linux-kernel - Re: [PATCH v6 01/38] minmax: Add in

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Message-ID: <ZMuqLhfsu0tV/tAT@casper.infradead.org>
Date:   Thu, 3 Aug 2023 14:22:54 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Phi Nguyen <phind.uet@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 01/38] minmax: Add in_range() macro

On Thu, Aug 03, 2023 at 09:00:35PM +0800, Phi Nguyen wrote:
> On 8/2/2023 11:13 PM, Matthew Wilcox (Oracle) wrote:
> > +static inline bool in_range64(u64 val, u64 start, u64 len)
> > +{
> > +	return (val - start) < len;
> > +}
> > +
> > +static inline bool in_range32(u32 val, u32 start, u32 len)
> > +{
> > +	return (val - start) < len;
> > +}
> > +
> 
> I think these two functions return wrong result if val is smaller than start
> and len is big enough.

How is it that you stopped reading at exactly the point where I explained
that this is intentional?

+/**
+ * in_range - Determine if a value lies within a range.
+ * @val: Value to test.
+ * @start: First value in range.
+ * @len: Number of values in range.
+ *
+ * This is more efficient than "if (start <= val && val < (start + len))".
+ * It also gives a different answer if @start + @len overflows the size of
+ * the type by a sufficient amount to encompass @val.  Decide for yourself
+ * which behaviour you want, or prove that start + len never overflow.
+ * Do not blindly replace one form with the other.
+ */