Fix undefined behaviour and sign extension issues in kstrtok

Prevent read beyond the end of sep if it's an empty string.

Change to internally use unsigned char to avoid unwanted sign
extension on platforms where 'char' is signed.

Fix undefined behaviour where pointers were made to the element
before the start of `str`:
https://www.securecoding.cert.org/confluence/display/c/ARR30-C.+Do+not+form+or+use+out-of-bounds+pointers+or+array+subscripts

Author: daviesrob

kstring.c

@@ -34,28 +34,29 @@ int ksprintf(kstring_t *s, const char *fmt, ...)
 	return l;
 }
 
-char *kstrtok(const char *str, const char *sep, ks_tokaux_t *aux)
+char *kstrtok(const char *str, const char *sep_in, ks_tokaux_t *aux)
 {
-	const char *p, *start;
+	const unsigned char *p, *start, *sep = (unsigned char *) sep_in;
 	if (sep) { // set up the table
 		if (str == 0 && aux->finished) return 0; // no need to set up if we have finished
 		aux->finished = 0;
-		if (sep[1]) {
+		if (sep[0] && sep[1]) {
 			aux->sep = -1;
 			aux->tab[0] = aux->tab[1] = aux->tab[2] = aux->tab[3] = 0;
 			for (p = sep; *p; ++p) aux->tab[*p>>6] |= 1ull<<(*p&0x3f);
 		} else aux->sep = sep[0];
 	}
 	if (aux->finished) return 0;
-	else if (str) aux->p = str - 1, aux->finished = 0;
+	else if (str) start = (unsigned char *) str, aux->finished = 0;
+	else start = (unsigned char *) aux->p + 1;
 	if (aux->sep < 0) {
-		for (p = start = aux->p + 1; *p; ++p)
+		for (p = start; *p; ++p)
 			if (aux->tab[*p>>6]>>(*p&0x3f)&1) break;
 	} else {
-		for (p = start = aux->p + 1; *p; ++p)
+		for (p = start; *p; ++p)
 			if (*p == aux->sep) break;
 	}
-	aux->p = p; // end of token
+	aux->p = (const char *) p; // end of token
 	if (*p == 0) aux->finished = 1; // no more tokens
 	return (char*)start;
 }