linux-kernel - Re: [PATCH] string.c: test *cmp for all possible 1-character strings

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Date:   Thu, 22 Dec 2022 16:15:52 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] string.c: test *cmp for all possible 1-character strings

On Thu, Dec 22, 2022 at 03:05:06PM +0100, Rasmus Villemoes wrote:
> The switch to -funsigned-char made a pre-existing bug on m68k more
> apparent. That is now fixed (by removing m68k's private strcmp(), see
> commit 7c0846125358), but we still have quite a few architectures that
> provide one or more of strcmp(), strncmp() and memcmp().
> 
> They probably all work fine for the cases where the input is all
> ASCII, and/or where the caller only wants to know about equality or
> not (i.e. only checks whether the return value is 0 or not).
> 
> Let's check that all these implementations also behave correctly for
> bytes with the high bit set, and provide the correct ordering -
> independent of us now building with -funsigned-char, the C standard
> says that these *cmp functions should consider the buffers as
> consisting of unsigned chars.
> 
> This is only intended to help find other latent bugs and can/should be
> ripped out again before v6.2, or perhaps moved to test_string.c in
> some form, but for now I think it's worth doing unconditionally.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  lib/string.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/lib/string.c b/lib/string.c
> index 4fb566ea610f..1718f96e8082 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -880,3 +880,30 @@ void *memchr_inv(const void *start, int c, size_t bytes)
>  	return check_bytes8(start, value, bytes % 8);
>  }
>  EXPORT_SYMBOL(memchr_inv);
> +
> +static int sign(int x)
> +{
> +	return (x > 0) - (x < 0);
> +}
> +
> +static int test_xxxcmp(void)
> +{
> +	char a[2], b[2];
> +	int i, j;
> +
> +	a[1] = b[1] = 0;
> +	for (i = 0; i < 256; ++i) {
> +		a[0] = i;
> +		for (j = 0; j < 256; ++j) {
> +			b[0] = j;
> +			WARN_ONCE(sign(strcmp(a, b)) != sign(i - j),
> +				  "strcmp() broken for (%2ph, %2ph)\n", a, b);
> +			WARN_ONCE(sign(memcmp(a, b, 2)) != sign(i - j),
> +				  "memcmp() broken for (%2ph, %2ph)\n", a, b);
> +			WARN_ONCE(sign(strncmp(a, b, 2)) != sign(i - j),
> +				  "strncmp() broken for (%2ph, %2ph)\n", a, b);
> +		}
> +	}
> +	return 0;
> +}
> +late_initcall(test_xxxcmp);

This probably belongs in some config-gated selftest file that can be
compiled out, rather than running unconditionally on every boot, right?

Jason