[PATCH 03/23] lib: uaccess: provide getline_from_user()

[Bartosz Golaszewski <brgl@bgdev.pl>]

From: Bartosz Golaszewski <bgolaszewski@baylibre.com>

Provide a uaccess helper that allows callers to copy a single line from
user memory. This is useful for debugfs write callbacks.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
---
 include/linux/uaccess.h |  3 +++
 lib/usercopy.c          | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 94b285411659..5aedd8ac5c31 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -333,6 +333,9 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
 		long count);
 long strnlen_user_nofault(const void __user *unsafe_addr, long count);
 
+ssize_t getline_from_user(char *dst, size_t dst_size,
+			  const char __user *src, size_t src_size);
+
 /**
  * get_kernel_nofault(): safely attempt to read from a location
  * @val: read into this variable
diff --git a/lib/usercopy.c b/lib/usercopy.c
index b26509f112f9..55aaaf93d847 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -87,3 +87,40 @@ int check_zeroed_user(const void __user *from, size_t size)
 	return -EFAULT;
 }
 EXPORT_SYMBOL(check_zeroed_user);
+
+/**
+ * getline_from_user - Copy a single line from user
+ * @dst: Where to copy the line to
+ * @dst_size: Size of the destination buffer
+ * @src: Where to copy the line from
+ * @src_size: Size of the source user buffer
+ *
+ * Copies a number of characters from given user buffer into the dst buffer.
+ * The number of bytes is limited to the lesser of the sizes of both buffers.
+ * If the copied string contains a newline, its first occurrence is replaced
+ * by a NULL byte in the destination buffer. Otherwise the function ensures
+ * the copied string is NULL-terminated.
+ *
+ * Returns the number of copied bytes or a negative error number on failure.
+ */
+
+ssize_t getline_from_user(char *dst, size_t dst_size,
+			  const char __user *src, size_t src_size)
+{
+	size_t size = min_t(size_t, dst_size, src_size);
+	char *c;
+	int ret;
+
+	ret = copy_from_user(dst, src, size);
+	if (ret)
+		return -EFAULT;
+
+	dst[size - 1] = '\0';
+
+	c = strchrnul(dst, '\n');
+	if (*c)
+		*c = '\0';
+
+	return c - dst;
+}
+EXPORT_SYMBOL(getline_from_user);
-- 
2.26.1