[PATCH] inotify: fix event loss on hardlinked files

People have run into a problem when they do this:

watch (file1, all_events);
watch (file2, some_events);

if file2 is a hard link to file1, some events will be missed because by
default we replace the mask.  The patch below adds a flag IN_MASK_ADD which
will cause inotify to add to the existing mask if present.

Signed-off-by: John McCutchan <ttb@tentacle.dhs.org>
Signed-off-by: Robert Love <rml@novell.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 9 insertions, 1 deletions

diff --git a/fs/inotify.c b/fs/inotify.c
index 2fd97ef547ff..a37e9fb1da58 100644
--- a/fs/inotify.c
+++ b/fs/inotify.c
@@ -931,6 +931,7 @@ asmlinkage long sys_inotify_add_watch(int fd, const char __user *path, u32 mask)
         struct nameidata nd;
         struct file *filp;
         int ret, fput_needed;
+        int mask_add = 0;
 
         filp = fget_light(fd, &fput_needed);
         if (unlikely(!filp))
@@ -953,6 +954,9 @@ asmlinkage long sys_inotify_add_watch(int fd, const char __user *path, u32 mask)
         down(&inode->inotify_sem);
         down(&dev->sem);
 
+        if (mask & IN_MASK_ADD)
+                mask_add = 1;
+
         /* don't let user-space set invalid bits: we don't want flags set */
         mask &= IN_ALL_EVENTS;
         if (unlikely(!mask)) {
@@ -966,7 +970,10 @@ asmlinkage long sys_inotify_add_watch(int fd, const char __user *path, u32 mask)
          */
         old = inode_find_dev(inode, dev);
         if (unlikely(old)) {
-                old->mask = mask;
+                if (mask_add)
+                        old->mask |= mask;
+                else
+                        old->mask = mask;
                 ret = old->wd;
                 goto out;
         }
diff --git a/include/linux/inotify.h b/include/linux/inotify.h
index 93bb3afe646b..ee5b239092ed 100644
--- a/include/linux/inotify.h
+++ b/include/linux/inotify.h
@@ -47,6 +47,7 @@ struct inotify_event {
 #define IN_MOVE                 (IN_MOVED_FROM | IN_MOVED_TO) /* moves */
 
 /* special flags */
+#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 */