linux-kernel - Re: [PATCH 4/4] x86: Pass kernel thread parameters in fork

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Date:	Mon, 23 May 2016 10:23:35 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Brian Gerst <brgerst@...il.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/4] x86: Pass kernel thread parameters in fork_frame

On Sat, May 21, 2016 at 12:04:51PM -0400, Brian Gerst wrote:
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -405,37 +405,29 @@ END(__switch_to_asm)
>   * A newly forked process directly context switches into this address.
>   *
>   * rax: prev task we switched from
> + * rbx: kernel thread func
> + * r12: kernel thread arg
>   */
>  ENTRY(ret_from_fork)
>  	movq	%rax, %rdi
>  	call	schedule_tail			/* rdi: 'prev' task parameter */
>  
> -	testb	$3, CS(%rsp)			/* from kernel_thread? */
> +	testq	%rbx, %rbx			/* from kernel_thread? */
>  	jnz	1f
>  
> -	/*
> -	 * We came from kernel_thread.  This code path is quite twisted, and
> -	 * someone should clean it up.
> -	 *
> -	 * copy_thread_tls stashes the function pointer in RBX and the
> -	 * parameter to be passed in RBP.  The called function is permitted
> -	 * to call do_execve and thereby jump to user mode.
> -	 */
> -	movq	RBP(%rsp), %rdi
> -	call	*RBX(%rsp)
> -	movq	%rax, RAX(%rsp)
> -
> -	/*
> -	 * Fall through as though we're exiting a syscall.  This makes a
> -	 * twisted sort of sense if we just called do_execve.
> -	 */
> -
> -1:
> +2:
>  	movq	%rsp, %rdi
>  	call	syscall_return_slowpath	/* returns with IRQs disabled */
>  	TRACE_IRQS_ON			/* user mode is traced as IRQS on */
>  	SWAPGS
>  	jmp	restore_regs_and_iret
> +
> +1:
> +	/* kernel thread */
> +	movq	%r12, %rdi
> +	call	*%rbx
> +	movq	%rax, RAX(%rsp)
> +	jmp	2b
>  END(ret_from_fork)

It seems really surprising that a kernel thread would be returning to
user space.  It would probably be a good idea to preserve the existing
comments about that.

-- 
Josh