Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | From | Andi Kleen <> | Subject | [PATCH] sched: Work around undefined behavior in sched class checking | Date | Tue, 4 May 2021 20:39:45 -0700 |
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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
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