[PATCH] x86: Terminate the kernel stacks for the unwinder

Always make sure RIP/EIP is 0 in the registers stored on the top
of the stack of a kernel thread. This makes sure the unwinder code
won't try a fallback but knows the stack has ended.

AK: this patch is a bit mysterious. in theory they should be terminated
anyways, but it seems to fix at least one crash. Anyways double termination
probably doesn't hurt.
Signed-off-by: Andi Kleen <ak@suse.de>

arch/i386/kernel/process.c

@@ -328,6 +328,7 @@ extern void kernel_thread_helper(void);
 int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
 {
 	struct pt_regs regs;
+	int err;
 
 	memset(&regs, 0, sizeof(regs));
 
@@ -342,7 +343,10 @@ int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
 	regs.eflags = X86_EFLAGS_IF | X86_EFLAGS_SF | X86_EFLAGS_PF | 0x2;
 
 	/* Ok, create the new process.. */
-	return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);
+	err = do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);
+	if (err == 0) /* terminate kernel stack */
+		task_pt_regs(current)->eip = 0;
+	return err;
 }
 EXPORT_SYMBOL(kernel_thread);
 

arch/x86_64/kernel/entry.S

@@ -978,6 +978,11 @@ ENTRY(kernel_thread)
 	call do_fork
 	movq %rax,RAX(%rsp)
 	xorl %edi,%edi
+	test %rax,%rax
+	jnz  1f
+	/* terminate stack in child */
+	movq %rdi,RIP(%rsp)
+1:
 
 	/*
 	 * It isn't worth to check for reschedule here,