linux-kernel - [PATCH] tick/nohz: unexport __init-annotated tick_nohz_full

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Date:   Mon, 27 Jun 2022 12:22:09 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: [PATCH] tick/nohz: unexport __init-annotated tick_nohz_full_setup()

EXPORT_SYMBOL and __init is a bad combination because the .init.text
section is freed up after the initialization. Hence, modules cannot
use symbols annotated __init. The access to a freed symbol may end up
with kernel panic.

modpost used to detect it, but it had been broken for a decade.

Commit 28438794aba4 ("modpost: fix section mismatch check for exported
init/exit sections") fixed it so modpost started to warn it again, then
this showed up:

    MODPOST vmlinux.symvers
  WARNING: modpost: vmlinux.o(___ksymtab_gpl+tick_nohz_full_setup+0x0): Section mismatch in reference from the variable __ksymtab_tick_nohz_full_setup to the function .init.text:tick_nohz_full_setup()
  The symbol tick_nohz_full_setup is exported and annotated __init
  Fix this by removing the __init annotation of tick_nohz_full_setup or drop the export.

Drop the export because tick_nohz_full_setup() is only called from the
built-in code in kernel/sched/isolation.c.

Fixes: ae9e557b5be2 ("time: Export tick start/stop functions for rcutorture")
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 kernel/time/tick-sched.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 58a11f859ac7..30049580cd62 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -526,7 +526,6 @@ void __init tick_nohz_full_setup(cpumask_var_t cpumask)
 	cpumask_copy(tick_nohz_full_mask, cpumask);
 	tick_nohz_full_running = true;
 }
-EXPORT_SYMBOL_GPL(tick_nohz_full_setup);
 
 static int tick_nohz_cpu_down(unsigned int cpu)
 {
-- 
2.32.0