From: Eric W. Biederman <ebiederm@xmission.com>
Date: Mon, 11 Apr 2022 21:17:28 +0000 (-0500)
Subject: fork: Explicity test for idle tasks in copy_thread
X-Git-Tag: v5.19-rc1~30^2~5
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=36cb0e1cda645ee645b85a6ce652cb46a16e14e5;p=thirdparty%2Flinux.git

fork: Explicity test for idle tasks in copy_thread

The architectures ia64 and parisc have special handling for the idle
thread in copy_process.  Add a flag named idle to kernel_clone_args
and use it to explicity test if an idle process is being created.

Fullfill the expectations of the rest of the copy_thread
implemetations and pass a function pointer in .stack from fork_idle().
This makes what is happening in copy_thread better defined, and is
useful to make idle threads less special.

Link: https://lkml.kernel.org/r/20220506141512.516114-3-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---

diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c
index 10d41ded05a5c..8f010ae818bc2 100644
--- a/arch/ia64/kernel/process.c
+++ b/arch/ia64/kernel/process.c
@@ -342,7 +342,7 @@ copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	ia64_drop_fpu(p);	/* don't pick up stale state from a CPU's fph */
 
 	if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) {
-		if (unlikely(!user_stack_base)) {
+		if (unlikely(args->idle)) {
 			/* fork_idle() called us */
 			return 0;
 		}
diff --git a/arch/parisc/kernel/process.c b/arch/parisc/kernel/process.c
index 129c17de45ba5..30a5874ca8458 100644
--- a/arch/parisc/kernel/process.c
+++ b/arch/parisc/kernel/process.c
@@ -224,7 +224,7 @@ copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) {
 		/* kernel thread */
 		memset(cregs, 0, sizeof(struct pt_regs));
-		if (!usp) /* idle thread */
+		if (args->idle) /* idle thread */
 			return 0;
 		/* Must exit via ret_from_kernel_thread in order
 		 * to call schedule_tail()
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index fcdcba231aac9..3d6b99ce54084 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -33,6 +33,7 @@ struct kernel_clone_args {
 	int cgroup;
 	int io_thread;
 	int kthread;
+	int idle;
 	struct cgroup *cgrp;
 	struct css_set *cset;
 };
diff --git a/kernel/fork.c b/kernel/fork.c
index d39a248a8d8db..93d77ee921ff5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2544,12 +2544,21 @@ static inline void init_idle_pids(struct task_struct *idle)
 	}
 }
 
+static int idle_dummy(void *dummy)
+{
+	/* This function is never called */
+	return 0;
+}
+
 struct task_struct * __init fork_idle(int cpu)
 {
 	struct task_struct *task;
 	struct kernel_clone_args args = {
 		.flags		= CLONE_VM,
+		.stack		= (unsigned long)&idle_dummy,
+		.stack_size	= (unsigned long)NULL,
 		.kthread	= 1,
+		.idle		= 1,
 	};
 
 	task = copy_process(&init_struct_pid, 0, cpu_to_node(cpu), &args);