Messages in this thread Patch in this message | | | Date | Wed, 4 Nov 2015 20:19:12 +0100 | From | Oleg Nesterov <> | Subject | [PATCH 1/1] signal: kill the obsolete SIGNAL_UNKILLABLE check in complete_signal() |
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complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy the thread group, today this is unnecessary and even not 100% correct.
After the commit f008faff0e27 ("signals: protect init from unwanted signals more") we rely on sig_task_ignored(), complete_signal(SIGKILL) can only see a SIGNAL_UNKILLABLE task if we actually want to kill it. And note that after the commit b3bfa0cba867 ("signals: protect cinit from blocked fatal signals") we do not drop SIGKILL dequeued by /sbin/init.
And it does not look right. fatal_signal_pending() should always imply that the whole thread group (except ->group_exit_task if it is not NULL) is killed, this check breaks the rule.
This explains WARN_ON(!JOBCTL_STOP_PENDING) in task_participate_group_stop() triggered by the test-case from Dmitry:
int main() { int pid = 1; ptrace(PTRACE_ATTACH, pid, 0, 0); ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_EXITKILL); sleep(1); return 0; }
do_signal_stop()->signal_group_exit() returns false because SIGNAL_GROUP_EXIT is not set, but task_set_jobctl_pending() checks fatal_signal_pending() and does not set JOBCTL_STOP_PENDING.
The test-case above needs root and (correctly) crashes the kernel, but we can trigger the same warning inside the container or using another test-case:
static int init(void *arg) { for (;;) pause(); }
int main(void) { char stack[16 * 1024];
for (;;) { int pid = clone(init, stack + sizeof(stack)/2, CLONE_NEWPID | SIGCHLD, NULL); assert(pid > 0);
assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0); assert(waitpid(-1, NULL, WSTOPPED) == pid);
assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0); assert(syscall(__NR_tkill, pid, SIGKILL) == 0); assert(pid == wait(NULL)); } }
Reported-by: Dmitry Vyukov <dvyukov@google.com> Signed-off-by: Oleg Nesterov <oleg@redhat.com> --- kernel/signal.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c index d64efad..9b6f385 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -914,7 +914,7 @@ static void complete_signal(int sig, struct task_struct *p, int group) * then start taking the whole group down immediately. */ if (sig_fatal(p, sig) && - !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) && + !(signal->flags & SIGNAL_GROUP_EXIT) && !sigismember(&t->real_blocked, sig) && (sig == SIGKILL || !t->ptrace)) { /* -- 1.5.5.1
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