lib/uklibparam: Minimum value of a signed parameter is rejected as out of range #1932

Describe the bug

uklibparam rejects the minimum value of every signed parameter type. For example, passing -128 to an __s8 parameter fails:

libuklibparam.test_s8=-128 --
ERR:  [libuklibparam] <parser.c @  479> Parameter libuklibparam.test_s8 (s8): Given number is out of range

The same happens for -32768 (__s16), -2147483648 (__s32), and -9223372036854775808 (__s64). Values one above the minimum are accepted.

Cause

do_check_num() in lib/uklibparam/parser.c compares the parsed magnitude with the type maximum before it applies the sign:

if (num > (unsigned long long) max)
	goto outofrange;

The magnitude of the minimum of a signed type is max + 1, so this check fails before the negative range check runs.

Steps to reproduce

1. Register a parameter with UK_LIBPARAM_PARAM(x, __s8, "...").
2. Boot with <prefix>.x=-128 --.
3. The parser prints "Given number is out of range" and x keeps its default value.

Expected behavior

x is set to -128.