From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756241Ab1GGTGZ (ORCPT ); Thu, 7 Jul 2011 15:06:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40151 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755658Ab1GGTGJ (ORCPT ); Thu, 7 Jul 2011 15:06:09 -0400 Date: Thu, 7 Jul 2011 21:03:24 +0200 From: Oleg Nesterov To: Tejun Heo , Linus Torvalds Cc: vda.linux@googlemail.com, jan.kratochvil@redhat.com, pedro@codesourcery.com, indan@nul.nu, bdonlan@gmail.com, linux-kernel@vger.kernel.org Subject: [PATCH 1/1] ptrace: fix ptrace_signal() && STOP_DEQUEUED interaction Message-ID: <20110707190324.GB25332@redhat.com> References: <20110707190301.GA25332@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110707190301.GA25332@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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)