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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 14 Jul 2020 08:27:14 -0700
Message-ID: <CAHk-=wgpYFEPMfYmNR-9SuPaeC432sC7nmZrismRHQEiR2GM4g@mail.gmail.com>
Subject: Re: [PATCH 4/6] uaccess: remove segment_eq
To:     Christoph Hellwig <hch@lst.de>
Cc:     Nick Hu <nickhu@andestech.com>, Greentime Hu <green.hu@gmail.com>,
        Vincent Chen <deanbo422@gmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-riscv@lists.infradead.org,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>
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Ack, just with a note:

On Tue, Jul 14, 2020 at 4:06 AM Christoph Hellwig <hch@lst.de> wrote:
>
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -33,7 +33,7 @@ static inline void set_fs(mm_segment_t fs)
>         set_thread_flag(TIF_FSCHECK);
>  }
>
> -#define segment_eq(a, b)       ((a).seg == (b).seg)
> +#define uaccess_kernel() (get_fs().seg == KERNEL_DS.seg)
>  #define user_addr_max() (current->thread.addr_limit.seg)

This "uaccess_kernel()" interface is a better model anyway, because at
least on x86 (and from a quick glance at others), we might avoid the
exact equality comparison, and instead do simpler/better things.

On x86-64, for example, checking whether the limit has the high bit
set is not only more flexible and correct, it's much cheaper too.

Of course, trying to get rid of all this means that it doesn't matter
so much, but it would probably have been good to do this part years
ago regardless.

                  Linus