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From: Brian Gerst <brgerst@gmail.com>
Date: Sun, 19 Jun 2016 18:01:29 -0400
Message-ID: <CAMzpN2jVhF1tx3TistjgRBa4AtRETwhiVWGLU90eaEKYLXoDTQ@mail.gmail.com>
Subject: Re: [PATCH v2 5/6] x86: Pass kernel thread parameters in fork_frame
To: Andy Lutomirski <luto@amacapital.net>
Cc: X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Denys Vlasenko <dvlasenk@redhat.com>, Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>
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On Sun, Jun 19, 2016 at 5:28 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Sat, Jun 18, 2016 at 1:56 PM, Brian Gerst <brgerst@gmail.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@gmail.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