[PATCH 1/1] ptrace: fix ptrace_signal() && STOP_DEQUEUED interaction

[Oleg Nesterov <oleg@redhat.com>]

Simple test-case,

	int main(void)
	{
		int pid, status;

		pid = fork();
		if (!pid) {
			pause();
			assert(0);
			return 0x23;
		}

		assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0);
		assert(wait(&status) == pid);
		assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);

		kill(pid, SIGCONT);	// <--- also clears STOP_DEQUEUD

		assert(ptrace(PTRACE_CONT, pid, 0,0) == 0);
		assert(wait(&status) == pid);
		assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGCONT);

		assert(ptrace(PTRACE_CONT, pid, 0, SIGSTOP) == 0);
		assert(wait(&status) == pid);
		assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);

		kill(pid, SIGKILL);
		return 0;
	}

Without the patch it hangs. After the patch SIGSTOP "injected" by the
tracer is not ignored and stops the tracee.

Note also that if this test-case uses, say, SIGWINCH instead of SIGCONT,
everything works without the patch. This can't be right, and this is
confusing.

The problem is that SIGSTOP (or any other sig_kernel_stop() signal) has
no effect without JOBCTL_STOP_DEQUEUED. This means it is simply ignored
after PTRACE_CONT unless JOBCTL_STOP_DEQUEUED was set "by accident", say
it wasn't cleared after initial SIGSTOP sent by PTRACE_ATTACH.

At first glance we could change ptrace_signal() to add STOP_DEQUEUED
after return from ptrace_stop(), but this is not right in case when the
tracer does not change the reported SIGSTOP and SIGCONT comes in between.
This is even more wrong with PT_SEIZED, SIGCONT adds JOBCTL_TRAP_NOTIFY
which will be "lost" during the TRAP_STOP | TRAP_NOTIFY report.

So lets add STOP_DEQUEUED _before_ we report the signal. It has no effect
unless sig_kernel_stop() == T after the tracer resumes us, and in the
latter case the pending STOP_DEQUEUED means no SIGCONT in between, we
should stop.

Note also that if SIGCONT was sent, PT_SEIZED tracee will correctly
report PTRACE_EVENT_STOP/SIGTRAP and thus the tracer can notice the fact
SIGSTOP was cancelled.

Also, move the current->ptrace check from ptrace_signal() to its caller,
get_signal_to_deliver(), this looks more natural.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 kernel/signal.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

--- ptrace/kernel/signal.c~1_ptrace_signal_stop_dequeued	2011-07-07 18:34:44.000000000 +0200
+++ ptrace/kernel/signal.c	2011-07-07 20:34:41.000000000 +0200
@@ -2084,12 +2084,13 @@ static void do_jobctl_trap(void)
 static int ptrace_signal(int signr, siginfo_t *info,
 			 struct pt_regs *regs, void *cookie)
 {
-	if (!current->ptrace)
-		return signr;
-
 	ptrace_signal_deliver(regs, cookie);
-
-	/* Let the debugger run.  */
+	/*
+	 * Debugger can change sig_kernel_stop(signr) from F to T,
+	 * in this case we should stop iff no SIGCONT in between.
+	 * Otherwise this JOBCTL_STOP_DEQUEUED has no effect.
+	 */
+	current->jobctl |= JOBCTL_STOP_DEQUEUED;
 	ptrace_stop(signr, CLD_TRAPPED, 0, info);
 
 	/* We're back.  Did the debugger cancel the sig?  */
@@ -2193,7 +2194,7 @@ relock:
 		if (!signr)
 			break; /* will return 0 */
 
-		if (signr != SIGKILL) {
+		if (unlikely(current->ptrace) && signr != SIGKILL) {
 			signr = ptrace_signal(signr, info,
 					      regs, cookie);
 			if (!signr)