linux-kernel - Re: [PATCH] epoll: try to be a _bit

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Date: Fri, 3 May 2024 22:24:28 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: keescook@...omium.org, axboe@...nel.dk, brauner@...nel.org,
	christian.koenig@....com, dri-devel@...ts.freedesktop.org,
	io-uring@...r.kernel.org, jack@...e.cz, laura@...bott.name,
	linaro-mm-sig@...ts.linaro.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	minhquangbui99@...il.com, sumit.semwal@...aro.org,
	syzbot+045b454ab35fd82a35fb@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

On Fri, May 03, 2024 at 02:11:30PM -0700, Linus Torvalds wrote:
> epoll is a mess, and does various invalid things in the name of
> performance.
> 
> Let's try to rein it in a bit. Something like this, perhaps?

> +/*
> + * The ffd.file pointer may be in the process of
> + * being torn down due to being closed, but we
> + * may not have finished eventpoll_release() yet.
> + *
> + * Technically, even with the atomic_long_inc_not_zero,
> + * the file may have been free'd and then gotten
> + * re-allocated to something else (since files are
> + * not RCU-delayed, they are SLAB_TYPESAFE_BY_RCU).

Can we get to ep_item_poll(epi, ...) after eventpoll_release_file()
got past __ep_remove()?  Because if we can, we have a worse problem -
epi freed under us.

If not, we couldn't possibly have reached ->release() yet, let
alone freeing anything.