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From:   Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
To:     linux-kernel@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>
Subject: [PATCH 2/2] sched: allow sched_getattr with old size
Date:   Tue,  3 Sep 2019 14:16:45 -0300
Message-Id: <20190903171645.28090-2-cascardo@canonical.com>
In-Reply-To: <20190903171645.28090-1-cascardo@canonical.com>
References: <20190903171645.28090-1-cascardo@canonical.com>
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After commit a509a7cd7974 (sched/uclamp: Extend sched_setattr() to support
utilization clamping), using sched_getattr with size 48 will return E2BIG.

This breaks, for example, chrt.
$ chrt -p $$
chrt: failed to get pid 26306's policy: Argument list too long
$

With this fix, when using the old size, the utilization clamping values will be
absent from sched_attr. When using the new size or some larger size, they will
be present.

After the fix, chrt works again.
$ chrt -p $$
pid 4649's current scheduling policy: SCHED_OTHER
pid 4649's current scheduling priority: 0
$

The drawback with this solution is that userspace will ignore there are
non-default utilization clamps, but it's arguable whether returning E2BIG in
this case makes sense when that same userspace doesn't know about those values
anyway.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Fixes: a509a7cd7974 (sched/uclamp: Extend sched_setattr() to support utilization clamping)
Cc: Patrick Bellasi <patrick.bellasi@arm.com>
---
 kernel/sched/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0fd67281e656..0ccc7fa80da6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5183,8 +5183,10 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 		attr.sched_nice = task_nice(p);
 
 #ifdef CONFIG_UCLAMP_TASK
-	attr.sched_util_min = p->uclamp_req[UCLAMP_MIN].value;
-	attr.sched_util_max = p->uclamp_req[UCLAMP_MAX].value;
+	if (size >= SCHED_ATTR_SIZE_VER1) {
+		attr.sched_util_min = p->uclamp_req[UCLAMP_MIN].value;
+		attr.sched_util_max = p->uclamp_req[UCLAMP_MAX].value;
+	}
 #endif
 
 	rcu_read_unlock();
-- 
2.20.1