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From:   Andi Kleen <ak@linux.intel.com>
To:     peterz@infradead.org
Cc:     linux-kernel@vger.kernel.org, Andi Kleen <andi@firstfloor.org>
Subject: [PATCH] sched: Work around undefined behavior in sched class checking
Date:   Tue,  4 May 2021 20:39:45 -0700
Message-Id: <20210505033945.1282851-1-ak@linux.intel.com>
MIME-Version: 1.0
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From: Andi Kleen <andi@firstfloor.org>

The scheduler initialization code checks that the scheduling
classes are consecutive in memory by comparing the end
addresses with the next address.

Technically in ISO C comparing symbol addresseses outside different objects
is undefined. With LTO gcc 10 tries to exploits this and creates an
unconditional BUG_ON in the scheduler initialization, resulting
in a boot hang.

Use RELOC_HIDE to make this work. This hides the symbols from gcc,
so the optimizer won't make these assumption. I also split
the BUG_ONs in multiple.

Signed-off-by: Andi Kleen <andi@firstfloor.org>
---
 kernel/sched/core.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 98191218d891..272a654f5293 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8086,12 +8086,20 @@ void __init sched_init(void)
 	unsigned long ptr = 0;
 	int i;
 
-	/* Make sure the linker didn't screw up */
-	BUG_ON(&idle_sched_class + 1 != &fair_sched_class ||
-	       &fair_sched_class + 1 != &rt_sched_class ||
-	       &rt_sched_class + 1   != &dl_sched_class);
+	/*
+	 * Make sure the linker didn't screw up.
+	 * We have to use RELOC_HIDE to prevent gcc from optimizing
+	 * them to true because they're technically undefined in ISO-C.
+	 */
+	BUG_ON(RELOC_HIDE(&idle_sched_class, 0) + 1 !=
+			RELOC_HIDE(&fair_sched_class, 0));
+	BUG_ON(RELOC_HIDE(&fair_sched_class, 0) + 1 !=
+			RELOC_HIDE(&rt_sched_class, 0));
+	BUG_ON(RELOC_HIDE(&rt_sched_class, 0) + 1 !=
+			RELOC_HIDE(&dl_sched_class, 0));
 #ifdef CONFIG_SMP
-	BUG_ON(&dl_sched_class + 1 != &stop_sched_class);
+	BUG_ON(RELOC_HIDE(&dl_sched_class, 0) + 1 !=
+			RELOC_HIDE(&stop_sched_class, 0));
 #endif
 
 	wait_bit_init();
-- 
2.25.4