3 files changed, 151 insertions, 0 deletions

diff --git a/lib/Kconfig b/lib/Kconfig
index 0e25c03939e3..98230ac3db29 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -16,6 +16,9 @@ config BITREVERSE
 config RATIONAL
         boolean
 
+config GENERIC_STRNCPY_FROM_USER
+        bool
+
 config GENERIC_FIND_FIRST_BIT
         bool
 
diff --git a/lib/Makefile b/lib/Makefile
index 74290c9e2864..b98df505f335 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -125,6 +125,8 @@ obj-$(CONFIG_CLZ_TAB) += clz_tab.o
 
 obj-$(CONFIG_DDR) += jedec_ddr_data.o
 
+obj-$(CONFIG_GENERIC_STRNCPY_FROM_USER) += strncpy_from_user.o
+
 hostprogs-y     := gen_crc32table
 clean-files     := crc32table.h
 
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
new file mode 100644
index 000000000000..c4c09b0e96ba
--- /dev/null
+++ b/lib/strncpy_from_user.c
@@ -0,0 +1,146 @@
+#include <linux/module.h>
+#include <linux/uaccess.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+
+#include <asm/byteorder.h>
+
+static inline long find_zero(unsigned long mask)
+{
+        long byte = 0;
+
+#ifdef __BIG_ENDIAN
+#ifdef CONFIG_64BIT
+        if (mask >> 32)
+                mask >>= 32;
+        else
+                byte = 4;
+#endif
+        if (mask >> 16)
+                mask >>= 16;
+        else
+                byte += 2;
+        return (mask >> 8) ? byte : byte + 1;
+#else
+#ifdef CONFIG_64BIT
+        if (!((unsigned int) mask)) {
+                mask >>= 32;
+                byte = 4;
+        }
+#endif
+        if (!(mask & 0xffff)) {
+                mask >>= 16;
+                byte += 2;
+        }
+        return (mask & 0xff) ? byte : byte + 1;
+#endif
+}
+
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+#define IS_UNALIGNED(src, dst)  0
+#else
+#define IS_UNALIGNED(src, dst)  \
+        (((long) dst | (long) src) & (sizeof(long) - 1))
+#endif
+
+/*
+ * 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)
+{
+        const unsigned long high_bits = REPEAT_BYTE(0xfe) + 1;
+        const unsigned long low_bits = REPEAT_BYTE(0x7f);
+        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 (IS_UNALIGNED(src, dst))
+                goto byte_at_a_time;
+
+        while (max >= sizeof(unsigned long)) {
+                unsigned long c, v, rhs;
+
+                /* Fall back to byte-at-a-time if we get a page fault */
+                if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
+                        break;
+                rhs = c | low_bits;
+                v = (c + high_bits) & ~rhs;
+                *(unsigned long *)(dst+res) = c;
+                if (v) {
+                        v = (c & low_bits) + low_bits;
+                        v = ~(v | rhs);
+                        return res + find_zero(v);
+                }
+                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 = user_addr_max();
+        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);