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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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