vfs: define kernel_read_file_from_path

This patch defines kernel_read_file_from_path(), a wrapper for the VFS
common kernel_read_file().

Changelog:
- revert error msg regression - reported by Sergey Senozhatsky
- Separated from the IMA patch

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>

fs/exec.c

@@ -884,6 +884,25 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
 }
 EXPORT_SYMBOL_GPL(kernel_read_file);
 
+int kernel_read_file_from_path(char *path, void **buf, loff_t *size,
+			       loff_t max_size, enum kernel_read_file_id id)
+{
+	struct file *file;
+	int ret;
+
+	if (!path || !*path)
+		return -EINVAL;
+
+	file = filp_open(path, O_RDONLY, 0);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	ret = kernel_read_file(file, buf, size, max_size, id);
+	fput(file);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kernel_read_file_from_path);
+
 ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
 {
 	ssize_t res = vfs_read(file, (void __user *)addr, len, &pos);

include/linux/fs.h

@@ -2583,6 +2583,8 @@ enum kernel_read_file_id {
 extern int kernel_read(struct file *, loff_t, char *, unsigned long);
 extern int kernel_read_file(struct file *, void **, loff_t *, loff_t,
 			    enum kernel_read_file_id);
+extern int kernel_read_file_from_path(char *, void **, loff_t *, loff_t,
+				      enum kernel_read_file_id);
 extern ssize_t kernel_write(struct file *, const char *, size_t, loff_t);
 extern ssize_t __kernel_write(struct file *, const char *, size_t, loff_t *);
 extern struct file * open_exec(const char *);