[PATCH RESEND 2 1/1] sched/syscalls: Allow setting niceness using sched_param struct

[Michael C. Pratt]

>From userspace, spawning a new process with, for example,
posix_spawn(), only allows the user to work with
the scheduling priority value defined by POSIX
in the sched_param struct.

However, sched_setparam() and similar syscalls lead to
__sched_setscheduler() which rejects any new value
for the priority other than 0 for non-RT schedule classes,
a behavior that existed since Linux 2.6 or earlier.

Linux translates the usage of the sched_param struct
into it's own internal sched_attr struct during the syscall,
but the user currently has no way to manage the other values
within the sched_attr struct using only POSIX functions.

The only other way to adjust niceness when using posix_spawn()
would be to set the value after the process has started,
but this introduces the risk of the process being dead
before the syscall can set the priority afterward.

To resolve this, allow the use of the priority value
originally from the POSIX sched_param struct in order to
set the niceness value instead of rejecting the priority value.

Edit the sched_get_priority_*() POSIX syscalls
in order to reflect the range of values accepted.

Cc: stable@xxxxxxxxxxxxxxx # Apply to kernel/sched/core.c
Signed-off-by: Michael C. Pratt <mcpratt@xxxxx>
---
 kernel/sched/syscalls.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 24f9f90b6574..43eb283e6281 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -785,6 +785,19 @@ static int _sched_setscheduler(struct task_struct *p, int policy,
 		attr.sched_policy = policy;
 	}
 
+	if (attr.sched_priority > MAX_PRIO-1)
+		return -EINVAL;
+
+	/*
+	 * If priority is set for SCHED_NORMAL or SCHED_BATCH,
+	 * set the niceness instead, but only for user calls.
+	 */
+	if (check && attr.sched_priority > MAX_RT_PRIO-1 &&
+	   ((policy != SETPARAM_POLICY && fair_policy(policy)) || fair_policy(p->policy))) {
+		attr.sched_nice = PRIO_TO_NICE(attr.sched_priority);
+		attr.sched_priority = 0;
+	}
+
 	return __sched_setscheduler(p, &attr, check, true);
 }
 /**
@@ -1532,9 +1545,11 @@ SYSCALL_DEFINE1(sched_get_priority_max, int, policy)
 	case SCHED_RR:
 		ret = MAX_RT_PRIO-1;
 		break;
-	case SCHED_DEADLINE:
 	case SCHED_NORMAL:
 	case SCHED_BATCH:
+		ret = MAX_PRIO-1;
+		break;
+	case SCHED_DEADLINE:
 	case SCHED_IDLE:
 	case SCHED_EXT:
 		ret = 0;
@@ -1560,9 +1575,11 @@ SYSCALL_DEFINE1(sched_get_priority_min, int, policy)
 	case SCHED_RR:
 		ret = 1;
 		break;
-	case SCHED_DEADLINE:
 	case SCHED_NORMAL:
 	case SCHED_BATCH:
+		ret = MAX_RT_PRIO;
+		break;
+	case SCHED_DEADLINE:
 	case SCHED_IDLE:
 	case SCHED_EXT:
 		ret = 0;

base-commit: 2d5404caa8c7bb5c4e0435f94b28834ae5456623
-- 
2.30.2