linux-hardening - [PATCH] sched: task_struct: Fill unconditional hole induced by sched

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Date:   Thu, 23 Sep 2021 19:54:50 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH] sched: task_struct: Fill unconditional hole induced by sched_entity

With struct sched_entity before the other sched entities, its alignment
won't induce a struct hole. This saves 64 bytes in defconfig task_struct:

Before:
	...
        unsigned int               rt_priority;          /*   120     4 */

        /* XXX 4 bytes hole, try to pack */

        /* --- cacheline 2 boundary (128 bytes) --- */
        const struct sched_class  * sched_class;         /*   128     8 */

        /* XXX 56 bytes hole, try to pack */

        /* --- cacheline 3 boundary (192 bytes) --- */
        struct sched_entity        se __attribute__((__aligned__(64))); /*   192   448 */
        /* --- cacheline 10 boundary (640 bytes) --- */
        struct sched_rt_entity     rt;                   /*   640    48 */
        struct sched_dl_entity     dl __attribute__((__aligned__(8))); /*   688   224 */
        /* --- cacheline 14 boundary (896 bytes) was 16 bytes ago --- */

After:
	...
        unsigned int               rt_priority;          /*   120     4 */

        /* XXX 4 bytes hole, try to pack */

        /* --- cacheline 2 boundary (128 bytes) --- */
        struct sched_entity        se __attribute__((__aligned__(64))); /*   128   448 */
        /* --- cacheline 9 boundary (576 bytes) --- */
        struct sched_rt_entity     rt;                   /*   576    48 */
        struct sched_dl_entity     dl __attribute__((__aligned__(8))); /*   624   224 */
        /* --- cacheline 13 boundary (832 bytes) was 16 bytes ago --- */

Summary diff:
-	/* size: 7040, cachelines: 110, members: 188 */
+	/* size: 6976, cachelines: 109, members: 188 */

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/linux/sched.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 39039ce8ac4c..27ed1d40028f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -775,10 +775,10 @@ struct task_struct {
 	int				normal_prio;
 	unsigned int			rt_priority;
 
-	const struct sched_class	*sched_class;
 	struct sched_entity		se;
 	struct sched_rt_entity		rt;
 	struct sched_dl_entity		dl;
+	const struct sched_class	*sched_class;
 
 #ifdef CONFIG_SCHED_CORE
 	struct rb_node			core_node;
-- 
2.30.2