[PATCH -V18 05/13] vfs: Support null pathname in readlink

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

From: NeilBrown <neilb@suse.de>

This enables to use readlink to get the link target name
from a file descriptor point to the link. This can be used
with open_by_handle syscall that returns a file descriptor for a link.
We can then use this file descriptor to get the target name.

This is similar to utimensat(2) interface

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
 fs/stat.c |   30 ++++++++++++++++++++++--------
 1 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/fs/stat.c b/fs/stat.c
index 12e90e2..3723a9f 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -289,26 +289,40 @@ SYSCALL_DEFINE2(newfstat, unsigned int, fd, struct stat __user *, statbuf)
 SYSCALL_DEFINE4(readlinkat, int, dfd, const char __user *, pathname,
 		char __user *, buf, int, bufsiz)
 {
-	struct path path;
-	int error;
+	int error = 0, fput_needed;
+	struct path path, *pp;
+	struct file *file = NULL;
 
 	if (bufsiz <= 0)
 		return -EINVAL;
 
-	error = user_path_at(dfd, pathname, 0, &path);
+	if (pathname == NULL && dfd != AT_FDCWD) {
+		file = fget_light(dfd, &fput_needed);
+
+		if (file)
+			pp = &file->f_path;
+		else
+			error = -EBADF;
+	} else {
+		error = user_path_at(dfd, pathname, 0, &path);
+		pp = &path;
+	}
 	if (!error) {
-		struct inode *inode = path.dentry->d_inode;
+		struct inode *inode = pp->dentry->d_inode;
 
 		error = -EINVAL;
 		if (inode->i_op->readlink) {
-			error = security_inode_readlink(path.dentry);
+			error = security_inode_readlink(pp->dentry);
 			if (!error) {
-				touch_atime(path.mnt, path.dentry);
-				error = inode->i_op->readlink(path.dentry,
+				touch_atime(pp->mnt, pp->dentry);
+				error = inode->i_op->readlink(pp->dentry,
 							      buf, bufsiz);
 			}
 		}
-		path_put(&path);
+		if (file)
+			fput_light(file, fput_needed);
+		else
+			path_put(&path);
 	}
 	return error;
 }
-- 
1.7.0.4