[PATCH] vfs: new O_NODE open flag

Miklos Szeredi
2009-09-24T14:53:01Z

Reseding this patch which I've forgotten about a bit, but David
Howells reminded me. Thanks, David.

It received positive responses from Linus and others at the last
submission.

Al, could you please take a look?

Thanks,
Miklos

---
From: Miklos Szeredi <miklos@szeredi.hu>

This patch adds a new open flag, O_NODE. This flag means: open just
the filesystem node instead of the object referenced by the node.

Opening a file will not call the driver's ->open() method and will not
have any side effect other than referencing the dentry/vfsmount from
the struct file pointer.

Currently O_NODE can only be used together with O_NOACCESS (value 3),
meaning that read(2) and write(2) will return EBADF and no permission
is necessary to open the file. This is logical for device nodes and
fifos. For directories we could allow O_RDONLY, and for regular files
any access mode, but this is not done yet.

O_NODE used together with O_NOFOLLOW will open a symlink node itself
instead of returning ELOOP. O_NOFOLLOW will not prevent following
mountpoints if used together with O_NODE. In theory we could allow
O_RDONLY for symlinks too and return the contents of the link on
read(2).

Filesystems which want to install special file operations for O_NODE
opens (e.g. ioctl) may set S_OPEN_NODE flag in the inode. This will
cause dentry_open() to call inode->i_fop->open, and the filesystem is
responsible for handling the O_NODE flag and installing the right
filesystem operations in file->f_op.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: David Howells <dhowells@redhat.com>
---
fs/file_table.c | 6 ++--
fs/locks.c | 3 ++
fs/namei.c | 65 ++++++++++++++++++++++++++------------------
fs/open.c | 12 +++++++-
include/asm-generic/fcntl.h | 4 ++
include/linux/fs.h | 1
6 files changed, 62 insertions(+), 29 deletions(-)

Index: linux-2.6/fs/file_table.c
===================================================================
--- linux-2.6.orig/fs/file_table.c 2009-09-24 15:56:41.000000000 +0200
+++ linux-2.6/fs/file_table.c 2009-09-24 16:36:35.000000000 +0200
@@ -281,8 +281,10 @@ void __fput(struct file *file)
file->f_op->release(inode, file);
security_file_free(file);
ima_file_free(file);
- if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL))
- cdev_put(inode->i_cdev);
+ if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL)) {
+ if (!(file->f_flags & O_NODE))
+ cdev_put(inode->i_cdev);
+ }
fops_put(file->f_op);
put_pid(file->f_owner.pid);
file_kill(file);
Index: linux-2.6/fs/locks.c
===================================================================
--- linux-2.6.orig/fs/locks.c 2009-09-24 15:56:41.000000000 +0200
+++ linux-2.6/fs/locks.c 2009-09-24 16:36:35.000000000 +0200
@@ -1183,6 +1183,9 @@ int __break_lease(struct inode *inode, u
unsigned long break_time;
int i_have_this_lease = 0;

+ if (!(mode & (FMODE_READ | FMODE_WRITE)))
+ return 0;
+
new_fl = lease_alloc(NULL, mode & FMODE_WRITE ? F_WRLCK : F_RDLCK);

lock_kernel();
Index: linux-2.6/fs/namei.c
===================================================================
--- linux-2.6.orig/fs/namei.c 2009-09-24 15:56:41.000000000 +0200
+++ linux-2.6/fs/namei.c 2009-09-24 16:36:36.000000000 +0200
@@ -1160,7 +1160,9 @@ static int path_lookup_open(int dfd, con
nd->intent.open.file = filp;
nd->intent.open.flags = open_flags;
nd->intent.open.create_mode = 0;
- err = do_path_lookup(dfd, name, lookup_flags|LOOKUP_OPEN, nd);
+ if (!(open_flags & O_NODE))
+ lookup_flags |= LOOKUP_OPEN;
+ err = do_path_lookup(dfd, name, lookup_flags, nd);
if (IS_ERR(nd->intent.open.file)) {
if (err == 0) {
err = PTR_ERR(nd->intent.open.file);
@@ -1511,22 +1513,27 @@ int may_open(struct path *path, int acc_
if (!inode)
return -ENOENT;

- switch (inode->i_mode & S_IFMT) {
- case S_IFLNK:
- return -ELOOP;
- case S_IFDIR:
- if (acc_mode & MAY_WRITE)
- return -EISDIR;
- break;
- case S_IFBLK:
- case S_IFCHR:
- if (path->mnt->mnt_flags & MNT_NODEV)
+ if ((flag & O_NODE)) {
+ if (acc_mode & (MAY_READ | MAY_WRITE))
return -EACCES;
- /*FALLTHRU*/
- case S_IFIFO:
- case S_IFSOCK:
- flag &= ~O_TRUNC;
- break;
+ } else {
+ switch (inode->i_mode & S_IFMT) {
+ case S_IFLNK:
+ return -ELOOP;
+ case S_IFDIR:
+ if (acc_mode & MAY_WRITE)
+ return -EISDIR;
+ break;
+ case S_IFBLK:
+ case S_IFCHR:
+ if (path->mnt->mnt_flags & MNT_NODEV)
+ return -EACCES;
+ /*FALLTHRU*/
+ case S_IFIFO:
+ case S_IFSOCK:
+ flag &= ~O_TRUNC;
+ break;
+ }
}

error = inode_permission(inode, acc_mode);
@@ -1639,14 +1646,16 @@ out_unlock:
* 11 - read-write
* for the internal routines (ie open_namei()/follow_link() etc)
* This is more logical, and also allows the 00 "no perm needed"
- * to be used for symlinks (where the permissions are checked
- * later).
+ * to be used for opening a symlink, pipe, socket or device with
+ * O_NODE.
*
*/
static inline int open_to_namei_flags(int flag)
{
if ((flag+1) & O_ACCMODE)
flag++;
+ else if (flag & O_NODE)
+ flag &= ~O_ACCMODE;
return flag;
}

