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Date:   Thu, 14 May 2020 12:27:42 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-parisc@...r.kernel.org,
        linux-um <linux-um@...ts.infradead.org>,
        Netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/18] maccess: remove strncpy_from_unsafe

On 5/14/20 11:44 AM, Masami Hiramatsu wrote:
> On Wed, 13 May 2020 19:43:24 -0700
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> On Wed, May 13, 2020 at 6:00 PM Masami Hiramatsu <mhiramat@...nel.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..
> 
> It seems TASK_SIZE_MAX is defined only on x86 and s390, what about
> comparing STACK_TOP_MAX with PAGE_OFFSET ?
> Anyway, I agree that the best way is introducing a CONFIG.

Agree, CONFIG knob that archs can select feels cleanest. Fwiw, I've cooked
up fixes for bpf side locally here and finishing up testing, will push out
later today.

Thanks,
Daniel

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