linux-kernel - Re: [PATCH v4] modules: wait do_free

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Date: Tue, 27 Feb 2024 08:30:44 -0800
From: Luis Chamberlain <mcgrof@...nel.org>
To: Changbin Du <changbin.du@...wei.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org,
	Xiaoyi Su <suxiaoyi@...wei.com>,
	Eric Chanudet <echanude@...hat.com>,
	Luis Chamberlain <mcgrof@...radead.org>
Subject: Re: [PATCH v4] modules: wait do_free_init correctly

On Tue, Feb 27, 2024 at 10:35:46AM +0800, Changbin Du wrote:
> The synchronization here is to ensure the ordering of freeing of a module
> init so that it happens before W+X checking. It is worth noting it is not
> that the freeing was not happening, it is just that our sanity checkers
> raced against the permission checkers which assume init memory is already
> gone.
> 
> Commit 1a7b7d922081 ("modules: Use vmalloc special flag") moved
> calling do_free_init() into a global workqueue instead of relying on it
> being called through call_rcu(..., do_free_init), which used to allowed us
> call do_free_init() asynchronously after the end of a subsequent grace
> period. The move to a global workqueue broke the gaurantees for code which
> needed to be sure the do_free_init() would complete with rcu_barrier().
> To fix this callers which used to rely on rcu_barrier() must now instead
> use flush_work(&init_free_wq).
> 
> Without this fix, we still could encounter false positive reports in W+X
> checking since the rcu_barrier() here can not ensure the ordering now.
> 
> Even worse, the rcu_barrier() can introduce significant delay. Eric Chanudet
> reported that the rcu_barrier introduces ~0.1s delay on a PREEMPT_RT kernel.
> 
>   [    0.291444] Freeing unused kernel memory: 5568K
>   [    0.402442] Run /sbin/init as init process
> 
> With this fix, the above delay can be eliminated.
> 
> Fixes: 1a7b7d922081 ("modules: Use vmalloc special flag")
> Signed-off-by: Changbin Du <changbin.du@...wei.com>
> Cc: Xiaoyi Su <suxiaoyi@...wei.com>
> Cc: Eric Chanudet <echanude@...hat.com>
> Cc: Luis Chamberlain <mcgrof@...radead.org>
> Tested-by: Eric Chanudet <echanude@...hat.com>

Acked-by: Luis Chamberlain <mcgrof@...nel.org>

  Luis