linux-kernel - Re: [PATCH RFC 1/1] module: delay kobject uevent until after module init call

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Message-ID: <4418e7a3-39e7-865e-9f2b-56ebf5782ac4@suse.com>
Date:   Thu, 3 Dec 2020 15:42:38 +0100
From:   Nicolas Morey-Chaisemartin <nmoreychaisemartin@...e.com>
To:     Jessica Yu <jeyu@...nel.org>, linux-kernel@...r.kernel.org,
        systemd-devel@...ts.freedesktop.org
Cc:     Franck Bui <fbui@...e.com>
Subject: Re: [PATCH RFC 1/1] module: delay kobject uevent until after module
 init call

I tested this on a system that had the script in patch#0 fail 100% of the time.
With this patch, I haven't seen any failure in a few hours.
Haven't seen any unexpected side effect either.

Tested-By: Nicolas Morey-Chaisemartin <nmoreychaisemartin@...e.com>

On 12/3/20 2:51 PM, Jessica Yu wrote:
> Apparently there has been a longstanding race between udev/systemd and
> the module loader. Currently, the module loader sends a uevent right
> after sysfs initialization, but before the module calls its init
> function. However, some udev rules expect that the module has
> initialized already upon receiving the uevent.
> 
> This race has been triggered recently (see link in references) in some
> systemd mount unit files. For instance, the configfs module creates the
> /sys/kernel/config mount point in its init function, however the module
> loader issues the uevent before this happens. sys-kernel-config.mount
> expects to be able to mount /sys/kernel/config upon receipt of the
> module loading uevent, but if the configfs module has not called its
> init function yet, then this directory will not exist and the mount unit
> fails. A similar situation exists for sys-fs-fuse-connections.mount, as
> the fuse sysfs mount point is created during the fuse module's init
> function. If udev is faster than module initialization then the mount
> unit would fail in a similar fashion.
> 
> To fix this race, delay the module KOBJ_ADD uevent until after the
> module has finished calling its init routine.
> 
> References: https://github.com/systemd/systemd/issues/17586
> Signed-off-by: Jessica Yu <jeyu@...nel.org>
> ---
>  kernel/module.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index a40ec708f8f2..e1dd0df57244 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1897,7 +1897,6 @@ static int mod_sysfs_init(struct module *mod)
>  	if (err)
>  		mod_kobject_put(mod);
>  
> -	/* delay uevent until full sysfs population */
>  out:
>  	return err;
>  }
> @@ -1934,7 +1933,6 @@ static int mod_sysfs_setup(struct module *mod,
>  	add_sect_attrs(mod, info);
>  	add_notes_attrs(mod, info);
>  
> -	kobject_uevent(&mod->mkobj.kobj, KOBJ_ADD);
>  	return 0;
>  
>  out_unreg_modinfo_attrs:
> @@ -3656,6 +3654,9 @@ static noinline int do_init_module(struct module *mod)
>  	blocking_notifier_call_chain(&module_notify_list,
>  				     MODULE_STATE_LIVE, mod);
>  
> +	/* Delay uevent until module has finished its init routine */
> +	kobject_uevent(&mod->mkobj.kobj, KOBJ_ADD);
> +
>  	/*
>  	 * We need to finish all async code before the module init sequence
>  	 * is done.  This has potential to deadlock.  For example, a newly
>