linux-kernel - Re: [PATCH] kernfs: Drop kernfs_rwsem while invoking lookup_positive

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Message-ID: <20250220203924.GL1977892@ZenIV>
Date: Thu, 20 Feb 2025 20:39:24 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: syzbot <syzbot+ecccecbc636b455f9084@...kaller.appspotmail.com>,
	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com, tj@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] kernfs: Drop kernfs_rwsem while invoking
 lookup_positive_unlocked().

On Tue, Feb 18, 2025 at 05:39:38PM +0100, Sebastian Andrzej Siewior wrote:

> +	scoped_guard(rcu) {
> +		knparent = find_next_ancestor(kn, NULL);
> +		if (WARN_ON(!knparent)) {
> +			dput(dentry);
> +			return ERR_PTR(-EINVAL);
> +		}

NAK.  dput() is blocking; you *can't* do that under rcu_read_lock().