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kernel-ark/include/linux/inotify.h
Al Viro 8f7b0ba1c8 Fix inotify watch removal/umount races 
Inotify watch removals suck violently.

To kick the watch out we need (in this order) inode->inotify_mutex and
ih->mutex.  That's fine if we have a hold on inode; however, for all
other cases we need to make damn sure we don't race with umount.  We can
*NOT* just grab a reference to a watch - inotify_unmount_inodes() will
happily sail past it and we'll end with reference to inode potentially
outliving its superblock.

Ideally we just want to grab an active reference to superblock if we
can; that will make sure we won't go into inotify_umount_inodes() until
we are done.  Cleanup is just deactivate_super().

However, that leaves a messy case - what if we *are* racing with
umount() and active references to superblock can't be acquired anymore?
We can bump ->s_count, grab ->s_umount, which will almost certainly wait
until the superblock is shut down and the watch in question is pining
for fjords.  That's fine, but there is a problem - we might have hit the
window between ->s_active getting to 0 / ->s_count - below S_BIAS (i.e.
the moment when superblock is past the point of no return and is heading
for shutdown) and the moment when deactivate_super() acquires
->s_umount.

We could just do drop_super() yield() and retry, but that's rather
antisocial and this stuff is luser-triggerable.  OTOH, having grabbed
->s_umount and having found that we'd got there first (i.e.  that
->s_root is non-NULL) we know that we won't race with
inotify_umount_inodes().

So we could grab a reference to watch and do the rest as above, just
with drop_super() instead of deactivate_super(), right? Wrong.  We had
to drop ih->mutex before we could grab ->s_umount.  So the watch
could've been gone already.

That still can be dealt with - we need to save watch->wd, do idr_find()
and compare its result with our pointer.  If they match, we either have
the damn thing still alive or we'd lost not one but two races at once,
the watch had been killed and a new one got created with the same ->wd
at the same address.  That couldn't have happened in inotify_destroy(),
but inotify_rm_wd() could run into that.  Still, "new one got created"
is not a problem - we have every right to kill it or leave it alone,
whatever's more convenient.

So we can use idr_find(...) == watch && watch->inode->i_sb == sb as
"grab it and kill it" check.  If it's been our original watch, we are
fine, if it's a newcomer - nevermind, just pretend that we'd won the
race and kill the fscker anyway; we are safe since we know that its
superblock won't be going away.

And yes, this is far beyond mere "not very pretty"; so's the entire
concept of inotify to start with.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Greg KH <greg@kroah.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-11-15 12:26:44 -08:00

