linux-kernel - Re: [PATCH v4] module: Add CONFIG_MODULE_DISABLE_INIT

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Message-ID: <ZSglMlKdvhpNq2Vy@bombadil.infradead.org>
Date:   Thu, 12 Oct 2023 09:56:18 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Joey Jiao <quic_jiangenj@...cinc.com>
Cc:     linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] module: Add CONFIG_MODULE_DISABLE_INIT_FREE option

On Thu, Oct 12, 2023 at 07:17:19AM +0530, Joey Jiao wrote:
>  
> +config MODULE_DISABLE_INIT_FREE
> +	bool "Disable freeing of init sections"
> +	default n
> +	help
> +	  By default, kernel will free init sections after module being fully
> +	  loaded.
> +
> +	  MODULE_DISABLE_INIT_FREE allows users to prevent the freeing of init
> +	  sections. This option is particularly helpful for syzkaller fuzzing,
> +	  ensuring that the module consistently loads into the same address
> +	  across reboots.

How and why does not free'ing init help with syzkaller exactly? I don't
see the relationship between not free'ing init and ensuring th emodule
loads into the same address. There could be many things which could
incur an address gets allocated from a module at another location which
a module can take. I cannot fathom how this simple toggle could ensure
modules following the address allocations accross reboots. That seems
like odd chance, not something actually deterministic.

> +
>  endif # MODULES
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 98fedfdb8db5..0f242b7b29fe 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -2593,7 +2593,8 @@ static noinline int do_init_module(struct module *mod)
>  	 * be cleaned up needs to sync with the queued work - ie
>  	 * rcu_barrier()
>  	 */
> -	if (llist_add(&freeinit->node, &init_free_list))
> +	if (llist_add(&freeinit->node, &init_free_list) &&
> +		!IS_ENABLED(CONFIG_MODULE_DISABLE_INIT_FREE))
>  		schedule_work(&init_free_wq);