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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 13 May 2020 19:43:24 -0700
Message-ID: <CAHk-=wjP8ysEZnNFi_+E1ZEFGpcbAN8kbYHrCnC93TX6XX+jEQ@mail.gmail.com>
Subject: Re: [PATCH 11/18] maccess: remove strncpy_from_unsafe
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Daniel Borkmann <daniel@iogearbox.net>,
        Christoph Hellwig <hch@lst.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-parisc@vger.kernel.org,
        linux-um <linux-um@lists.infradead.org>,
        Netdev <netdev@vger.kernel.org>, bpf@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, May 13, 2020 at 6:00 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> > But we should likely at least disallow it entirely on platforms where
> > we really can't - or pick one hardcoded choice. On sparc, you really
> > _have_ to specify one or the other.
>
> OK. BTW, is there any way to detect the kernel/user space overlap on
> memory layout statically? If there, I can do it. (I don't like
> "if (CONFIG_X86)" thing....)
> Or, maybe we need CONFIG_ARCH_OVERLAP_ADDRESS_SPACE?

I think it would be better to have a CONFIG variable that
architectures can just 'select' to show that they are ok with separate
kernel and user addresses.

Because I don't think we have any way to say that right now as-is. You
can probably come up with hacky ways to approximate it, ie something
like

    if (TASK_SIZE_MAX > PAGE_OFFSET)
        .... they overlap ..

which would almost work, but..

                 Linus