New kind of open files - "location only".

New flag for open(2) - O_PATH.  Semantics:
	* pathname is resolved, but the file itself is _NOT_ opened
as far as filesystem is concerned.
	* almost all operations on the resulting descriptors shall
fail with -EBADF.  Exceptions are:
	1) operations on descriptors themselves (i.e.
		close(), dup(), dup2(), dup3(), fcntl(fd, F_DUPFD),
		fcntl(fd, F_DUPFD_CLOEXEC, ...), fcntl(fd, F_GETFD),
		fcntl(fd, F_SETFD, ...))
	2) fcntl(fd, F_GETFL), for a common non-destructive way to
		check if descriptor is open
	3) "dfd" arguments of ...at(2) syscalls, i.e. the starting
		points of pathname resolution
	* closing such descriptor does *NOT* affect dnotify or
posix locks.
	* permissions are checked as usual along the way to file;
no permission checks are applied to the file itself.  Of course,
giving such thing to syscall will result in permission checks (at
the moment it means checking that starting point of ....at() is
a directory and caller has exec permissions on it).

fget() and fget_light() return NULL on such descriptors; use of
fget_raw() and fget_raw_light() is needed to get them.  That protects
existing code from dealing with those things.

There are two things still missing (they come in the next commits):
one is handling of symlinks (right now we refuse to open them that
way; see the next commit for semantics related to those) and another
is descriptor passing via SCM_RIGHTS datagrams.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 29 insertions, 6 deletions

diff --git a/fs/open.c b/fs/open.c
index 48afc5c139d2..14a51de01f54 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -669,11 +669,16 @@ static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
                                         int (*open)(struct inode *, struct file *),
                                         const struct cred *cred)
 {
+        static const struct file_operations empty_fops = {};
         struct inode *inode;
         int error;
 
         f->f_mode = OPEN_FMODE(f->f_flags) | FMODE_LSEEK |
                                 FMODE_PREAD | FMODE_PWRITE;
+
+        if (unlikely(f->f_flags & O_PATH))
+                f->f_mode = FMODE_PATH;
+
         inode = dentry->d_inode;
         if (f->f_mode & FMODE_WRITE) {
                 error = __get_file_write_access(inode, mnt);
@@ -687,9 +692,15 @@ static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
         f->f_path.dentry = dentry;
         f->f_path.mnt = mnt;
         f->f_pos = 0;
-        f->f_op = fops_get(inode->i_fop);
         file_sb_list_add(f, inode->i_sb);
 
+        if (unlikely(f->f_mode & FMODE_PATH)) {
+                f->f_op = &empty_fops;
+                return f;
+        }
+
+        f->f_op = fops_get(inode->i_fop);
+
         error = security_dentry_open(f, cred);
         if (error)
                 goto cleanup_all;
@@ -911,9 +922,18 @@ static inline int build_open_flags(int flags, int mode, struct open_flags *op)
         if (flags & __O_SYNC)
                 flags |= O_DSYNC;
 
-        op->open_flag = flags;
+        /*
+         * If we have O_PATH in the open flag. Then we
+         * cannot have anything other than the below set of flags
+         */
+        if (flags & O_PATH) {
+                flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH;
+                acc_mode = 0;
+        } else {
+                acc_mode = MAY_OPEN | ACC_MODE(flags);
+        }
 
-        acc_mode = MAY_OPEN | ACC_MODE(flags);
+        op->open_flag = flags;
 
         /* O_TRUNC implies we need access checks for write permissions */
         if (flags & O_TRUNC)
@@ -926,7 +946,8 @@ static inline int build_open_flags(int flags, int mode, struct open_flags *op)
 
         op->acc_mode = acc_mode;
 
-        op->intent = LOOKUP_OPEN;
+        op->intent = flags & O_PATH ? 0 : LOOKUP_OPEN;
+
         if (flags & O_CREAT) {
                 op->intent |= LOOKUP_CREATE;
                 if (flags & O_EXCL)
@@ -1053,8 +1074,10 @@ int filp_close(struct file *filp, fl_owner_t id)
         if (filp->f_op && filp->f_op->flush)
                 retval = filp->f_op->flush(filp, id);
 
-        dnotify_flush(filp, id);
-        locks_remove_posix(filp, id);
+        if (likely(!(filp->f_mode & FMODE_PATH))) {
+                dnotify_flush(filp, id);
+                locks_remove_posix(filp, id);
+        }
         fput(filp);
         return retval;
 }