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
