[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <35584a9f-f4c2-423a-8bb8-2c729cedb6fe@yandex.ru>
Date: Wed, 6 Mar 2024 21:07:53 +0300
From: Dmitry Antipov <dmantipov@...dex.ru>
To: Wen Gu <guwen@...ux.alibaba.com>,
"wenjia@...ux.ibm.com" <wenjia@...ux.ibm.com>
Cc: "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>,
"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"jaka@...ux.ibm.com" <jaka@...ux.ibm.com>
Subject: Re: [lvc-project] [PATCH] [RFC] net: smc: fix fasync leak in
smc_release()
On 3/6/24 17:45, Wen Gu wrote:
> IIUC, the fallback (or more precisely the private_data change) essentially
> always happens when the lock_sock(smc->sk) is held, except in smc_listen_work()
> or smc_listen_decline(), but at that moment, userspace program can not yet
> acquire this new socket to add fasync entries to the fasync_list.
>
> So IMHO, the above patch should work, since it checks the private_data under
> the lock_sock(sk). But if I missed something, please correct me.
Well, the whole picture is somewhat more complicated. Consider the
following diagram (an underlying kernel socket is in [], e.g. [smc->sk]):
Thread 0 Thread 1
ioctl(sock, FIOASYNC, [1])
...
sock = filp->private_data;
lock_sock(sock [smc->sk]);
sock_fasync(sock, ..., 1) ; new fasync_struct linked to smc->sk
release_sock(sock [smc->sk]);
...
lock_sock([smc->sk]);
...
smc_switch_to_fallback()
...
smc->clcsock->file->private_data = smc->clcsock;
...
release_sock([smc->sk]);
ioctl(sock, FIOASYNC, [0])
...
sock = filp->private_data;
lock_sock(sock [smc->clcsock]);
sock_fasync(sock, ..., 0) ; nothing to unlink from smc->clcsock
; since fasync entry was linked to smc->sk
release_sock(sock [smc->clcsock]);
...
close(sock [smc->clcsock]);
__fput(...);
file->f_op->fasync(sock, [0]) ; always failed -
; should use
; smc->sk instead
file->f_op->release()
...
smc_restore_fallback_changes()
...
file->private_data = smc->sk.sk_socket;
That is, smc_restore_fallback_changes() restores filp->private_data to
smc->sk. If __fput() would have called file->f_op->release() _before_
file->f_op->fasync(), the fix would be as simple as adding
smc->sk.sk_socket->wq.fasync_list = smc->clcsock->wq.fasync_list;
to smc_restore_fallback_changes(). But since file->f_op->fasync() is called
before file->f_op->release(), the former always makes an attempt to unlink fasync
entry from smc->clcsock instead of smc->sk, thus introducing the memory leak.
And an idea with shared wait queue was intended in attempt to eliminate
this chicken-egg lookalike problem completely.
Dmitry
Powered by blists - more mailing lists