linux-kernel - [PATCH] preempt: Move PREEMPT_RT before PREEMPT in vermagic.

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Message-ID: <20241205160602.3lIAsJRT@linutronix.de>
Date: Thu, 5 Dec 2024 17:06:02 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Cc: Ben Segall <bsegall@...gle.com>, Clark Williams <clrkwllms@...nel.org>,
	Daniel Gomez <da.gomez@...sung.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Luis Chamberlain <mcgrof@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Petr Pavlu <petr.pavlu@...e.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] preempt: Move PREEMPT_RT before PREEMPT in vermagic.

Since the dynamic preemption has been enabled for PREEMPT_RT we have now
CONFIG_PREEMPT and CONFIG_PREEMPT_RT set simultaneously. This affects
the vermagic strings which comes now PREEMPT with PREEMPT_RT enabled.

The PREEMPT_RT module usually can not be loaded on a PREEMPT kernel
because some symbols are missing.
However if the symbols are fine then it continues and it crashes later.
The problem is that the struct module has a different layout and the
num_exentries or init members are at a different position leading to a
crash later on. This is not necessary caught by the size check in
elf_validity_cache_index_mod() because the mem member has an alignment
requirement of __module_memory_align which is big enough keep the total
size unchanged. Therefore we should keep the string accurate instead of
removing it.

Move the PREEMPT_RT check before the PREEMPT so that it takes precedence
if both symbols are enabled.

Fixes: 35772d627b55c ("sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/vermagic.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/vermagic.h b/include/linux/vermagic.h
index a54046bf37e55..939ceabcaf06f 100644
--- a/include/linux/vermagic.h
+++ b/include/linux/vermagic.h
@@ -15,10 +15,10 @@
 #else
 #define MODULE_VERMAGIC_SMP ""
 #endif
-#ifdef CONFIG_PREEMPT_BUILD
-#define MODULE_VERMAGIC_PREEMPT "preempt "
-#elif defined(CONFIG_PREEMPT_RT)
+#ifdef CONFIG_PREEMPT_RT
 #define MODULE_VERMAGIC_PREEMPT "preempt_rt "
+#elif defined(CONFIG_PREEMPT_BUILD)
+#define MODULE_VERMAGIC_PREEMPT "preempt "
 #else
 #define MODULE_VERMAGIC_PREEMPT ""
 #endif
-- 
2.45.2