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@linux-foundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.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

On Sun, Jun 26, 2022 at 8:22 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> Drop the export because tick_nohz_full_setup() is only called from the
> built-in code in kernel/sched/isolation.c.

Thanks, applied.

              Linus

On Mon, Jun 27, 2022 at 12:22:09PM +0900, Masahiro Yamada wrote:
> 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@linux-foundation.org>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Tested-by: Paul E. McKenney <paulmck@kernel.org>

I went through my email and notes, and cannot find whatever it was that
possessed me to add tick_nohz_full_setup() to ae9e557b5be2.  The other
two symbols have clear motivations.  Apologies for the noise!!!

							Thanx, Paul

> ---
> 
>  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
>