@@ -1734,7 +1743,9 @@ struct file *do_filp_open(int dfd, const
nd.intent.open.create_mode = mode;
dir = nd.path.dentry;
nd.flags &= ~LOOKUP_PARENT;
- nd.flags |= LOOKUP_CREATE | LOOKUP_OPEN;
+ nd.flags |= LOOKUP_CREATE;
+ if (!(open_flag & O_NODE))
+ nd.flags |= LOOKUP_OPEN;
if (flag & O_EXCL)
nd.flags |= LOOKUP_EXCL;
mutex_lock(&dir->d_inode->i_mutex);
@@ -1793,19 +1804,24 @@ do_last:

if (__follow_mount(&path)) {
error = -ELOOP;
- if (flag & O_NOFOLLOW)
+ if ((flag & O_NOFOLLOW) & !(flag & O_NODE))
goto exit_dput;
}

error = -ENOENT;
if (!path.dentry->d_inode)
goto exit_dput;
- if (path.dentry->d_inode->i_op->follow_link)
- goto do_link;
+ if (path.dentry->d_inode->i_op->follow_link) {
+ if (!(flag & O_NOFOLLOW))
+ goto do_link;
+ error = -ELOOP;
+ if (!(flag & O_NODE))
+ goto exit_dput;
+ }

path_to_nameidata(&path, &nd);
error = -EISDIR;
- if (path.dentry->d_inode && S_ISDIR(path.dentry->d_inode->i_mode))
+ if (S_ISDIR(path.dentry->d_inode->i_mode))
goto exit;
ok:
/*
@@ -1859,9 +1875,6 @@ exit_parent:
return ERR_PTR(error);

do_link:
- error = -ELOOP;
- if (flag & O_NOFOLLOW)
- goto exit_dput;
/*
* This is subtle. Instead of calling do_follow_link() we do the
* thing by hands. The reason is that this way we have zero link_count
Index: linux-2.6/fs/open.c
===================================================================
--- linux-2.6.orig/fs/open.c 2009-09-24 15:56:41.000000000 +0200
+++ linux-2.6/fs/open.c 2009-09-24 16:36:36.000000000 +0200
@@ -804,12 +804,17 @@ static inline int __get_file_write_acces
return error;
}

+static const struct file_operations default_node_fops = {
+ .llseek = no_llseek,
+};
+
static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
int flags, struct file *f,
int (*open)(struct inode *, struct file *),
const struct cred *cred)
{
struct inode *inode;
+ const struct file_operations *fops;
int error;

f->f_flags = flags;
@@ -828,7 +833,12 @@ static struct file *__dentry_open(struct
f->f_path.dentry = dentry;
f->f_path.mnt = mnt;
f->f_pos = 0;
- f->f_op = fops_get(inode->i_fop);
+
+ fops = inode->i_fop;
+ if ((flags & O_NODE) && !(inode->i_flags & S_OPEN_NODE))
+ fops = &default_node_fops;
+ f->f_op = fops_get(fops);
+
file_move(f, &inode->i_sb->s_files);

error = security_dentry_open(f, cred);
Index: linux-2.6/include/asm-generic/fcntl.h
===================================================================
--- linux-2.6.orig/include/asm-generic/fcntl.h 2009-09-24 15:56:41.000000000 +0200
+++ linux-2.6/include/asm-generic/fcntl.h 2009-09-24 16:36:36.000000000 +0200
@@ -9,6 +9,7 @@
#define O_RDONLY 00000000
#define O_WRONLY 00000001
#define O_RDWR 00000002
+#define O_NOACCESS 00000003
#ifndef O_CREAT
#define O_CREAT 00000100 /* not fcntl */
#endif
@@ -51,6 +52,9 @@
#ifndef O_CLOEXEC
#define O_CLOEXEC 02000000 /* set close_on_exec */
#endif
+#ifndef O_NODE
+#define O_NODE 04000000 /* open filesystem node, not device */
+#endif
#ifndef O_NDELAY
#define O_NDELAY O_NONBLOCK
#endif
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h 2009-09-24 15:56:41.000000000 +0200
+++ linux-2.6/include/linux/fs.h 2009-09-24 16:36:36.000000000 +0200
@@ -231,6 +231,7 @@ struct inodes_stat_t {
#define S_NOCMTIME 128 /* Do not update file c/mtime */
#define S_SWAPFILE 256 /* Do not truncate: swapon got its bmaps */
#define S_PRIVATE 512 /* Inode is fs-internal */
+#define S_OPEN_NODE 1024 /* Fs is prepared for O_NODE opens */

/*
* Note that nosuid etc flags are inode-specific: setting some file-system