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/*
* Inode based directory notification for Linux
*
* Copyright (C) 2005 John McCutchan
*/
#ifndef _LINUX_INOTIFY_H
#define _LINUX_INOTIFY_H
/* For O_CLOEXEC and O_NONBLOCK */
#include <linux/fcntl.h>
#include <linux/types.h>
/*
* struct inotify_event - structure read from the inotify device for each event
*
* When you are watching a directory, you will receive the filename for events
* such as IN_CREATE, IN_DELETE, IN_OPEN, IN_CLOSE, ..., relative to the wd.
*/
struct inotify_event {
__s32 wd; /* watch descriptor */
__u32 mask; /* watch mask */
__u32 cookie; /* cookie to synchronize two events */
__u32 len; /* length (including nulls) of name */
char name[0]; /* stub for possible name */
};
/* the following are legal, implemented events that user-space can watch for */
#define IN_ACCESS 0x00000001 /* File was accessed */
#define IN_MODIFY 0x00000002 /* File was modified */
#define IN_ATTRIB 0x00000004 /* Metadata changed */
#define IN_CLOSE_WRITE 0x00000008 /* Writtable file was closed */
#define IN_CLOSE_NOWRITE 0x00000010 /* Unwrittable file closed */
#define IN_OPEN 0x00000020 /* File was opened */
#define IN_MOVED_FROM 0x00000040 /* File was moved from X */
#define IN_MOVED_TO 0x00000080 /* File was moved to Y */
#define IN_CREATE 0x00000100 /* Subfile was created */
#define IN_DELETE 0x00000200 /* Subfile was deleted */
#define IN_DELETE_SELF 0x00000400 /* Self was deleted */
#define IN_MOVE_SELF 0x00000800 /* Self was moved */
/* the following are legal events. they are sent as needed to any watch */
#define IN_UNMOUNT 0x00002000 /* Backing fs was unmounted */
#define IN_Q_OVERFLOW 0x00004000 /* Event queued overflowed */
#define IN_IGNORED 0x00008000 /* File was ignored */
/* helper events */
#define IN_CLOSE (IN_CLOSE_WRITE | IN_CLOSE_NOWRITE) /* close */
#define IN_MOVE (IN_MOVED_FROM | IN_MOVED_TO) /* moves */
/* special flags */
#define IN_ONLYDIR 0x01000000 /* only watch the path if it is a directory */
#define IN_DONT_FOLLOW 0x02000000 /* don't follow a sym link */
#define IN_MASK_ADD 0x20000000 /* add to the mask of an already existing watch */
#define IN_ISDIR 0x40000000 /* event occurred against dir */
#define IN_ONESHOT 0x80000000 /* only send event once */
/*
* All of the events - we build the list by hand so that we can add flags in
* the future and not break backward compatibility. Apps will get only the
* events that they originally wanted. Be sure to add new events here!
*/
#define IN_ALL_EVENTS (IN_ACCESS | IN_MODIFY | IN_ATTRIB | IN_CLOSE_WRITE | \
IN_CLOSE_NOWRITE | IN_OPEN | IN_MOVED_FROM | \
IN_MOVED_TO | IN_DELETE | IN_CREATE | IN_DELETE_SELF | \
IN_MOVE_SELF)
/* Flags for sys_inotify_init1. */
#define IN_CLOEXEC O_CLOEXEC
#define IN_NONBLOCK O_NONBLOCK
#ifdef __KERNEL__
#include <linux/dcache.h>
#include <linux/fs.h>
/*
* struct inotify_watch - represents a watch request on a specific inode
*
* h_list is protected by ih->mutex of the associated inotify_handle.
* i_list, mask are protected by inode->inotify_mutex of the associated inode.
* ih, inode, and wd are never written to once the watch is created.
*
* Callers must use the established inotify interfaces to access inotify_watch
* contents. The content of this structure is private to the inotify
* implementation.
*/
struct inotify_watch {
struct list_head h_list; /* entry in inotify_handle's list */
struct list_head i_list; /* entry in inode's list */
atomic_t count; /* reference count */
struct inotify_handle *ih; /* associated inotify handle */
struct inode *inode; /* associated inode */
__s32 wd; /* watch descriptor */
__u32 mask; /* event mask for this watch */
};
struct inotify_operations {
void (*handle_event)(struct inotify_watch *, u32, u32, u32,
const char *, struct inode *);
void (*destroy_watch)(struct inotify_watch *);
};
#ifdef CONFIG_INOTIFY
/* Kernel API for producing events */
extern void inotify_d_instantiate(struct dentry *, struct inode *);
extern void inotify_d_move(struct dentry *);
extern void inotify_inode_queue_event(struct inode *, __u32, __u32,
const char *, struct inode *);
extern void inotify_dentry_parent_queue_event(struct dentry *, __u32, __u32,
const char *);
extern void inotify_unmount_inodes(struct list_head *);
extern void inotify_inode_is_dead(struct inode *);
extern u32 inotify_get_cookie(void);
/* Kernel Consumer API */
extern struct inotify_handle *inotify_init(const struct inotify_operations *);
extern void inotify_init_watch(struct inotify_watch *);
extern void inotify_destroy(struct inotify_handle *);
extern __s32 inotify_find_watch(struct inotify_handle *, struct inode *,
struct inotify_watch **);
extern __s32 inotify_find_update_watch(struct inotify_handle *, struct inode *,
u32);
extern __s32 inotify_add_watch(struct inotify_handle *, struct inotify_watch *,
struct inode *, __u32);
extern __s32 inotify_clone_watch(struct inotify_watch *, struct inotify_watch *);
extern void inotify_evict_watch(struct inotify_watch *);
extern int inotify_rm_watch(struct inotify_handle *, struct inotify_watch *);
extern int inotify_rm_wd(struct inotify_handle *, __u32);
extern void inotify_remove_watch_locked(struct inotify_handle *,
struct inotify_watch *);
extern void get_inotify_watch(struct inotify_watch *);
extern void put_inotify_watch(struct inotify_watch *);
extern int pin_inotify_watch(struct inotify_watch *);
extern void unpin_inotify_watch(struct inotify_watch *);
#else
static inline void inotify_d_instantiate(struct dentry *dentry,
struct inode *inode)
{
}
static inline void inotify_d_move(struct dentry *dentry)
{
}
static inline void inotify_inode_queue_event(struct inode *inode,
__u32 mask, __u32 cookie,
const char *filename,
struct inode *n_inode)
{
}
static inline void inotify_dentry_parent_queue_event(struct dentry *dentry,
__u32 mask, __u32 cookie,
const char *filename)
{
}
static inline void inotify_unmount_inodes(struct list_head *list)
{
}
static inline void inotify_inode_is_dead(struct inode *inode)
{
}
static inline u32 inotify_get_cookie(void)
{
return 0;
}
static inline struct inotify_handle *inotify_init(const struct inotify_operations *ops)
{
return ERR_PTR(-EOPNOTSUPP);
}
static inline void inotify_init_watch(struct inotify_watch *watch)
{
}
static inline void inotify_destroy(struct inotify_handle *ih)
{
}
static inline __s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode,
struct inotify_watch **watchp)
{
return -EOPNOTSUPP;
}
static inline __s32 inotify_find_update_watch(struct inotify_handle *ih,
struct inode *inode, u32 mask)
{
return -EOPNOTSUPP;
}
static inline __s32 inotify_add_watch(struct inotify_handle *ih,
struct inotify_watch *watch,
struct inode *inode, __u32 mask)
{
return -EOPNOTSUPP;
}
static inline int inotify_rm_watch(struct inotify_handle *ih,
struct inotify_watch *watch)
{
return -EOPNOTSUPP;
}
static inline int inotify_rm_wd(struct inotify_handle *ih, __u32 wd)
{
return -EOPNOTSUPP;
}
static inline void inotify_remove_watch_locked(struct inotify_handle *ih,
struct inotify_watch *watch)
{
}
static inline void get_inotify_watch(struct inotify_watch *watch)
{
}
static inline void put_inotify_watch(struct inotify_watch *watch)
{
}
extern inline int pin_inotify_watch(struct inotify_watch *watch)
{
return 0;
}
extern inline void unpin_inotify_watch(struct inotify_watch *watch)
{
}
#endif /* CONFIG_INOTIFY */
#endif /* __KERNEL __ */
#endif /* _LINUX_INOTIFY_H */
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