linux-kernel - epoll may leak events on dup

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Date:   Sat, 30 Oct 2021 10:03:19 +0000
From:   Sargun Dhillon <sargun@...gun.me>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, willy@...radead.org, arnd@...nel.org,
        Willem de Bruijn <willemb@...gle.com>
Subject: epoll may leak events on dup

I discovered an interesting behaviour in epoll today. If I register the same 
file twice, under two different file descriptor numbers, and then I close one of 
the two file descriptors, epoll "leaks" the first event. This is fine, because 
one would think I could just go ahead and remove the event, but alas, that isn't 
the case. Some example python code follows to show the issue at hand.

I'm not sure if this is really considered a "bug" or just "interesting epoll
behaviour", but in my opinion this is kind of a bug, especially because leaks
may happen by accident -- especially if files are not immediately freed.

I'm also not sure why epoll events are registered by file, and not just fd.
Is the expectation that you can share a single epoll amongst multiple
"users" and register different files that have the same file descriptor
number (at least for purposes other than CRIU). Maybe someone can shed
light on the behaviour.

(FWIW: On closing of all references the file, the callback finally occurs
clearing the event)

#!/usr/bin/env python3
import select
import socket
import os

n = 2
sockets = []
epoll = select.epoll()

for i in range(n):
        sockets.append(socket.socket(socket.AF_INET6, socket.SOCK_DGRAM))
        sockets[-1].bind(('::1', 0))

for i in sockets:
        epoll.register(i)

clones = [os.dup(i.fileno()) for i in sockets]

epoll.poll()
# Returns: [(4, 4), (5, 4)]

for i in clones:
        epoll.register(i)

epoll.poll()
# Returns: [(4, 4), (5, 4), (6, 4), (7, 4)]

socket0name = sockets[0].getsockname()
socket0fileno = sockets[0].fileno()
print(f'Closing {socket0fileno}')
os.close(socket0fileno)

epoll.poll()
# Returns: [(4, 4), (5, 4), (6, 4), (7, 4)]

badsocket = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
badsocket.sendto(b'test', (socket0name[0], socket0name[1]))

epoll.poll()
# Returns [(4, 5), (5, 4), (6, 5), (7, 4)]
# Indicating fd 4, and 6 have events.

# Oh whoops, I forgot to unregister 4...
epoll.unregister(4)

# Returns:
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# FileNotFoundError: [Errno 2] No such file or directory

print(open(f'/proc/{os.getpid()}/fdinfo/{epoll.fileno()}').read())
# Returns:
# pos:  0
# flags:        02000002
# mnt_id:       15
# ino:  10994
# tfd:        5 events:       1f data:                5  pos:0 ino:30ffc5 sdev:8
# tfd:        7 events:       1f data:                7  pos:0 ino:30ffc5 sdev:8
# tfd:        4 events:       1f data:                4  pos:0 ino:30ffc4 sdev:8
# tfd:        6 events:       1f data:                6  pos:0 ino:30ffc4 sdev:8