powerpc: copy_thread differentiate kthreads and user mode threads

When copy_thread is given a kernel function to run in arg->fn, this
does not necessarily mean it is a kernel thread. User threads can be
created this way (e.g., kernel_init, see also x86's copy_thread()).
These threads run a kernel function which may call kernel_execve()
and return, which returns like a userspace exec(2) syscall.

Kernel threads are to  be differentiated with PF_KTHREAD, will always
have arg->fn set, and should never return from that function, instead
calling kthread_exit() to exit.

Create separate paths for the kthread and user kernel thread creation
logic. The kthread path will never exit and does not require a user
interrupt frame, so it gets a minimal stack frame.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230325122904.2375060-6-npiggin@gmail.com

diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
index cf13d84..bf5dde1 100644 (file)
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -176,9 +176,11 @@ static inline bool test_thread_local_flags(unsigned int flags)
 #ifdef CONFIG_COMPAT
 #define is_32bit_task()        (test_thread_flag(TIF_32BIT))
 #define is_tsk_32bit_task(tsk) (test_tsk_thread_flag(tsk, TIF_32BIT))
+#define clear_tsk_compat_task(tsk) (clear_tsk_thread_flag(p, TIF_32BIT))
 #else
 #define is_32bit_task()        (IS_ENABLED(CONFIG_PPC32))
 #define is_tsk_32bit_task(tsk) (IS_ENABLED(CONFIG_PPC32))
+#define clear_tsk_compat_task(tsk) do { } while (0)
 #endif
 
 #if defined(CONFIG_PPC64)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 3b34bd9..88898ca 100644 (file)
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1738,10 +1738,7 @@ static void setup_ksp_vsid(struct task_struct *p, unsigned long sp)
  */
 int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 {
-       unsigned long clone_flags = args->flags;
-       unsigned long usp = args->stack;
-       unsigned long tls = args->tls;
-       struct pt_regs *childregs, *kregs;
+       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);
@@ -1754,45 +1751,76 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 
        klp_init_thread_info(p);
 
-       /* Create initial stack frame. */
-       sp -= STACK_USER_INT_FRAME_SIZE;
-       *(unsigned long *)(sp + STACK_INT_FRAME_MARKER) = STACK_FRAME_REGS_MARKER;
-
-       /* Copy registers */
-       childregs = (struct pt_regs *)(sp + STACK_INT_FRAME_REGS);
-       if (unlikely(args->fn)) {
+       if (unlikely(p->flags & PF_KTHREAD)) {
                /* kernel thread */
+
+               /* Create initial minimum stack frame. */
+               sp -= STACK_FRAME_MIN_SIZE;
                ((unsigned long *)sp)[0] = 0;
-               memset(childregs, 0, sizeof(struct pt_regs));
-               childregs->gpr[1] = sp + STACK_USER_INT_FRAME_SIZE;
-#ifdef CONFIG_PPC64
-               clear_tsk_thread_flag(p, TIF_32BIT);
-               childregs->softe = IRQS_ENABLED;
-#endif
-               p->thread.regs = NULL;  /* no user register state */
-               ti->flags |= _TIF_RESTOREALL;
+
                f = ret_from_kernel_thread;
+               p->thread.regs = NULL;  /* no user register state */
+               clear_tsk_compat_task(p);
        } else {
                /* user thread */
-               struct pt_regs *regs = current_pt_regs();
-               *childregs = *regs;
-               if (usp)
-                       childregs->gpr[1] = usp;
-               ((unsigned long *)sp)[0] = childregs->gpr[1];
-               p->thread.regs = childregs;
-               if (clone_flags & CLONE_SETTLS) {
-                       if (!is_32bit_task())
-                               childregs->gpr[13] = tls;
+               struct pt_regs *childregs;
+
+               /* Create initial user return stack frame. */
+               sp -= STACK_USER_INT_FRAME_SIZE;
+               *(unsigned long *)(sp + STACK_INT_FRAME_MARKER) = STACK_FRAME_REGS_MARKER;
+
+               childregs = (struct pt_regs *)(sp + STACK_INT_FRAME_REGS);
+
+               if (unlikely(args->fn)) {
+                       /*
+                        * A user space thread, but it first runs a kernel
+                        * thread, and then returns as though it had called
+                        * execve rather than fork, so user regs will be
+                        * filled in (e.g., by kernel_execve()).
+                        */
+                       ((unsigned long *)sp)[0] = 0;
+                       memset(childregs, 0, sizeof(struct pt_regs));
+#ifdef CONFIG_PPC64
+                       childregs->softe = IRQS_ENABLED;
+#endif
+                       ti->flags |= _TIF_RESTOREALL;
+                       f = ret_from_kernel_thread;
+               } else {
+                       struct pt_regs *regs = current_pt_regs();
+                       unsigned long clone_flags = args->flags;
+                       unsigned long usp = args->stack;
+
+                       /* Copy registers */
+                       *childregs = *regs;
+                       if (usp)
+                               childregs->gpr[1] = usp;
+                       ((unsigned long *)sp)[0] = childregs->gpr[1];
+#ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
+                       WARN_ON_ONCE(childregs->softe != IRQS_ENABLED);
+#endif
+                       if (clone_flags & CLONE_SETTLS) {
+                               unsigned long tls = args->tls;
+
+                               if (!is_32bit_task())
+                                       childregs->gpr[13] = tls;
+                               else
+                                       childregs->gpr[2] = tls;
+                       }
+
+                       if (trap_is_scv(regs))
+                               f = ret_from_fork_scv;
                        else
-                               childregs->gpr[2] = tls;
+                               f = ret_from_fork;
                }
 
-               if (trap_is_scv(regs))
-                       f = ret_from_fork_scv;
-               else
-                       f = ret_from_fork;
+#ifdef CONFIG_PPC64
+               if (cpu_has_feature(CPU_FTR_HAS_PPR))
+                       childregs->ppr = DEFAULT_PPR;
+#endif
+
+               childregs->msr &= ~(MSR_FP|MSR_VEC|MSR_VSX);
+               p->thread.regs = childregs;
        }
-       childregs->msr &= ~(MSR_FP|MSR_VEC|MSR_VSX);
 
        /*
         * The way this works is that at some point in the future
@@ -1843,8 +1871,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
                p->thread.dscr_inherit = current->thread.dscr_inherit;
                p->thread.dscr = mfspr(SPRN_DSCR);
        }
-       if (cpu_has_feature(CPU_FTR_HAS_PPR))
-               childregs->ppr = DEFAULT_PPR;
 
        p->thread.tidr = 0;
 #endif