fork: Generalize PF_IO_WORKER handling

Add fn and fn_arg members into struct kernel_clone_args and test for
them in copy_thread (instead of testing for PF_KTHREAD | PF_IO_WORKER).
This allows any task that wants to be a user space task that only runs
in kernel mode to use this functionality.

The code on x86 is an exception and still retains a PF_KTHREAD test
because x86 unlikely everything else handles kthreads slightly
differently than user space tasks that start with a function.

The functions that created tasks that start with a function
have been updated to set ".fn" and ".fn_arg" instead of
".stack" and ".stack_size".  These functions are fork_idle(),
create_io_thread(), kernel_thread(), and user_mode_thread().

Link: https://lkml.kernel.org/r/20220506141512.516114-4-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

4 files changed, 13 insertions, 14 deletions

diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h
index 99a8820e8cc4..b2486b2cbc6e 100644
--- a/arch/x86/include/asm/fpu/sched.h
+++ b/arch/x86/include/asm/fpu/sched.h
@@ -11,7 +11,7 @@
 
 extern void save_fpregs_to_fpstate(struct fpu *fpu);
 extern void fpu__drop(struct fpu *fpu);
-extern int  fpu_clone(struct task_struct *dst, unsigned long clone_flags);
+extern int  fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal);
 extern void fpu_flush_thread(void);
 
 /*
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index b5f0d2ff47e4..c08eb0fdd11f 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -78,13 +78,13 @@ static inline void update_task_stack(struct task_struct *task)
 }
 
 static inline void kthread_frame_init(struct inactive_task_frame *frame,
-				      unsigned long fun, unsigned long arg)
+				      int (*fun)(void *), void *arg)
 {
-	frame->bx = fun;
+	frame->bx = (unsigned long)fun;
 #ifdef CONFIG_X86_32
-	frame->di = arg;
+	frame->di = (unsigned long)arg;
 #else
-	frame->r12 = arg;
+	frame->r12 = (unsigned long)arg;
 #endif
 }
 
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index c049561f373a..fbade5a3975b 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -556,7 +556,7 @@ static inline void fpu_inherit_perms(struct fpu *dst_fpu)
 }
 
 /* Clone current's FPU state on fork */
-int fpu_clone(struct task_struct *dst, unsigned long clone_flags)
+int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal)
 {
 	struct fpu *src_fpu = &current->thread.fpu;
 	struct fpu *dst_fpu = &dst->thread.fpu;
@@ -579,7 +579,7 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags)
 	 * No FPU state inheritance for kernel threads and IO
 	 * worker threads.
 	 */
-	if (dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
+	if (minimal) {
 		/* Clear out the minimal state */
 		memcpy(&dst_fpu->fpstate->regs, &init_fpstate.regs,
 		       init_fpstate_copy_size());
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 0fce52b10dc4..d20eaad52a85 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -134,7 +134,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 {
 	unsigned long clone_flags = args->flags;
 	unsigned long sp = args->stack;
-	unsigned long arg = args->stack_size;
 	unsigned long tls = args->tls;
 	struct inactive_task_frame *frame;
 	struct fork_frame *fork_frame;
@@ -172,13 +171,13 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	frame->flags = X86_EFLAGS_FIXED;
 #endif
 
-	fpu_clone(p, clone_flags);
+	fpu_clone(p, clone_flags, args->fn);
 
 	/* Kernel thread ? */
 	if (unlikely(p->flags & PF_KTHREAD)) {
 		p->thread.pkru = pkru_get_init_value();
 		memset(childregs, 0, sizeof(struct pt_regs));
-		kthread_frame_init(frame, sp, arg);
+		kthread_frame_init(frame, args->fn, args->fn_arg);
 		return 0;
 	}
 
@@ -198,10 +197,10 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	task_user_gs(p) = get_user_gs(current_pt_regs());
 #endif
 
-	if (unlikely(p->flags & PF_IO_WORKER)) {
+	if (unlikely(args->fn)) {
 		/*
-		 * An IO thread is a user space thread, but it doesn't
-		 * return to ret_after_fork().
+		 * A user space thread, but it doesn't return to
+		 * ret_after_fork().
 		 *
 		 * In order to indicate that to tools like gdb,
 		 * we reset the stack and instruction pointers.
@@ -211,7 +210,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		 */
 		childregs->sp = 0;
 		childregs->ip = 0;
-		kthread_frame_init(frame, sp, arg);
+		kthread_frame_init(frame, args->fn, args->fn_arg);
 		return 0;
 	}