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

[Petr Mladek]

On Fri 2023-06-23 18:55:29, Brian Gerst wrote:
> When kCFI is enabled, special handling is needed for the indirect call
> to the kernel thread function.  Rewrite the ret_from_fork() function in
> C so that the compiler can properly handle the indirect call.

This patch broke livepatching. Kthreads never have a reliable stack.
It works when I revert it.

See also below.

> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -284,36 +284,19 @@ SYM_FUNC_END(__switch_to_asm)
>   * r12: kernel thread arg
>   */
>  .pushsection .text, "ax"
> -	__FUNC_ALIGN
> -SYM_CODE_START_NOALIGN(ret_from_fork)
> -	UNWIND_HINT_END_OF_STACK
> +SYM_CODE_START(ret_from_fork_asm)
> +	UNWIND_HINT_REGS
>  	ANNOTATE_NOENDBR // copy_thread
>  	CALL_DEPTH_ACCOUNT
> -	movq	%rax, %rdi
> -	call	schedule_tail			/* rdi: 'prev' task parameter */
>  
> -	testq	%rbx, %rbx			/* from kernel_thread? */
> -	jnz	1f				/* kernel threads are uncommon */
> +	movq	%rax, %rdi		/* prev */
> +	movq	%rsp, %rsi		/* regs */
> +	movq	%rbx, %rdx		/* fn */
> +	movq	%r12, %rcx		/* fn_arg */
> +	call	ret_from_fork
>  
> -2:
> -	UNWIND_HINT_REGS
> -	movq	%rsp, %rdi
> -	call	syscall_exit_to_user_mode	/* returns with IRQs disabled */
>  	jmp	swapgs_restore_regs_and_return_to_usermode
> -
> -1:
> -	/* kernel thread */
> -	UNWIND_HINT_END_OF_STACK

I think that it might be related to removal of this line.
The following intructions are going to call fn(fn_arg).
See below.

> -	movq	%r12, %rdi
> -	CALL_NOSPEC rbx
> -	/*
> -	 * A kernel thread is allowed to return here after successfully
> -	 * calling kernel_execve().  Exit to userspace to complete the execve()
> -	 * syscall.
> -	 */
> -	movq	$0, RAX(%rsp)
> -	jmp	2b
> -SYM_CODE_END(ret_from_fork)
> +SYM_CODE_END(ret_from_fork_asm)
>  .popsection
>  
>  .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index 5c91305d09d2..f42dbf17f52b 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -12,7 +12,9 @@ struct task_struct *__switch_to_asm(struct task_struct *prev,
>  __visible struct task_struct *__switch_to(struct task_struct *prev,
>  					  struct task_struct *next);
>  
> -asmlinkage void ret_from_fork(void);
> +asmlinkage void ret_from_fork_asm(void);
> +__visible void ret_from_fork(struct task_struct *prev, struct pt_regs *regs,
> +			     int (*fn)(void *), void *fn_arg);
>  
>  /*
>   * This is the structure pointed to by thread.sp for an inactive task.  The
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index cc7a642f8c9d..001e6dad9a48 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -136,6 +137,25 @@ static int set_new_tls(struct task_struct *p, unsigned long tls)
>  		return do_set_thread_area_64(p, ARCH_SET_FS, tls);
>  }
>  
> +__visible noinstr void ret_from_fork(struct task_struct *prev, struct pt_regs *regs,
> +				     int (*fn)(void *), void *fn_arg)
> +{
> +	schedule_tail(prev);
> +
> +	/* Is this a kernel thread? */
> +	if (unlikely(fn)) {
> +		fn(fn_arg);

This is the related code but it does not include the annotation
about the end of the stack.

Honestly, I am not familiar with the stack unwinder and how this is
supposed to work.

I hope that Josh or anyone else might know better.

> +		/*
> +		 * A kernel thread is allowed to return here after successfully
> +		 * calling kernel_execve().  Exit to userspace to complete the
> +		 * execve() syscall.
> +		 */
> +		regs->ax = 0;
> +	}
> +
> +	syscall_exit_to_user_mode(regs);
> +}
> +
>  int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>  {
>  	unsigned long clone_flags = args->flags;

Best Regards,
Petr