sparc: Add full proper error handling to strncpy_from_user().

Linus removed the end-of-address-space hackery from
fs/namei.c:do_getname() so we really have to validate these edge
conditions and cannot cheat any more (as x86 used to as well).

Move to a common C implementation like x86 did.  And if both
src and dst are sufficiently aligned we'll do word at a time
copies and checks as well.

Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/lib/usercopy.c

@@ -1,4 +1,6 @@
 #include <linux/module.h>
+#include <linux/uaccess.h>
+#include <linux/errno.h>
 #include <linux/bug.h>
 
 void copy_from_user_overflow(void)
@@ -6,3 +8,133 @@ void copy_from_user_overflow(void)
 	WARN(1, "Buffer overflow detected!\n");
 }
 EXPORT_SYMBOL(copy_from_user_overflow);
+
+#define REPEAT_BYTE(x)	((~0ul / 0xff) * (x))
+
+/* Return the high bit set in the first byte that is a zero */
+static inline unsigned long has_zero(unsigned long a)
+{
+	return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);
+}
+
+static inline long find_zero(unsigned long c)
+{
+#ifdef CONFIG_64BIT
+	if (!(c & 0xff00000000000000UL))
+		return 0;
+	if (!(c & 0x00ff000000000000UL))
+		return 1;
+	if (!(c & 0x0000ff0000000000UL))
+		return 2;
+	if (!(c & 0x000000ff00000000UL))
+		return 3;
+#define __OFF 4
+#else
+#define __OFF 0
+#endif
+	if (!(c & 0xff000000))
+		return __OFF + 0;
+	if (!(c & 0x00ff0000))
+		return __OFF + 1;
+	if (!(c & 0x0000ff00))
+		return __OFF + 2;
+	return __OFF + 3;
+#undef __OFF
+}
+
+/*
+ * Do a strncpy, return length of string without final '\0'.
+ * 'count' is the user-supplied count (return 'count' if we
+ * hit it), 'max' is the address space maximum (and we return
+ * -EFAULT if we hit it).
+ */
+static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
+{
+	long res = 0;
+
+	/*
+	 * Truncate 'max' to the user-specified limit, so that
+	 * we only have one limit we need to check in the loop
+	 */
+	if (max > count)
+		max = count;
+
+	if (((long) dst | (long) src) & (sizeof(long) - 1))
+		goto byte_at_a_time;
+
+	while (max >= sizeof(unsigned long)) {
+		unsigned long c;
+
+		/* Fall back to byte-at-a-time if we get a page fault */
+		if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
+			break;
+		*(unsigned long *)(dst+res) = c;
+		if (has_zero(c))
+			return res + find_zero(c);
+		res += sizeof(unsigned long);
+		max -= sizeof(unsigned long);
+	}
+
+byte_at_a_time:
+	while (max) {
+		char c;
+
+		if (unlikely(__get_user(c,src+res)))
+			return -EFAULT;
+		dst[res] = c;
+		if (!c)
+			return res;
+		res++;
+		max--;
+	}
+
+	/*
+	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
+	 * too? If so, that's ok - we got as much as the user asked for.
+	 */
+	if (res >= count)
+		return res;
+
+	/*
+	 * Nope: we hit the address space limit, and we still had more
+	 * characters the caller would have wanted. That's an EFAULT.
+	 */
+	return -EFAULT;
+}
+
+/**
+ * strncpy_from_user: - Copy a NUL terminated string from userspace.
+ * @dst:   Destination address, in kernel space.  This buffer must be at
+ *         least @count bytes long.
+ * @src:   Source address, in user space.
+ * @count: Maximum number of bytes to copy, including the trailing NUL.
+ *
+ * Copies a NUL-terminated string from userspace to kernel space.
+ *
+ * On success, returns the length of the string (not including the trailing
+ * NUL).
+ *
+ * If access to userspace fails, returns -EFAULT (some data may have been
+ * copied).
+ *
+ * If @count is smaller than the length of the string, copies @count bytes
+ * and returns @count.
+ */
+long strncpy_from_user(char *dst, const char __user *src, long count)
+{
+	unsigned long max_addr, src_addr;
+
+	if (unlikely(count <= 0))
+		return 0;
+
+	max_addr = ~0UL;
+	if (likely(segment_eq(get_fs(), USER_DS)))
+		max_addr = STACK_TOP;
+	src_addr = (unsigned long)src;
+	if (likely(src_addr < max_addr)) {
+		unsigned long max = max_addr - src_addr;
+		return do_strncpy_from_user(dst, src, count, max);
+	}
+	return -EFAULT;
+}
+EXPORT_SYMBOL(strncpy_from_user);