Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 20, 2023 at 07:22:08AM +0200, Peter Zijlstra wrote:

> > I'm not sure that would be worth the trouble / cleverness.  The hint is
> > straightforward IMO.
> 
> I tried, it doesn't work, clearly I missed something.

FWIW, I tried the below. That should make user_mode() true for the
kernel thread regset, and while the kernel did boot, it still fails the
livepatch self-test.

The difference seems to be that END_OF_STACK terminates it right there,
while REGS thinks its a valid frame and only terminates on user_mode()
when unwinding one more frame. The frame at REGS clearly isn't very
sane.


diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 72015dba72ab..45a400b16b80 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -232,6 +232,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		 * It does the same kernel frame setup to return to a kernel
 		 * function that a kernel thread does.
 		 */
+		childregs->cs = 3;
 		childregs->sp = 0;
 		childregs->ip = 0;
 		kthread_frame_init(frame, args->fn, args->fn_arg);