linux-kernel - Re: [PATCH] tools/nolibc/string: fix memcmp() implementation

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Message-ID: <20221013065603.GB16609@1wt.eu>
Date:   Thu, 13 Oct 2022 08:56:03 +0200
From:   Willy Tarreau <w@....eu>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tools/nolibc/string: fix memcmp() implementation

Hi Rasmus,

On Mon, Oct 10, 2022 at 01:36:06PM +0200, Rasmus Villemoes wrote:
> The C standard says that memcmp() must treat the buffers as consisting
> of "unsigned chars". If char happens to be unsigned, the casts are ok,
> but then obviously the c1 variable can never contain a negative
> value. And when char is signed, the casts are wrong, and there's still
> a problem with using an 8-bit quantity to hold the difference, because
> that can range from -255 to +255.
> 
> For example, assuming char is signed, comparing two 1-byte buffers,
> one containing 0x00 and another 0x80, the current implementation would
> return -128 for both memcmp(a, b, 1) and memcmp(b, a, 1), whereas one
> of those should of course return something positive.

You're totally right of course, thank you for spotting this one! I'm
queuing it now.

Regards,
Willy