make default ->i_fop have ->open() fail with ENXIO

As it is, default ->i_fop has NULL ->open() (along with all other methods).
The only case where it matters is reopening (via procfs symlink) a file that
didn't get its ->f_op from ->i_fop - anything else will have ->i_fop assigned
to something sane (default would fail on read/write/ioctl/etc.).

	Unfortunately, such case exists - alloc_file() users, especially
anon_get_file() ones.  There we have tons of opened files of very different
kinds sharing the same inode.  As the result, attempt to reopen those via
procfs succeeds and you get a descriptor you can't do anything with.

	Moreover, in case of sockets we set ->i_fop that will only be used
on such reopen attempts - and put a failing ->open() into it to make sure
those do not succeed.

	It would be simpler to put such ->open() into default ->i_fop and leave
it unchanged both for anon inode (as we do anyway) and for socket ones.  Result:
	* everything going through do_dentry_open() works as it used to
	* sock_no_open() kludge is gone
	* attempts to reopen anon-inode files fail as they really ought to
	* ditto for aio_private_file()
	* ditto for perfmon - this one actually tried to imitate sock_no_open()
trick, but failed to set ->i_fop, so in the current tree reopens succeed and
yield completely useless descriptor.  Intent clearly had been to fail with
-ENXIO on such reopens; now it actually does.
	* everything else that used alloc_file() keeps working - it has ->i_fop
set for its inodes anyway

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

1 files changed, 8 insertions, 3 deletions

diff --git a/fs/inode.c b/fs/inode.c
index 26753ba7b6d6..5b83ef7fc8d5 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -114,6 +114,11 @@ int proc_nr_inodes(struct ctl_table *table, int write,
 }
 #endif
 
+static int no_open(struct inode *inode, struct file *file)
+{
+        return -ENXIO;
+}
+
 /**
  * inode_init_always - perform inode structure intialisation
  * @sb: superblock inode belongs to
@@ -125,7 +130,7 @@ int proc_nr_inodes(struct ctl_table *table, int write,
 int inode_init_always(struct super_block *sb, struct inode *inode)
 {
         static const struct inode_operations empty_iops;
-        static const struct file_operations empty_fops;
+        static const struct file_operations no_open_fops = {.open = no_open};
         struct address_space *const mapping = &inode->i_data;
 
         inode->i_sb = sb;
@@ -133,7 +138,7 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
         inode->i_flags = 0;
         atomic_set(&inode->i_count, 1);
         inode->i_op = &empty_iops;
-        inode->i_fop = &empty_fops;
+        inode->i_fop = &no_open_fops;
         inode->__i_nlink = 1;
         inode->i_opflags = 0;
         i_uid_write(inode, 0);
@@ -1801,7 +1806,7 @@ void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
         } else if (S_ISFIFO(mode))
                 inode->i_fop = &pipefifo_fops;
         else if (S_ISSOCK(mode))
-                inode->i_fop = &bad_sock_fops;
+                ;       /* leave it no_open_fops */
         else
                 printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o) for"
                                   " inode %s:%lu\n", mode, inode->i_sb->s_id,