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    SubjectRe: [RFC][PATCH 11/11] signal: Ignore all but multi-process signals that come in during fork.
    On 07/10, Eric W. Biederman wrote:
    >
    > @@ -1602,6 +1603,20 @@ static __latent_entropy struct task_struct *copy_process(
    > {
    > int retval;
    > struct task_struct *p;
    > + unsigned seq;
    > +
    > + /*
    > + * Signals that are delivered to multiple processes need to be
    > + * delivered to just the parent before the fork or both the
    > + * parent and the child after the fork. Cache the multiple
    > + * process signal sequence number so we can detect any of
    > + * these signals that happen during the fork. In the unlikely
    > + * event a signal comes in while fork is starting and restart
    > + * fork to handle the signal.
    > + */
    > + seq = read_seqcount_begin(&current->signal->multi_process_seq);
    > + if (signal_pending(current))
    > + return ERR_PTR(-ERESTARTNOINTR);
    >
    > /*
    > * Don't allow sharing the root directory with processes in a different
    > @@ -1930,8 +1945,8 @@ static __latent_entropy struct task_struct *copy_process(
    > * A fatal signal pending means that current will exit, so the new
    > * thread can't slip out of an OOM kill (or normal SIGKILL).
    > */
    > - recalc_sigpending();
    > - if (signal_pending(current)) {
    > + if (read_seqcount_retry(&current->signal->multi_process_seq, seq) ||
    > + fatal_signal_pending(current)) {
    > retval = -ERESTARTNOINTR;
    > goto bad_fork_cancel_cgroup;

    So once again, I think this is not right, see the discussion on bugzilla.

    If signal_pending() == T we simply can't know if copy_process() can succeed or not.
    I have already mentioned the races with stop/freeze, but I think there are more.

    And in fact I think that the fact that signal_wake_up() helps to avoid the races
    with fork() is useful. Say, we could add signal_wake_up() into syscall_regfunc()
    and kill syscall_tracepoint_update(). Not that I think this particular change makes
    any sense, but it can work.



    That is why I tried to sugest another approach. copy_process() should always fail
    if signal_pending() == T, just the "real" signal should not disturb the forking
    thread unless the signal is fatal or multi-process.

    This also makes another difference in multi-threaded case, a signal with a handler
    sent to a forking process will be re-targeted to another thread which can handle it;
    with your patch this signal will be "blocked" until fork() finishes or until another
    thread gets TIF_SIGPENDING. Not that I think this is that important, but still.

    Oleg.

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