linux-kernel - Re: [PATCH] clocksource: hyper-v: unexport __init-annotated hv_init

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Date:   Mon, 06 Jun 2022 08:59:48 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        "K . Y . Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        linux-hyperv@...r.kernel.org
Subject: Re: [PATCH] clocksource: hyper-v: unexport __init-annotated
 hv_init_clocksource()

Masahiro Yamada <masahiroy@...nel.org> writes:

> 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 has been broken for a decade.
>
> Recently, I fixed modpost so it started to warn it again, then this
> showed up in linux-next builds.
>
> There are two ways to fix it:
>
>   - Remove __init
>   - Remove EXPORT_SYMBOL
>
> I chose the latter for this case because the only in-tree call-site,
> arch/x86/kernel/cpu/mshyperv.c is never compiled as modular.
> (CONFIG_HYPERVISOR_GUEST is boolean)
>
> Fixes: dd2cb348613b ("clocksource/drivers: Continue making Hyper-V clocksource ISA agnostic")
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
>
>  drivers/clocksource/hyperv_timer.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index ff188ab68496..bb47610bbd1c 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -565,4 +565,3 @@ void __init hv_init_clocksource(void)
>  	hv_sched_clock_offset = hv_read_reference_counter();
>  	hv_setup_sched_clock(read_hv_sched_clock_msr);
>  }
> -EXPORT_SYMBOL_GPL(hv_init_clocksource);

hv_init_clocksource() is not called from modules indeed, 

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly