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

[Petr Mladek]

On Wed 2023-07-19 22:15:38, Peter Zijlstra wrote:
> On Wed, Jul 19, 2023 at 10:02:22PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 19, 2023 at 05:21:11PM +0200, Petr Mladek wrote:
> > 
> > > This patch broke livepatching. Kthreads never have a reliable stack.
> > > It works when I revert it.
> > 
> > > > +SYM_CODE_START(ret_from_fork_asm)
> > > > +	UNWIND_HINT_REGS
> > 
> > It works again when I change the above hint to UNWIND_HINT_END_OF_STACK,
> > so yeah. Doing this makes objtool unhappy with something else though,
> > so I'll go prod at things with something sharp...
> 
> 
> The below cures things; Josh, did I miss anything?

I can confirm that it solved the problem. Feel free to use:

Tested-by: Petr Mladek <pmladek@xxxxxxxx>

Thanks a lot for the quick fix.

Best Regards,
Petr

> ---
>  arch/x86/entry/entry_64.S | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 91f6818884fa..cfe7882ea9ae 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -285,7 +285,14 @@ SYM_FUNC_END(__switch_to_asm)
>   */
>  .pushsection .text, "ax"
>  SYM_CODE_START(ret_from_fork_asm)
> -	UNWIND_HINT_REGS
> +	/*
> +	 * This is the start of the kernel stack; even through there's a regs
> +	 * set at the top, there is no real exception frame and one cannot
> +	 * unwind further. This is the end.
> +	 *
> +	 * This ensures stack unwinds of kernel threads hit a known good state.
> +	 */
> +	UNWIND_HINT_END_OF_STACK
>  	ANNOTATE_NOENDBR // copy_thread
>  	CALL_DEPTH_ACCOUNT
>  
> @@ -295,6 +302,11 @@ SYM_CODE_START(ret_from_fork_asm)
>  	movq	%r12, %rcx		/* fn_arg */
>  	call	ret_from_fork
>  
> +	/*
> +	 * Set the stack state to what is expected for the target function
> +	 * -- also it is not wrong.
> +	 */
> +	UNWIND_HINT_REGS
>  	jmp	swapgs_restore_regs_and_return_to_usermode
>  SYM_CODE_END(ret_from_fork_asm)
>  .popsection