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    Subject[RFC PATCH 3/3] ptrace: dont send SIGSTOP on auto-attach if PT_SEIZED
    The fake SIGSTOP during attach has numerous problems. PTRACE_SEIZE
    is already fine, but we have basically the same problems is SIGSTOP
    is sent on auto-attach, the tracer can't know if this signal signal
    should be cancelled or not.

    Change ptrace_event() to set JOBCTL_TRAP_STOP if the new child is
    PT_SEIZED, this triggers the PTRACE_EVENT_STOP report.

    Thereafter a PT_SEIZED task can never report the bogus SIGSTOP.

    Test-case:

    #define PTRACE_SEIZE 0x4206
    #define PTRACE_SEIZE_DEVEL 0x80000000
    #define PTRACE_EVENT_STOP 7
    #define WEVENT(s) ((s & 0xFF0000) >> 16)

    int main(void)
    {
    int child, grand_child, status;
    long message;

    child = fork();
    if (!child) {
    kill(getpid(), SIGSTOP);
    fork();
    assert(0);
    return 0x23;
    }

    assert(ptrace(PTRACE_SEIZE, child, 0,PTRACE_SEIZE_DEVEL) == 0);
    assert(wait(&status) == child);
    assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);

    assert(ptrace(PTRACE_SETOPTIONS, child, 0, PTRACE_O_TRACEFORK) == 0);

    assert(ptrace(PTRACE_CONT, child, 0,0) == 0);
    assert(waitpid(child, &status, 0) == child);
    assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
    assert(WEVENT(status) == PTRACE_EVENT_FORK);

    assert(ptrace(PTRACE_GETEVENTMSG, child, 0, &message) == 0);
    grand_child = message;

    assert(waitpid(grand_child, &status, 0) == grand_child);
    assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
    assert(WEVENT(status) == PTRACE_EVENT_STOP);

    kill(child, SIGKILL);
    kill(grand_child, SIGKILL);
    return 0;
    }

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

    include/linux/ptrace.h | 13 +++++++------
    1 file changed, 7 insertions(+), 6 deletions(-)

    --- ptrace/include/linux/ptrace.h~5_clone_no_stop_if_seized 2011-07-08 18:32:05.000000000 +0200
    +++ ptrace/include/linux/ptrace.h 2011-07-08 18:32:53.000000000 +0200
    @@ -228,7 +228,11 @@ static inline void ptrace_init_task(stru
    child->ptrace = current->ptrace;
    __ptrace_link(child, current->parent);

    - sigaddset(&child->pending.signal, SIGSTOP);
    + if (child->ptrace & PT_SEIZED)
    + task_set_jobctl_pending(child, JOBCTL_TRAP_STOP);
    + else
    + sigaddset(&child->pending.signal, SIGSTOP);
    +
    set_tsk_thread_flag(child, TIF_SIGPENDING);
    }
    }


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