linux-kernel - Re: [PATCH v2 5/6] x86: Pass kernel thread parameters in fork

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Message-ID: <CALCETrXZyyLzHDZSO8e137WjNhHk7951Cetm6vKSmOoQusLG=g@mail.gmail.com>
Date:	Sun, 19 Jun 2016 14:28:17 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 5/6] x86: Pass kernel thread parameters in fork_frame

On Sat, Jun 18, 2016 at 1:56 PM, Brian Gerst <brgerst@...il.com> wrote:
> Instead of setting up a fake pt_regs context, put the kernel thread
> function pointer and arg into the unused callee-restored registers
> of struct fork_frame.

This seems generally okay.

>
> Signed-off-by: Brian Gerst <brgerst@...il.com>

> @@ -146,19 +147,12 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,


>         if (unlikely(p->flags & PF_KTHREAD)) {
>                 /* kernel thread */
>                 memset(childregs, 0, sizeof(struct pt_regs));
> -               frame->ret_addr = (unsigned long) ret_from_kernel_thread;
> -               task_user_gs(p) = __KERNEL_STACK_CANARY;
> -               childregs->ds = __USER_DS;
> -               childregs->es = __USER_DS;
> -               childregs->fs = __KERNEL_PERCPU;

Is the idea that do_execve promises to initialize all these fields to
something sensible if the kernel thread in question tries to return to
user mode?

--Andy