[PATCH] lib: fix buggy strcmp and strncmp

[Tom Rini]

On Wed, Oct 05, 2022 at 11:09:25AM +0200, Rasmus Villemoes wrote:

> There are two problems with both strcmp and strncmp:
> 
> (1) The C standard is clear that the contents should be compared as
> "unsigned char":
> 
>   The sign of a nonzero value returned by the comparison functions
>   memcmp, strcmp, and strncmp is determined by the sign of the
>   difference between the values of the first pair of characters (both
>   interpreted as unsigned char) that differ in the objects being
>   compared.
> 
> (2) The difference between two char (or unsigned char) values can
> range from -255 to +255; so that's (due to integer promotion) the
> range of values we could get in the *cs-*ct expressions, but when that
> is then shoe-horned into an 8-bit quantity the sign may of course
> change.
> 
> The impact is somewhat limited by the way these functions
> are used in practice:
> 
> - Most of the time, one is only interested in equality (or for
>   strncmp, "starts with"), and the existing functions do correctly
>   return 0 if and only if the strings are equal [for strncmp, up to
>   the given bound].
> 
> - Also most of the time, the strings being compared only consist of
>   ASCII characters, i.e. have values in the range [0, 127], and in
>   that case it doesn't matter if they are interpreted as signed or
>   unsigned char, and the possible difference range is bounded to
>   [-127, 127] which does fit the signed char.
> 
> For size, one could implement strcmp() in terms of strncmp() - just
> make it "return strncmp(a, b, (size_t)-1);". However, performance of
> strcmp() does matter somewhat, since it is used all over when parsing
> and matching DT nodes and properties, so let's find some other place
> to save those ~30 bytes.
> 
> Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>

Applied to u-boot/master, thanks!

-- 
Tom
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