Re: [PATCH AUTOSEL 5.2 039/249] signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig

From: Eric W. Biederman
Date: Wed Jul 24 2019 - 00:46:13 EST

Steve French <smfrench@xxxxxxxxx> 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,

+ sigdelset(&mask, SIGKILL);
sigprocmask(SIG_BLOCK, &mask, &oldmask);

/* Generate a rfc1002 marker for SMB2+ */