linux-kernel - Re: [PATCH 6/6] eventfs: clean up dentry ops and add revalidate function

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Message-ID: <20240131024626.GQ2087318@ZenIV>
Date: Wed, 31 Jan 2024 02:46:26 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] eventfs: clean up dentry ops and add revalidate
 function

On Tue, Jan 30, 2024 at 06:37:49PM -0800, Linus Torvalds wrote:
> On Tue, 30 Jan 2024 at 17:12, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > > + *
> > > + * Note that d_revalidate is called potentially under RCU,
> > > + * so it can't take the eventfs mutex etc. It's fine - if
> > > + * we open a file just as it's marked dead, things will
> > > + * still work just fine, and just see the old stale case.
> >
> > Looks like use after free, unless freeing ei is RCU-delayed...
> 
> We hold the ref to the ei in the very dentry that is doing d_revalidate().

What's to stop ->d_revalidate() from being called in parallel with
__dentry_kill()?