Merge branch 'siginfo-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace

Pull kernel thread signal handling fix from Eric Biederman:
"I overlooked the fact that kernel threads are created with all signals
set to SIG_IGN, and accidentally caused a regression in cifs and drbd
when replacing force_sig with send_sig.

This is my fix for that regression. I add a new function
allow_kernel_signal which allows kernel threads to receive signals
sent from the kernel, but continues to ignore all signals sent from
userspace. This ensures the user space interface for cifs and drbd
remain the same.

These kernel threads depend on blocking networking calls which block
until something is received or a signal is pending. Making receiving
of signals somewhat necessary for these kernel threads.

Perhaps someday we can cleanup those interfaces and remove
allow_kernel_signal. If not allow_kernel_signal is pretty trivial and
clearly documents what is going on so I don't think we will mind
carrying it"

* 'siginfo-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
signal: Allow cifs and drbd to receive their terminating signals

drivers/block/drbd/drbd_main.c

···
 		 thi->name[0],
 		 resource->name);
 
+	allow_kernel_signal(DRBD_SIGKILL);
+	allow_kernel_signal(SIGXCPU);
 restart:
 	retval = thi->function(thi);
 

fs/cifs/connect.c

···
 		mempool_resize(cifs_req_poolp, length + cifs_min_rcv);
 
 	set_freezable();
-	allow_signal(SIGKILL);
+	allow_kernel_signal(SIGKILL);
 	while (server->tcpStatus != CifsExiting) {
 		if (try_to_freeze())
 			continue;

include/linux/signal.h

···
 extern void exit_signals(struct task_struct *tsk);
 extern void kernel_sigaction(int, __sighandler_t);
 
+#define SIG_KTHREAD ((__force __sighandler_t)2)
+#define SIG_KTHREAD_KERNEL ((__force __sighandler_t)3)
+
 static inline void allow_signal(int sig)
 {
 	/*
···
 	 * know it'll be handled, so that they don't get converted to
 	 * SIGKILL or just silently dropped.
 	 */
-	kernel_sigaction(sig, (__force __sighandler_t)2);
+	kernel_sigaction(sig, SIG_KTHREAD);
+}
+
+static inline void allow_kernel_signal(int sig)
+{
+	/*
+	 * Kernel threads handle their own signals. Let the signal code
+	 * know signals sent by the kernel will be handled, so that they
+	 * don't get silently dropped.
+	 */
+	kernel_sigaction(sig, SIG_KTHREAD_KERNEL);
 }
 
 static inline void disallow_signal(int sig)

kernel/signal.c

···
 	    handler == SIG_DFL && !(force && sig_kernel_only(sig)))
 		return true;
 
+	/* Only allow kernel generated signals to this kthread */
+	if (unlikely((t->flags & PF_KTHREAD) &&
+		     (handler == SIG_KTHREAD_KERNEL) && !force))
+		return true;
+
 	return sig_handler_ignored(handler, sig);
 }