powerpc: differentiate kthread from user kernel thread start

Kernel created user threads start similarly to kernel threads in that
they call a kernel function after first returning from _switch, so
they share ret_from_kernel_thread for this. Kernel threads never return
from that function though, whereas user threads often do (although some
don't, e.g., IO threads).

Split these startup functions in two, and catch kernel threads that
improperly return from their function. This is intended to make the
complicated code a little bit easier to understand.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230325122904.2375060-7-npiggin@gmail.com

arch/powerpc/kernel/entry_32.S

@@ -186,8 +186,8 @@ ret_from_fork:
 	li	r3,0	/* fork() return value */
 	b	ret_from_syscall
 
-	.globl	ret_from_kernel_thread
-ret_from_kernel_thread:
+	.globl	ret_from_kernel_user_thread
+ret_from_kernel_user_thread:
 	bl	schedule_tail
 	mtctr	r14
 	mr	r3,r15
@@ -196,6 +196,22 @@ ret_from_kernel_thread:
 	li	r3,0
 	b	ret_from_syscall
 
+	.globl	start_kernel_thread
+start_kernel_thread:
+	bl	schedule_tail
+	mtctr	r14
+	mr	r3,r15
+	PPC440EP_ERR42
+	bctrl
+	/*
+	 * This must not return. We actually want to BUG here, not WARN,
+	 * because BUG will exit the process which is what the kernel thread
+	 * should have done, which may give some hope of continuing.
+	 */
+100:	trap
+	EMIT_BUG_ENTRY 100b,__FILE__,__LINE__,0
+
+
 /*
  * This routine switches between two different tasks.  The process
  * state of one is saved on its kernel stack.  Then the state

arch/powerpc/kernel/interrupt_64.S

@@ -739,7 +739,7 @@ _GLOBAL(ret_from_fork)
 	li	r3,0	/* fork() return value */
 	b	.Lsyscall_exit
 
-_GLOBAL(ret_from_kernel_thread)
+_GLOBAL(ret_from_kernel_user_thread)
 	bl	schedule_tail
 	mtctr	r14
 	mr	r3,r15
@@ -749,3 +749,19 @@ _GLOBAL(ret_from_kernel_thread)
 	bctrl
 	li	r3,0
 	b	.Lsyscall_exit
+
+_GLOBAL(start_kernel_thread)
+	bl	schedule_tail
+	mtctr	r14
+	mr	r3,r15
+#ifdef CONFIG_PPC64_ELF_ABI_V2
+	mr	r12,r14
+#endif
+	bctrl
+	/*
+	 * This must not return. We actually want to BUG here, not WARN,
+	 * because BUG will exit the process which is what the kernel thread
+	 * should have done, which may give some hope of continuing.
+	 */
+100:	trap
+	EMIT_BUG_ENTRY 100b,__FILE__,__LINE__,0

arch/powerpc/kernel/process.c

@@ -1741,7 +1741,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	struct pt_regs *kregs; /* Switch frame regs */
 	extern void ret_from_fork(void);
 	extern void ret_from_fork_scv(void);
-	extern void ret_from_kernel_thread(void);
+	extern void ret_from_kernel_user_thread(void);
+	extern void start_kernel_thread(void);
 	void (*f)(void);
 	unsigned long sp = (unsigned long)task_stack_page(p) + THREAD_SIZE;
 	struct thread_info *ti = task_thread_info(p);
@@ -1758,7 +1759,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		sp -= STACK_FRAME_MIN_SIZE;
 		((unsigned long *)sp)[0] = 0;
 
-		f = ret_from_kernel_thread;
+		f = start_kernel_thread;
 		p->thread.regs = NULL;	/* no user register state */
 		clear_tsk_compat_task(p);
 	} else {
@@ -1784,7 +1785,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 			childregs->softe = IRQS_ENABLED;
 #endif
 			ti->flags |= _TIF_RESTOREALL;
-			f = ret_from_kernel_thread;
+			f = ret_from_kernel_user_thread;
 		} else {
 			struct pt_regs *regs = current_pt_regs();
 			unsigned long clone_flags = args->flags;