linux-kernel - Re: [PATCH AUTOSEL 5.2 039/249] signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force

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Message-ID: <87v9vs43pq.fsf@xmission.com>
Date:   Tue, 23 Jul 2019 20:32:33 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Steve French <smfrench@...il.com>
Cc:     ronnie sahlberg <ronniesahlberg@...il.com>,
        Sasha Levin <sashal@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stable <stable@...r.kernel.org>,
        Namjae Jeon <namjae.jeon@...sung.com>,
        Jeff Layton <jlayton@...marydata.com>,
        linux-cifs <linux-cifs@...r.kernel.org>
Subject: Re: [PATCH AUTOSEL 5.2 039/249] signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig

Steve French <smfrench@...il.com> writes:

> Very easy to see what caused the regression with this global change:
>
> mount (which launches "cifsd" thread to read the socket)
> umount (which kills the "cifsd" thread)
> rmmod   (rmmod now fails since "cifsd" thread is still active)
>
> mount launches a thread to read from the socket ("cifsd")
> umount is supposed to kill that thread (but with the patch
> "signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of
> force_sig" that no longer works).  So the regression is that after
> unmount you still see the "cifsd" thread, and the reason that cifsd
> thread is still around is that that patch no longer force kills the
> process (see line 2652 of fs/cifs/connect.c) which regresses module
> removal.
>
> -               force_sig(SIGKILL, task);
> +               send_sig(SIGKILL, task, 1);
>
> The comment in the changeset indicates "The signal SIGKILL can not be
> ignored" but obviously it can be ignored - at least on 5.3-rc1 it is
> being ignored.
>
> If send_sig(SIGKILL ...) doesn't work and if force_sig(SIGKILL, task)
> is removed and no longer possible - how do we kill a helper process
> ...

I think I see what is happening.  It looks like as well as misuinsg
force_sig, cifs is also violating the invariant that keeps SIGKILL out
of the blocked signal set.

For that force_sig will act differently.  I did not consider it because
that is never supposed to happen.

Can someone test this code below and confirm the issue goes away?

diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index 5d6d44bfe10a..2a782ebc7b65 100644
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -347,6 +347,7 @@ __smb_send_rqst(struct TCP_Server_Info *server, int num_rqst,
 	 */
 
 	sigfillset(&mask);
+	sigdelset(&mask, SIGKILL);
 	sigprocmask(SIG_BLOCK, &mask, &oldmask);
 
 	/* Generate a rfc1002 marker for SMB2+ */


Eric