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

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Date:	Sun, 19 Jun 2016 18:01:29 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
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 Sun, Jun 19, 2016 at 5:28 PM, Andy Lutomirski <luto@...capital.net> wrote:
> 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

Yes, do_execve() should be setting the full pt_regs.

--
Brian Gerst