lib/cmdline: fix get_option() for strings starting with hyphen

When string doesn't have an integer and starts from hyphen get_option()
may return interesting results.  Fix it to return 0.

The simple_strtoull() is used due to absence of simple_strtoul() in a boot
code on some architectures.

Note, the Fixes tag below is rather for anthropological curiosity.

Link: https://lkml.kernel.org/r/20201112180732.75589-4-andriy.shevchenko@linux.intel.com
Fixes: f68565831e72 ("Import 2.4.0-test2pre3")
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Vitor Massaru Iha <vitor@massaru.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/cmdline.c

@@ -45,6 +45,9 @@ static int get_range(char **str, int *pint, int n)
  *	1 - int found, no subsequent comma
  *	2 - int found including a subsequent comma
  *	3 - hyphen found to denote a range
+ *
+ *	Leading hyphen without integer is no integer case, but we consume it
+ *	for the sake of simplification.
  */
 
 int get_option(char **str, int *pint)
@@ -53,7 +56,10 @@ int get_option(char **str, int *pint)
 
 	if (!cur || !(*cur))
 		return 0;
-	*pint = simple_strtol(cur, str, 0);
+	if (*cur == '-')
+		*pint = -simple_strtoull(++cur, str, 0);
+	else
+		*pint = simple_strtoull(cur, str, 0);
 	if (cur == *str)
 		return 0;
 	if (**str == ',') {