATCH v2] sched/deadline: Fix sched_getattr() for DL tasks

[PATCH v2] sched/deadline: Fix sched_getattr() for DL tasks

[Quentin Perret]

In its current state, sched_getattr() can report incorrect sched_flags
if called on a deadline task.

Firstly, if the reset_on_fork flag is set on a deadline task using
sched_setattr() with SCHED_FLAG_RESET_ON_FORK | SCHED_FLAG_KEEP_PARAMS,
p->sched_reset_on_fork will be set but __setscheduler() will bail out
early without updating the dl_se->flags. Consequently, if sched_getattr
is then called on the same task, __getparam_dl() will override
kattr.sched_flags with the now out-of-date copy in dl_se->flags and
report a stale reset_on_fork value to userspace.

And secondly, sched_getattr() currently reports SCHED_FLAG_SUGOV as set
if called on a schedutil worker, despite this flag being a kernel-only
value that is not exposed in UAPI headers.

To fix both of these problems, make sure to only copy the flags that are
relevant to sched_deadline to dl_se->flags, and filter them out when
reporting them back to userspace.

Signed-off-by: Quentin Perret <qperret@google.com>
---
v1 can be found here:
https://lore.kernel.org/r/20210727101103.2729607-1-qperret@google.com/

Changes since v1:
 - squashed the two patches together
 - moved the SCHED_FLAG_SUGOV filtering to __getparam_dl()
---
 kernel/sched/deadline.c | 7 ++++---
 kernel/sched/sched.h    | 2 ++
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index aaacd6cfd42f..358a8a2acfc2 100644
--- a/kernel/sched/deadline.c
+++ b/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 & ~SCHED_FLAG_SUGOV;
 }
 
 /*
@@ -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;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 14a41a243f7b..ad3aee63db26 100644
--- a/kernel/sched/sched.h
+++ b/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
-- 
2.32.0.554.ge1b32706d8-goog

Re: [PATCH v2] sched/deadline: Fix sched_getattr() for DL tasks

[Juri Lelli]

Hi,

On 29/07/21 18:52, Quentin Perret wrote:
> In its current state, sched_getattr() can report incorrect sched_flags
> if called on a deadline task.
> 
> Firstly, if the reset_on_fork flag is set on a deadline task using
> sched_setattr() with SCHED_FLAG_RESET_ON_FORK | SCHED_FLAG_KEEP_PARAMS,
> p->sched_reset_on_fork will be set but __setscheduler() will bail out
> early without updating the dl_se->flags. Consequently, if sched_getattr
> is then called on the same task, __getparam_dl() will override
> kattr.sched_flags with the now out-of-date copy in dl_se->flags and
> report a stale reset_on_fork value to userspace.
> 
> And secondly, sched_getattr() currently reports SCHED_FLAG_SUGOV as set
> if called on a schedutil worker, despite this flag being a kernel-only
> value that is not exposed in UAPI headers.
> 
> To fix both of these problems, make sure to only copy the flags that are
> relevant to sched_deadline to dl_se->flags, and filter them out when
> reporting them back to userspace.
> 
> Signed-off-by: Quentin Perret <qperret@google.com>

Looks good to me.

Acked-by: Juri Lelli <juri.lelli@redhat.com>

Thanks!
Juri