Re: [PATCH 1/1] inotify.7: Bug 77111 - watch descriptor reuse

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

[Adding Jan and Eric to the CC, in case they want to comment.]

Background is bugs https://bugzilla.kernel.org/show_bug.cgi?id=76851
and https://bugzilla.kernel.org/show_bug.cgi?id=77111 . The point is:

1. When an inotify watch descriptor is removed, pending unread 
   events remain pending.
2. When allocating a new watch descriptor, a past WD may 
   be recycled.
3. In theory, it could happen that events left over at 1 could
   be interpreted as though they belonged to the filesystem
   object watch in step 2.

On 05/29/2014 07:39 PM, Heinrich Schuchardt wrote:
> 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_watch 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.

It's unfortunate, but I'm not sure that it's very serious. I mean, in 
order to trigger this bug you need to

0. Remove your watch descriptor (wd1),
1. Leave some unread events for wd1 on the queue. and in the meantime,
2. Cycle through INT_MAX watch descriptors until you reuse wd1.

Unless I'm missing something, the chances of that are vanishingly small.
However, that's not to say that the issue shouldn't be documented. Rather,
if the issue is as unlikely to be hit as it appears to me in the above
formulation, then I thing the tome of the write-up should indicate
that the problem is more theoretical than practical. Perhaps Jan or Eric
has a comment about this.

> Actually in rare cases watch descriptor IDs are returned by inotify_add_watch
> for which events are still on the inotify queue.
> 
> cf. https://bugzilla.kernel.org/show_bug.cgi?id=77111
> 
> Signed-off-by: Heinrich Schuchardt <xypron.glpk-Mmb7MZpHnFY@public.gmane.org>
> ---
>  man7/inotify.7 | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/man7/inotify.7 b/man7/inotify.7
> index 1fc83f8..b1c2f6f 100644
> --- a/man7/inotify.7
> +++ b/man7/inotify.7
> @@ -728,6 +728,18 @@ and the accompanying
>  event might be fetched only on the next
>  .BR read (2).
>  .SH BUGS
> +As of Linux 3.14,
> +the following bug exists:
> +.IP * 3
> +.\" FIXME https://bugzilla.kernel.org/show_bug.cgi?id=77111
> +.BR inotify_add_watch (2)
> +may return a watch descriptor ID released by a prior call to
> +.BR inotify_rm_watch (2)
> +even if events for this watch descriptor still exist on the inotify queue.
> +As a workaround the inotify file descriptor can be read until the queue is
> +empty before calling
> +.BR inotify_add_watch (2).
> +.PP

Perhaps a wording something like this is more appropriate:

[[
As at Linux 3.15, the a possible bug exists in the following scenario:

1. When a watch descriptor is removed, any pending unread events for 
   that watch descriptor remain available to read. 
2. As watch descriptors are subsequently allocated with 
   inotify_ad_watch(), the kernel cycles through the range of possible
   watch descriptors 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.
3. 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 recycle watch descriptor.

However, this bug is perhaps more theoretical, than practical.
The range for watch descriptors is from 0 to INT_MAX, so that in
order to trigger the bug, an application must leave unread events
on the inotify queue while recycling INT_MAX watch descriptors.
]]

>  .\" FIXME kernel commit 611da04f7a31b2208e838be55a42c7a1310ae321
>  .\" implies that unmount events were buggy 2.6.11 to 2.6.36
>  .\"
> @@ -745,7 +757,7 @@ However, as an unintended effect of other changes,
>  since Linux 2.6.36, an
>  .B IN_IGNORED
>  event is generated in this case.
> -
> +.PP

(Not sure why you added this piece?)

>  Before kernel 2.6.25,
>  .\" commit 1c17d18e3775485bf1e0ce79575eb637a94494a2
>  the kernel code that was intended to coalesce successive identical events

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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