linux-kernel - Re: [PATCH v3?] proc_sysctl: remove rcu_dereference() for accessing ->sysctl

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Message-ID: <20250704020348.GN1880847@ZenIV>
Date: Fri, 4 Jul 2025 03:03:48 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neil@...wn.name>
Cc: Kees Cook <kees@...nel.org>, Joel Granados <joel.granados@...nel.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3?] proc_sysctl: remove rcu_dereference() for accessing
 ->sysctl

On Fri, Jul 04, 2025 at 11:39:16AM +1000, NeilBrown wrote:
> On Fri, 04 Jul 2025, Al Viro wrote:
> > On Mon, Jun 16, 2025 at 12:49:51PM +1000, NeilBrown wrote:
> > 
> > > The reality is that ->sysctl does not need rcu protection.  There is no
> > > concurrent update except that it can be set to NULL which is pointless.
> > 
> > I would rather *not* leave a dangling pointer there, and yes, it can
> > end up being dangling.  kfree_rcu() from inside the ->evict_inode()
> > may very well happen earlier than (also RCU-delayed) freeing of struct
> > inode itself.
> 
> In that case could we move the proc_sys_evict_inode() call from
> proc_evict_inode() to proc_free_inode(), and replace kfree_rcu() with
> kfree()?

proc_free_inode() can be called from softirq context, so we'd need to
touch all sysctl_lock users for that.  Definitely a larger patch that
way, if nothing else...