linux-kernel - Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from

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Date:   Tue, 14 Jan 2020 21:42:15 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user

On Tue, Jan 14, 2020 at 9:08 PM Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:

> diff --git a/arch/arc/include/asm/word-at-a-time.h b/arch/arc/include/asm/word-at-a-time.h
> new file mode 100644
> index 000000000000..00e92be70987
> --- /dev/null
> +++ b/arch/arc/include/asm/word-at-a-time.h
> @@ -0,0 +1,49 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2020 Synopsys Inc.
> + */
> +#ifndef __ASM_ARC_WORD_AT_A_TIME_H
> +#define __ASM_ARC_WORD_AT_A_TIME_H
> +
> +#ifdef __LITTLE_ENDIAN__
> +
> +#include <linux/kernel.h>
> +
> +struct word_at_a_time {
> +       const unsigned long one_bits, high_bits;
> +};

What's wrong with the generic version on little-endian? Any
chance you can find a way to make it work as well for you as
this copy?

> +static inline unsigned long find_zero(unsigned long mask)
> +{
> +#ifdef CONFIG_64BIT
> +       return fls64(mask) >> 3;
> +#else
> +       return fls(mask) >> 3;
> +#endif

The CONFIG_64BIT check not be needed, unless you are adding
support for 64-bit ARC really soon.

       Arnd