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    Subject[PATCH 04/11] ptrace: implement PTRACE_INTERRUPT
    Date
    Currently, there's no way to trap a running ptracee short of sending a
    signal which has various side effects. This patch implements
    PTRACE_INTERRUPT which traps ptracee without any signal or job control
    related side effect.

    The implementation is almost trivial. It uses the same trap site and
    event as PTRACE_SEIZE. A new trap flag JOBCTL_TRAP_INTERRUPT is
    added, which is set on PTRACE_INTERRUPT and cleared when tracee
    commits to INTERRUPT trap. As INTERRUPT should be useable regardless
    of the current state of tracee, task_is_traced() test in
    ptrace_check_attach() is skipped for INTERRUPT.

    PTRACE_INTERRUPT is available iff tracee is attached with
    PTRACE_SEIZE.

    Test program follows.

    #define PTRACE_SEIZE 0x4206
    #define PTRACE_INTERRUPT 0x4207

    #define PTRACE_SEIZE_DEVEL 0x80000000

    static const struct timespec ts100ms = { .tv_nsec = 100000000 };
    static const struct timespec ts1s = { .tv_sec = 1 };
    static const struct timespec ts3s = { .tv_sec = 3 };

    int main(int argc, char **argv)
    {
    pid_t tracee;

    tracee = fork();
    if (tracee == 0) {
    nanosleep(&ts100ms, NULL);
    while (1) {
    printf("tracee: alive pid=%d\n", getpid());
    nanosleep(&ts1s, NULL);
    }
    }

    if (argc > 1)
    kill(tracee, SIGSTOP);

    nanosleep(&ts100ms, NULL);

    ptrace(PTRACE_SEIZE, tracee, NULL,
    (void *)(unsigned long)PTRACE_SEIZE_DEVEL);
    waitid(P_PID, tracee, NULL, WSTOPPED);
    ptrace(PTRACE_CONT, tracee, NULL, NULL);
    nanosleep(&ts3s, NULL);

    printf("tracer: INTERRUPT and DETACH\n");
    ptrace(PTRACE_INTERRUPT, tracee, NULL, NULL);
    waitid(P_PID, tracee, NULL, WSTOPPED);
    ptrace(PTRACE_DETACH, tracee, NULL, NULL);
    nanosleep(&ts3s, NULL);

    printf("tracer: exiting\n");
    kill(tracee, SIGKILL);
    return 0;
    }

    When called without argument, tracee is seized from running state,
    continued, interrupted and then detached back to running state.

    # ./test-interrupt
    tracee: alive pid=4546
    tracee: alive pid=4546
    tracee: alive pid=4546
    tracer: INTERRUPT and DETACH
    tracee: alive pid=4546
    tracee: alive pid=4546
    tracee: alive pid=4546
    tracer: exiting

    When called with argument, it's the same but tracee is detached back
    to stopped state.

    # ./test-interrupt 1
    tracee: alive pid=4548
    tracee: alive pid=4548
    tracee: alive pid=4548
    tracer: INTERRUPT and DETACH
    tracer: exiting

    Before PTRACE_INTERRUPT, once the tracee was continued, there was no
    easy way to do PTRACE_DETACH without causing side effect as tracee
    couldn't be trapped without side effect.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    ---
    include/linux/ptrace.h | 1 +
    include/linux/sched.h | 3 ++-
    kernel/ptrace.c | 23 +++++++++++++++++++++--
    kernel/signal.c | 4 ++++
    4 files changed, 28 insertions(+), 3 deletions(-)

    diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
    index 8de301a..5b6128b 100644
    --- a/include/linux/ptrace.h
    +++ b/include/linux/ptrace.h
    @@ -48,6 +48,7 @@
    #define PTRACE_SETREGSET 0x4205

    #define PTRACE_SEIZE 0x4206
    +#define PTRACE_INTERRUPT 0x4207

    /* flags in @data for PTRACE_SEIZE */
    #define PTRACE_SEIZE_DEVEL 0x80000000 /* temp flag for development */
    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 2f383eb..221ab51 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -1785,9 +1785,10 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
    #define JOBCTL_STOP_PENDING (1 << 17) /* task should stop for group stop */
    #define JOBCTL_STOP_CONSUME (1 << 18) /* consume group stop count */
    #define JOBCTL_TRAP_SEIZE (1 << 19) /* trap for seize */
    +#define JOBCTL_TRAP_INTERRUPT (1 << 20) /* trap for interrupt */
    #define JOBCTL_TRAPPING (1 << 22) /* switching to TRACED */

    -#define JOBCTL_TRAP_MASK JOBCTL_TRAP_SEIZE
    +#define JOBCTL_TRAP_MASK (JOBCTL_TRAP_SEIZE | JOBCTL_TRAP_INTERRUPT)
    #define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)

    extern void task_clear_jobctl_stop_pending(struct task_struct *task);
    diff --git a/kernel/ptrace.c b/kernel/ptrace.c
    index 0f0121a..1262a36 100644
    --- a/kernel/ptrace.c
    +++ b/kernel/ptrace.c
    @@ -693,6 +693,23 @@ int ptrace_request(struct task_struct *child, long request,
    ret = ptrace_setsiginfo(child, &siginfo);
    break;

    + case PTRACE_INTERRUPT:
    + if (!likely(child->ptrace & PT_SEIZED))
    + break;
    + /*
    + * Stop tracee without any side-effect on signal or job
    + * control. If @child is already trapped, the current trap
    + * is not disturbed and INTERRUPT trap will happen after
    + * the current trap is ended with PTRACE_CONT. Note that
    + * other traps may happen before the scheduled INTERRUPT.
    + */
    + spin_lock(&child->sighand->siglock);
    + child->jobctl |= JOBCTL_TRAP_INTERRUPT;
    + signal_wake_up(child, 0);
    + spin_unlock(&child->sighand->siglock);
    + ret = 0;
    + break;
    +
    case PTRACE_DETACH: /* detach a process that was attached. */
    ret = ptrace_detach(child, data);
    break;
    @@ -818,7 +835,8 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
    goto out_put_task_struct;
    }

    - ret = ptrace_check_attach(child, request == PTRACE_KILL);
    + ret = ptrace_check_attach(child, request == PTRACE_KILL ||
    + request == PTRACE_INTERRUPT);
    if (ret < 0)
    goto out_put_task_struct;

    @@ -960,7 +978,8 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
    goto out_put_task_struct;
    }

    - ret = ptrace_check_attach(child, request == PTRACE_KILL);
    + ret = ptrace_check_attach(child, request == PTRACE_KILL ||
    + request == PTRACE_INTERRUPT);
    if (!ret)
    ret = compat_arch_ptrace(child, request, addr, data);

    diff --git a/kernel/signal.c b/kernel/signal.c
    index 9249230..7add912 100644
    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -1711,6 +1711,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
    __releases(&current->sighand->siglock)
    __acquires(&current->sighand->siglock)
    {
    + bool is_intr = exit_code == (SIGTRAP | (PTRACE_EVENT_INTERRUPT << 8));
    bool gstop_done = false;

    if (arch_ptrace_stop_needed(exit_code, info)) {
    @@ -1760,6 +1761,9 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
    task_clear_jobctl_trapping(current);
    current->jobctl &= ~JOBCTL_TRAP_SEIZE;

    + if (is_intr)
    + current->jobctl &= ~JOBCTL_TRAP_INTERRUPT;
    +
    spin_unlock_irq(&current->sighand->siglock);
    read_lock(&tasklist_lock);
    if (may_ptrace_stop()) {
    --
    1.7.1


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