coredump: simplify zap_process()

After commit 0258b5f ("coredump: Limit coredumps to a single thread
group") zap_process() doesn't need the "task_struct *start" arg,
zap_threads() can pass "signal_struct *signal" instead.

This simplifies the code and allows to use __for_each_thread() which
is slightly more efficient.

Link: https://lkml.kernel.org/r/20240625140311.GA20787@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: oleg-nesterov

fs/coredump.c

@@ -361,17 +361,16 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
 	return ispipe;
 }
 
-static int zap_process(struct task_struct *start, int exit_code)
+static int zap_process(struct signal_struct *signal, int exit_code)
 {
 	struct task_struct *t;
 	int nr = 0;
 
-	/* Allow SIGKILL, see prepare_signal() */
-	start->signal->flags = SIGNAL_GROUP_EXIT;
-	start->signal->group_exit_code = exit_code;
-	start->signal->group_stop_count = 0;
+	signal->flags = SIGNAL_GROUP_EXIT;
+	signal->group_exit_code = exit_code;
+	signal->group_stop_count = 0;
 
-	for_each_thread(start, t) {
+	__for_each_thread(signal, t) {
 		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
 		if (t != current && !(t->flags & PF_POSTCOREDUMP)) {
 			sigaddset(&t->pending.signal, SIGKILL);
@@ -391,8 +390,9 @@ static int zap_threads(struct task_struct *tsk,
 
 	spin_lock_irq(&tsk->sighand->siglock);
 	if (!(signal->flags & SIGNAL_GROUP_EXIT) && !signal->group_exec_task) {
+		/* Allow SIGKILL, see prepare_signal() */
 		signal->core_state = core_state;
-		nr = zap_process(tsk, exit_code);
+		nr = zap_process(signal, exit_code);
 		clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
 		tsk->flags |= PF_DUMPCORE;
 		atomic_set(&core_state->nr_threads, nr);