From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-Id: <20120105224403.138492777@clark.kroah.org> Date: Thu, 05 Jan 2012 14:44:57 -0800 From: Greg KH To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Tejun Heo , Oleg Nesterov Subject: [72/73] ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE race In-Reply-To: <20120105224632.GA14659@kroah.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: 3.0-stable review patch. If anyone has any objections, please let me know. ------------------ Content-Length: 2354 Lines: 86 From: Oleg Nesterov commit 50b8d257486a45cba7b65ca978986ed216bbcc10 upstream. Test-case: int main(void) { int pid, status; pid = fork(); if (!pid) { for (;;) { if (!fork()) return 0; if (waitpid(-1, &status, 0) < 0) { printf("ERR!! wait: %m\n"); return 0; } } } assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0); assert(waitpid(-1, NULL, 0) == pid); assert(ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_TRACEFORK) == 0); do { ptrace(PTRACE_CONT, pid, 0, 0); pid = waitpid(-1, NULL, 0); } while (pid > 0); return 1; } It fails because ->real_parent sees its child in EXIT_DEAD state while the tracer is going to change the state back to EXIT_ZOMBIE in wait_task_zombie(). The offending commit is 823b018e which moved the EXIT_DEAD check, but in fact we should not blame it. The original code was not correct as well because it didn't take ptrace_reparented() into account and because we can't really trust ->ptrace. This patch adds the additional check to close this particular race but it doesn't solve the whole problem. We simply can't rely on ->ptrace in this case, it can be cleared if the tracer is multithreaded by the exiting ->parent. I think we should kill EXIT_DEAD altogether, we should always remove the soon-to-be-reaped child from ->children or at least we should never do the DEAD->ZOMBIE transition. But this is too complex for 3.2. Reported-and-tested-by: Denys Vlasenko Tested-by: Lukasz Michalik Acked-by: Tejun Heo Signed-off-by: Oleg Nesterov Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- kernel/exit.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1553,8 +1553,15 @@ static int wait_consider_task(struct wai } /* dead body doesn't have much to contribute */ - if (p->exit_state == EXIT_DEAD) + if (unlikely(p->exit_state == EXIT_DEAD)) { + /* + * But do not ignore this task until the tracer does + * wait_task_zombie()->do_notify_parent(). + */ + if (likely(!ptrace) && unlikely(ptrace_reparented(p))) + wo->notask_error = 0; return 0; + } /* slay zombie? */ if (p->exit_state == EXIT_ZOMBIE) {