linux-kernel - Re: [PATCH 1/2] bitmap: new bitmap_copy_safe and bitmap_{from,to}

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Date:   Tue, 9 Jan 2018 00:29:23 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Yury Norov <ynorov@...iumnetworks.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        David Decotigny <decot@...glers.com>,
        "David S . Miller" <davem@...emloft.net>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 1/2] bitmap: new bitmap_copy_safe and bitmap_{from,to}_arr32

On 28 December 2017 at 16:00, Yury Norov <ynorov@...iumnetworks.com> wrote:
>
> In this patch, bitmap_copy_safe and bitmap_{from,to}_arr32 are introduced.
>
> 'Safe' in bitmap_copy_safe() stands for clearing unused bits in bitmap
> beyond last bit till the end of last word. It is useful for hardening
> API when bitmap is assumed to be exposed to userspace.

I agree completely with getting rid of the complexity of the u32array
functions, and also think they should simply be implemented as a
memcpy() when possible.

I'm not a fan of the _safe suffix, though. It doesn't say what it's
safe from. For example, one possible interpretation is that it allows
src or dst to be NULL (becoming a noop in such a case). Why not say
what it does? _clear_tail, _clear_rest, something like that. Or maybe,
can we simply make bitmap_copy behave that way? Hm, probably not, a
bit too many users to check they'd all be ok with that.

Rasmus