[PATCH -V18 04/13] vfs: Allow handle based open on symlinks

Aneesh Kumar K.V
2010-08-20T01:52:11Z

The patch update may_open to allow handle based open on symlinks.
The file handle based API use file descritor returned from open_by_handle_at
to do different file system operations. To find the link target name we
need to get a file descriptor on symlinks.

We should be able to read the link target using file handle. The exact
usecase is with respect to implementing READLINK operation on a
userspace NFS server. The request contain the file handle and the
response include target name.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
fs/namei.c | 24 ++++++++++++++++++++++--
include/asm-generic/fcntl.h | 1 +
include/linux/fs.h | 1 +
3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1291dfc..2c79363 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1447,7 +1447,20 @@ int may_open(struct path *path, int acc_mode, int flag)

switch (inode->i_mode & S_IFMT) {
case S_IFLNK:
- return -ELOOP;
+ /*
+ * Allow only if acc_mode contain
+ * open link request.
+ */
+ if (!(acc_mode & MAY_OPEN_LINK))
+ return -ELOOP;
+ /*
+ * Allow open on symlink only with
+ * open flag O_NOACCESS. We don't
+ * allow read/write on symlinks
+ */
+ if (flag != O_NOACCESS)
+ return -ELOOP;
+ break;
case S_IFDIR:
if (acc_mode & MAY_WRITE)
return -EISDIR;
@@ -2038,8 +2051,15 @@ long do_handle_open(int mountdirfd,
*/
if (open_flag & __O_SYNC)
open_flag |= O_DSYNC;
+ /*
+ * Handle based API allow open on a symlink
+ */
+ if (S_ISLNK(path->dentry->d_inode->i_mode))
+ acc_mode = MAY_OPEN_LINK;
+ else
+ acc_mode = MAY_OPEN;

- acc_mode = MAY_OPEN | ACC_MODE(open_flag);
+ acc_mode |= ACC_MODE(open_flag);

/* O_TRUNC implies we need access checks for write permissions */
if (open_flag & O_TRUNC)
diff --git a/include/asm-generic/fcntl.h b/include/asm-generic/fcntl.h
index a70b2d2..9d8f455 100644
--- a/include/asm-generic/fcntl.h
+++ b/include/asm-generic/fcntl.h
@@ -19,6 +19,7 @@
#define O_RDONLY 00000000
#define O_WRONLY 00000001
#define O_RDWR 00000002
+#define O_NOACCESS 00000003 /* No read/write access */
#ifndef O_CREAT
#define O_CREAT 00000100 /* not fcntl */
#endif
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e9471c7..3eb3571 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -55,6 +55,7 @@ struct inodes_stat_t {
#define MAY_ACCESS 16
#define MAY_OPEN 32
#define MAY_CHDIR 64
+#define MAY_OPEN_LINK 128

/*
* flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond
--
1.7.0.4