s390/uaccess: fix strncpy_from_user/strnlen_user zero maxlen case

If the maximum length specified for the to be accessed string for
strncpy_from_user() and strnlen_user() is zero the following incorrect
values would be returned or incorrect memory accesses would happen:

strnlen_user_std() and strnlen_user_pt() incorrectly return "1"
strncpy_from_user_pt() would incorrectly access "dst[maxlen - 1]"
strncpy_from_user_mvcos() would incorrectly return "-EFAULT"

Fix all these oddities by adding early checks.
Reviewed-by: NGerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/lib/uaccess_mvcos.c

@@ -184,6 +184,8 @@ static size_t strncpy_from_user_mvcos(size_t count, const char __user *src,
 {
 	size_t done, len, offset, len_str;
 
+	if (unlikely(!count))
+		return 0;
 	done = 0;
 	do {
 		offset = (size_t)src & ~PAGE_MASK;

arch/s390/lib/uaccess_pt.c

@@ -172,6 +172,8 @@ static size_t strnlen_user_pt(size_t count, const char __user *src)
 	unsigned long offset, done, len, kaddr;
 	size_t len_str;
 
+	if (unlikely(!count))
+		return 0;
 	if (segment_eq(get_fs(), KERNEL_DS))
 		return strnlen((const char __kernel __force *) src, count) + 1;
 	done = 0;
@@ -202,6 +204,8 @@ static size_t strncpy_from_user_pt(size_t count, const char __user *src,
 {
 	size_t n = strnlen_user_pt(count, src);
 
+	if (unlikely(!count))
+		return 0;
 	if (!n)
 		return -EFAULT;
 	if (n > count)

arch/s390/lib/uaccess_std.c

@@ -188,6 +188,8 @@ size_t strnlen_user_std(size_t size, const char __user *src)
 	register unsigned long reg0 asm("0") = 0UL;
 	unsigned long tmp1, tmp2;
 
+	if (unlikely(!size))
+		return 0;
 	asm volatile(
 		"   la    %2,0(%1)\n"
 		"   la    %3,0(%0,%1)\n"