linux-kernel - Re: [RFC PATCH 2/8] thread_info: allow custom in-task thread

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Date:   Fri, 16 Sep 2016 11:33:26 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>, james.morse@....com,
        Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        lorenzo.pieralisi@....com, Andrew Lutomirski <luto@...nel.org>,
        suzuki.poulose@....com,
        Takahiro Akashi <takahiro.akashi@...aro.org>,
        Will Deacon <will.deacon@....com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH 2/8] thread_info: allow custom in-task thread_info

On Thu, Sep 15, 2016 at 11:37:47AM -0700, Andy Lutomirski wrote:
> On Thu, Sep 15, 2016 at 6:49 AM, Mark Rutland <mark.rutland@....com> wrote:
> > Currently, task_struct is defined in <linux/sched.h>, which (indirectly)
> > pulls in a number of low-level arch headers such as <asm/preempt.h>
> > through a number of other headers. Thus, code and structures in these
> > headers can't rely on the definition of task_struct. Some of these
> > headers are necessary for the definition of task_struct, so moving
> > task_struct into its own header is insufficient tio avoid circular
> > includes.
> 
> The flippant answer is to fix the headers, but I tried that myself and
> gave up :(

Agreed; likewise (though I gave up quicker, I suspect). :(

Longer-term I'd still hope that we can do this.

> But how about this slightly less duplicative alternative:
> 
> struct thread_info {
> #ifdef arch_thread_info
>   struct arch_thread_info arch_ti;
> #endif
> };

I'm happy to have an arch_thread_info.

Just to check, what do you mean to happen with the flags field? Should
that always be in the generic thread_info? e.g.

struct thread_info {
	u32 flags;
#ifdef arch_thread_info
	struct arch_thread_info arch_ti;
#endif
};

Thanks,
Mark,