From 45cb24a1da53beb70f09efccc0373f6a47a9efe0 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 23 Mar 2011 10:37:01 +0100 Subject: [PATCH] job control: Allow access to job control events through ptracees Currently a real parent can't access job control stopped/continued events through a ptraced child. This utterly breaks job control when the children are ptraced. For example, if a program is run from an interactive shell and then strace(1) attaches to it, pressing ^Z would send SIGTSTP and strace(1) would notice it but the shell has no way to tell whether the child entered job control stop and thus can't tell when to take over the terminal - leading to awkward lone ^Z on the terminal. Because the job control and ptrace stopped states are independent, there is no reason to prevent real parents from accessing the stopped state regardless of ptrace. The continued state isn't separate but ptracers don't have any use for them as ptracees can never resume without explicit command from their ptracers, so as long as ptracers don't consume it, it should be fine. Although this is a behavior change, because the previous behavior is utterly broken when viewed from real parents and the change is only visible to real parents, I don't think it's necessary to make this behavior optional. One situation to be careful about is when a task from the real parent's group is ptracing. The parent group is the recipient of both ptrace and job control stop events and one stop can be reported as both job control and ptrace stops. As this can break the current ptrace users, suppress job control stopped events for these cases. If a real parent ptracer wants to know about both job control and ptrace stops, it can create a separate process to serve the role of real parent. Note that this only updates wait(2) side of things. The real parent can access the states via wait(2) but still is not properly notified (woken up and delivered signal). Test case polls wait(2) with WNOHANG to work around. Notification will be updated by future patches. Test case follows. #include #include #include #include #include #include #include int main(void) { const struct timespec ts100ms = { .tv_nsec = 100000000 }; pid_t tracee, tracer; siginfo_t si; int i; tracee = fork(); if (tracee == 0) { while (1) { printf("tracee: SIGSTOP\n"); raise(SIGSTOP); nanosleep(&ts100ms, NULL); printf("tracee: SIGCONT\n"); raise(SIGCONT); nanosleep(&ts100ms, NULL); } } waitid(P_PID, tracee, &si, WSTOPPED | WNOHANG | WNOWAIT); tracer = fork(); if (tracer == 0) { nanosleep(&ts100ms, NULL); ptrace(PTRACE_ATTACH, tracee, NULL, NULL); for (i = 0; i < 11; i++) { si.si_pid = 0; waitid(P_PID, tracee, &si, WSTOPPED); if (si.si_pid && si.si_code == CLD_TRAPPED) ptrace(PTRACE_CONT, tracee, NULL, (void *)(long)si.si_status); } printf("tracer: EXITING\n"); return 0; } while (1) { si.si_pid = 0; waitid(P_PID, tracee, &si, WSTOPPED | WCONTINUED | WEXITED | WNOHANG); if (si.si_pid) printf("mommy : WAIT status=%02d code=%02d\n", si.si_status, si.si_code); nanosleep(&ts100ms, NULL); } return 0; } Before the patch, while ptraced, the parent can't see any job control events. tracee: SIGSTOP mommy : WAIT status=19 code=05 tracee: SIGCONT tracee: SIGSTOP tracee: SIGCONT tracee: SIGSTOP tracee: SIGCONT tracee: SIGSTOP tracer: EXITING mommy : WAIT status=19 code=05 ^C After the patch, tracee: SIGSTOP mommy : WAIT status=19 code=05 tracee: SIGCONT mommy : WAIT status=18 code=06 tracee: SIGSTOP mommy : WAIT status=19 code=05 tracee: SIGCONT mommy : WAIT status=18 code=06 tracee: SIGSTOP mommy : WAIT status=19 code=05 tracee: SIGCONT mommy : WAIT status=18 code=06 tracee: SIGSTOP tracer: EXITING mommy : WAIT status=19 code=05 ^C -v2: Oleg pointed out that wait(2) should be suppressed for the real parent's group instead of only the real parent task itself. Updated accordingly. Signed-off-by: Tejun Heo Acked-by: Oleg Nesterov --- kernel/exit.c | 43 ++++++++++++++++++++++++++++++++++--------- 1 file changed, 34 insertions(+), 9 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 84d13d6bb30b..1a0f10f0a4db 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1541,17 +1541,19 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, if (p->exit_state == EXIT_DEAD) return 0; - if (likely(!ptrace) && unlikely(task_ptrace(p))) { - /* - * This child is hidden by ptrace. - * We aren't allowed to see it now, but eventually we will. - */ - wo->notask_error = 0; - return 0; - } - /* slay zombie? */ if (p->exit_state == EXIT_ZOMBIE) { + /* + * A zombie ptracee is only visible to its ptracer. + * Notification and reaping will be cascaded to the real + * parent when the ptracer detaches. + */ + if (likely(!ptrace) && unlikely(task_ptrace(p))) { + /* it will become visible, clear notask_error */ + wo->notask_error = 0; + return 0; + } + /* we don't reap group leaders with subthreads */ if (!delay_group_leader(p)) return wait_task_zombie(wo, p); @@ -1579,6 +1581,20 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, if (likely(!ptrace) || (wo->wo_flags & (WCONTINUED | WEXITED))) wo->notask_error = 0; } else { + /* + * If @p is ptraced by a task in its real parent's group, + * hide group stop/continued state when looking at @p as + * the real parent; otherwise, a single stop can be + * reported twice as group and ptrace stops. + * + * If a ptracer wants to distinguish the two events for its + * own children, it should create a separate process which + * takes the role of real parent. + */ + if (likely(!ptrace) && task_ptrace(p) && + same_thread_group(p->parent, p->real_parent)) + return 0; + /* * @p is alive and it's gonna stop, continue or exit, so * there always is something to wait for. @@ -1586,9 +1602,18 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, wo->notask_error = 0; } + /* + * Wait for stopped. Depending on @ptrace, different stopped state + * is used and the two don't interact with each other. + */ if (task_stopped_code(p, ptrace)) return wait_task_stopped(wo, ptrace, p); + /* + * Wait for continued. There's only one continued state and the + * ptracer can consume it which can confuse the real parent. Don't + * use WCONTINUED from ptracer. You don't need or want it. + */ return wait_task_continued(wo, p); }