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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Thu, 10 Feb 2022 15:53:52 +0100
Message-ID: <CAK8P3a1v37=1Knrctnvqrb-igMsAvwPySFkzYfFvtLrb3DOzHQ@mail.gmail.com>
Subject: Re: [PATCH] microblaze: remove CONFIG_SET_FS
To:     David Laight <David.Laight@aculab.com>
Cc:     Michal Simek <monstr@monstr.eu>,
        Christoph Hellwig <hch@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, Feb 10, 2022 at 3:21 PM David Laight <David.Laight@aculab.com> wrote:
> From: Arnd Bergmann Sent: 10 February 2022 13:30
>
> > static inline int __range_ok(unsigned long addr, unsigned long size)
> > {
> >         return size <= TASK_SIZE && addr <= (TASK_SIZE - size);
> > }
> >
> > since 'size' is usually constant, so this turns into a single comparison
> > against a compile-time constant.
>
> Hmmm... maybe there should be a comment that it is the same as
> the more obvious:
>         (addr <= TASK_SIZE && addr <= TASK_SIZE - size)
> but is better for constant size.
> (Provided TASK_SIZE is a constant.)
>
> I'm sure Linus was 'unhappy' about checking against 2^63 for
> 32bit processes on a 64bit kernel.
>
> Hmmm compat code that has 32bit addr/size needn't even call
> access_ok() - it can never access kernel memory at all.

I suppose the generic function should compare against
TASK_SIZE_MAX or user_addr_max() then, to make it a
constant while TASK_SIZE potentially depends on compat
mode.

        Arnd