[PATCH] mips: fix endless loop when processing signals for kernel tasks

[dimm <dmitry.adamushko@xxxxxxxxx>]

From: Dmitry Adamushko <dmitry.adamushka_ext@xxxxxxxxxxxxxx>

The problem occurs [1] when a kernel-mode task returns from a system
call with a pending signal.

A real-life scenario is a child of 'khelper' returning from a failed
kernel_execve() in ____call_usermodehelper() [ kernel/kmod.c ].
kernel_execve() fails due to a pending SIGKILL, which is the result of
"kill -9 -1" (at least, busybox's init does it upon reboot).

The loop is as follows:

* syscall_exit_work:
 - work_pending:            // start_of_the_loop
 - work_notifysig:
   - do_notify_resume()
     - do_signal()
       - if (!user_mode(regs)) return;
 - resume_userspace         // TIF_SIGPENDING is still set
 - work_pending             // so we call work_pending => goto 
                            // start_of_the_loop

More information can be found in another LKML thread:
http://www.serverphorums.com/read.php?12,457826

[1] The problem was also reproduced on !CONFIG_VM86 x86, and the
following fix was accepted.

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=29a2e2836ff9ea65a603c89df217f4198973a74f

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxx

--- arch/mips/kernel/entry.S.old	2012-04-05 10:57:12.500976124 +0200
+++ arch/mips/kernel/entry.S	2012-04-05 11:21:24.128174358 +0200
@@ -36,6 +36,11 @@ FEXPORT(ret_from_exception)
 FEXPORT(ret_from_irq)
 	LONG_S	s0, TI_REGS($28)
 FEXPORT(__ret_from_irq)
+/*
+ * We can be coming here from a syscall done in the kernel space,
+ * e.g. a failed kernel_execve().
+ */
+resume_userspace_check:
 	LONG_L	t0, PT_STATUS(sp)		# returning to kernel mode?
 	andi	t0, t0, KU_USER
 	beqz	t0, resume_kernel
@@ -162,7 +167,7 @@ work_notifysig:				# deal with pending s
 	move	a0, sp
 	li	a1, 0
 	jal	do_notify_resume	# a2 already loaded
-	j	resume_userspace
+	j	resume_userspace_check
 
 FEXPORT(syscall_exit_work_partial)
 	SAVE_STATIC