[PATCH 4/4] x86, 64-bit: Use user_mode() to determine new stack pointer in copy_thread()

[Brian Gerst <brgerst@gmail.com>]

Use user_mode() instead of a magic value for sp to determine when returning
to kernel mode.  This simplifies kernel_thread().

Signed-off-by: Brian Gerst <brgerst@gmail.com>
---
 arch/x86/kernel/process.c    |    6 +-----
 arch/x86/kernel/process_64.c |    5 +++--
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 71a22c9..4ebe3e4 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -255,7 +255,6 @@ extern void kernel_thread_helper(void);
 int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 {
 	struct pt_regs regs;
-	unsigned long newsp;
 
 	memset(&regs, 0, sizeof(regs));
 
@@ -267,9 +266,6 @@ int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 	regs.es = __USER_DS;
 	regs.fs = __KERNEL_PERCPU;
 	regs.gs = __KERNEL_STACK_CANARY;
-	newsp = 0;
-#else
-	newsp = ~0UL;
 #endif
 
 	regs.orig_ax = -1;
@@ -278,7 +274,7 @@ int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 	regs.flags = X86_EFLAGS_IF | 2;
 
 	/* Ok, create the new process.. */
-	return do_fork(flags | CLONE_VM | CLONE_UNTRACED, newsp, &regs, 0, NULL, NULL);
+	return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);
 }
 EXPORT_SYMBOL(kernel_thread);
 
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 8151a82..a9be0d5 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -283,8 +283,9 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 	*childregs = *regs;
 
 	childregs->ax = 0;
-	childregs->sp = sp;
-	if (sp == ~0UL)
+	if (user_mode(regs))
+		childregs->sp = sp;
+	else
 		childregs->sp = (unsigned long)childregs;
 
 	p->thread.sp = (unsigned long) childregs;
-- 
1.6.2.5