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inotify.7: BUGS: Note possible bug triggered by watch descriptor reuse 

Watch descriptor IDs are returned by inotify_add_watch().
When calling inotify_rm_watch() an IN_IGNORE is placed on the
inotify queue pointing to the ID of the removed watch.

inotify_add_watchi() should not return a watch descriptor ID for
which events are still on the queue but should return an
unused ID.

Unfortunately, the existing Kernel code does not provide such a
guarantee.

Actually, in rare cases watch descriptor IDs are returned by
inotify_add_watch() for which events are still on the inotify
queue.

See https://bugzilla.kernel.org/show_bug.cgi?id=77111

Cowritten-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Heinrich Schuchardt 2014-06-22 16:45:23 +02:00 committed by Michael Kerrisk
parent 76eac093e3
commit ec976a53a3
1 changed files with 18 additions and 0 deletions
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@ -754,6 +754,24 @@ if the older had not yet been read)
instead checked if the most recent event could be coalesced with the
.I oldest
unread event.
As of Linux 3.15.1, the following bug exists:
.\" FIXME https://bugzilla.kernel.org/show_bug.cgi?id=77111
When a watch descriptor is removed by calling
.BR inotify_rm_watch (2),
any pending unread events for that watch descriptor remain available to read.
As watch descriptors are subsequently allocated with
.BR inotify_add_watch (2),
the kernel cycles through the range of possible watch descriptors (0 to
.BR INT_MAX )
incrementally.
When allocating a free watch descriptor, no check is made to see whether that
watch descriptor number has any pending unread events in the inotify queue.
Thus, it can happen that a watch descriptor is reallocated even
when pending unread events exist for a previous incarnation of
that watch descriptor number, with the result that the application
might then read those events and interpret them as belonging to
the file associated with the newly recycled watch descriptor.
.SH EXAMPLE
The following program demonstrates the usage of the inotify API.
It marks the directories passed as a command-line arguments