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

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Date: Fri, 3 May 2024 23:07:44 +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 11:01:45PM +0100, Al Viro wrote:

> Having ->poll() instance itself grab reference is really asking for problem,
> even on the boxen that have CONFIG_EPOLL turned off.  select(2) is enough;
> it will take care of references grabbed by __pollwait(), but it doesn't
> know anything about the references driver has stashed hell knows where for
> hell knows how long.

Suppose your program calls select() on a pipe and dmabuf, sees data to be read
from pipe, reads it, closes both pipe and dmabuf and exits.

Would you expect that dmabuf file would stick around for hell knows how long
after that?  I would certainly be very surprised by running into that...