uml: definitively kill subprocesses on panic

In a stock 2.6.22.6 kernel, poweroff a user mode linux guest (2.6.22.6 running
in skas0 mode) will halt the host linux.  I think the reason is the kernel
thread abort because of a bug.  Then the sys_reboot in process of user mode
linux guest is not trapped by the user mode linux kernel and is executed by
host.  I think it is better to make sure all of our children process to quit
when user mode linux kernel abort.

[ jdike - the kernel process needs to ignore SIGTERM, plus the waitpid/kill
loop is needed to make sure that all of our children are dead before the
kernel exits ]

Signed-off-by: Lepton Wu <ytht.net@gmail.com>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 39 insertions, 1 deletions

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index e60d6e6c5a..d77c81d706 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -177,7 +177,7 @@ static int userspace_tramp(void *stack)
 
         ptrace(PTRACE_TRACEME, 0, 0, 0);
 
-        init_new_thread_signals();
+        signal(SIGTERM, SIG_DFL);
         err = set_interval();
         if (err)
                 panic("userspace_tramp - setting timer failed, errno = %d\n",
diff --git a/arch/um/os-Linux/util.c b/arch/um/os-Linux/util.c
index 7cbcf484e1..ef095436a7 100644
--- a/arch/um/os-Linux/util.c
+++ b/arch/um/os-Linux/util.c
@@ -105,6 +105,44 @@ int setjmp_wrapper(void (*proc)(void *, void *), ...)
 
 void os_dump_core(void)
 {
+        int pid;
+
         signal(SIGSEGV, SIG_DFL);
+
+        /*
+         * We are about to SIGTERM this entire process group to ensure that
+         * nothing is around to run after the kernel exits.  The
+         * kernel wants to abort, not die through SIGTERM, so we
+         * ignore it here.
+         */
+
+        signal(SIGTERM, SIG_IGN);
+        kill(0, SIGTERM);
+        /*
+         * Most of the other processes associated with this UML are
+         * likely sTopped, so give them a SIGCONT so they see the
+         * SIGTERM.
+         */
+        kill(0, SIGCONT);
+
+        /*
+         * Now, having sent signals to everyone but us, make sure they
+         * die by ptrace.  Processes can survive what's been done to
+         * them so far - the mechanism I understand is receiving a
+         * SIGSEGV and segfaulting immediately upon return.  There is
+         * always a SIGSEGV pending, and (I'm guessing) signals are
+         * processed in numeric order so the SIGTERM (signal 15 vs
+         * SIGSEGV being signal 11) is never handled.
+         *
+         * Run a waitpid loop until we get some kind of error.
+         * Hopefully, it's ECHILD, but there's not a lot we can do if
+         * it's something else.  Tell os_kill_ptraced_process not to
+         * wait for the child to report its death because there's
+         * nothing reasonable to do if that fails.
+         */
+
+        while ((pid = waitpid(-1, NULL, WNOHANG)) > 0)
+                os_kill_ptraced_process(pid, 0);
+
         abort();
 }