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>

arch/ia64/kernel/perfmon.c

@@ -2145,22 +2145,12 @@ pfm_close(struct inode *inode, struct file *filp)
 	return 0;
 }
 
-static int
-pfm_no_open(struct inode *irrelevant, struct file *dontcare)
-{
-	DPRINT(("pfm_no_open called\n"));
-	return -ENXIO;
-}
-
-
-
 static const struct file_operations pfm_file_ops = {
 	.llseek		= no_llseek,
 	.read		= pfm_read,
 	.write		= pfm_write,
 	.poll		= pfm_poll,
 	.unlocked_ioctl = pfm_ioctl,
-	.open		= pfm_no_open,	/* special open code to disallow open via /proc */
 	.fasync		= pfm_fasync,
 	.release	= pfm_close,
 	.flush		= pfm_flush

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,

include/linux/fs.h

@@ -2151,7 +2151,6 @@ static inline int sb_is_blkdev_sb(struct super_block *sb)
 extern int sync_filesystem(struct super_block *);
 extern const struct file_operations def_blk_fops;
 extern const struct file_operations def_chr_fops;
-extern const struct file_operations bad_sock_fops;
 #ifdef CONFIG_BLOCK
 extern int ioctl_by_bdev(struct block_device *, unsigned, unsigned long);
 extern int blkdev_ioctl(struct block_device *, fmode_t, unsigned, unsigned long);

net/Makefile

@@ -5,8 +5,6 @@
 # Rewritten to use lists instead of if-statements.
 #
 
-obj-y	:= nonet.o
-
 obj-$(CONFIG_NET)		:= socket.o core/
 
 tmp-$(CONFIG_COMPAT) 		:= compat.o

net/nonet.c

-/*
- * net/nonet.c
- *
- * Dummy functions to allow us to configure network support entirely
- * out of the kernel.
- *
- * Distributed under the terms of the GNU GPL version 2.
- * Copyright (c) Matthew Wilcox 2003
- */
-
-#include <linux/module.h>
-#include <linux/errno.h>
-#include <linux/fs.h>
-#include <linux/init.h>
-#include <linux/kernel.h>
-
-static int sock_no_open(struct inode *irrelevant, struct file *dontcare)
-{
-	return -ENXIO;
-}
-
-const struct file_operations bad_sock_fops = {
-	.owner = THIS_MODULE,
-	.open = sock_no_open,
-	.llseek = noop_llseek,
-};

net/socket.c

@@ -113,7 +113,6 @@ unsigned int sysctl_net_busy_read __read_mostly;
 unsigned int sysctl_net_busy_poll __read_mostly;
 #endif
 
-static int sock_no_open(struct inode *irrelevant, struct file *dontcare);
 static ssize_t sock_aio_read(struct kiocb *iocb, const struct iovec *iov,
 			 unsigned long nr_segs, loff_t pos);
 static ssize_t sock_aio_write(struct kiocb *iocb, const struct iovec *iov,
@@ -151,7 +150,6 @@ static const struct file_operations socket_file_ops = {
 	.compat_ioctl = compat_sock_ioctl,
 #endif
 	.mmap =		sock_mmap,
-	.open =		sock_no_open,	/* special open code to disallow open via /proc */
 	.release =	sock_close,
 	.fasync =	sock_fasync,
 	.sendpage =	sock_sendpage,
@@ -559,23 +557,6 @@ static struct socket *sock_alloc(void)
 	return sock;
 }
 
-/*
- *	In theory you can't get an open on this inode, but /proc provides
- *	a back door. Remember to keep it shut otherwise you'll let the
- *	creepy crawlies in.
- */
-
-static int sock_no_open(struct inode *irrelevant, struct file *dontcare)
-{
-	return -ENXIO;
-}
-
-const struct file_operations bad_sock_fops = {
-	.owner = THIS_MODULE,
-	.open = sock_no_open,
-	.llseek = noop_llseek,
-};
-
 /**
  *	sock_release	-	close a socket
  *	@sock: socket to close