[PATCH] fork: fix default_timer_slack_ns inheritance from RT parent

Per prctl(2), "Timer slack is not applied to threads that are scheduled
under a real-time scheduling policy." RT tasks' timer_slack_ns is
forcibly 0 - not a user-chosen value but a "not applicable" state.

When copy_process() sets the child's default_timer_slack_ns from the
parent's timer_slack_ns, it inherits this forced 0 for RT parents. This
corrupts the child's reset target, making prctl(PR_SET_TIMERSLACK, 0)
and sched_setscheduler() back to NORMAL unable to restore a meaningful
default.

Fix this by using default_timer_slack_ns (which preserves the pre-RT
value) when the parent is RT/DL. For non-RT parents, timer_slack_ns is
a meaningful user value and the existing behavior is preserved.

Fixes: ed4fb6d7ef68 ("hrtimer: Use and report correct timerslack values for realtime tasks")
Depends-on: 63c1a12bc0e0 ("sched: restore timer_slack_ns when resetting RT policy on fork")
Reported-by: Qiaoting.Lin <[email protected]>
Signed-off-by: Guanyou.Chen <[email protected]>
Signed-off-by: Chunhui.Li <[email protected]>
---
 kernel/fork.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 65113a304518..bc4df18bfd90 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -23,6 +23,7 @@
 #include <linux/sched/task_stack.h>
 #include <linux/sched/cputime.h>
 #include <linux/sched/ext.h>
+#include <linux/sched/rt.h>
 #include <linux/seq_file.h>
 #include <linux/rtmutex.h>
 #include <linux/init.h>
@@ -2133,7 +2134,10 @@ __latent_entropy struct task_struct *copy_process(
 	retval = -EAGAIN;
 #endif
 
-	p->default_timer_slack_ns = current->timer_slack_ns;
+	if (rt_or_dl_task_policy(current))
+		p->default_timer_slack_ns = current->default_timer_slack_ns;
+	else
+		p->default_timer_slack_ns = current->timer_slack_ns;
 
 #ifdef CONFIG_PSI
 	p->psi_flags = 0;
-- 
2.34.1