Re: fanotify man pages for review

[Michael Kerrisk (man-pages)]

On 05/05/2014 08:07 PM, Jan Kara wrote:
>   Hello,
> 
>   below are some minor comments regarding the manpages.
> 
>> diff --git a/man2/fanotify_mark.2 b/man2/fanotify_mark.2
>> new file mode 100644
>> index 0000000..693eff8
>> --- /dev/null
>> +++ b/man2/fanotify_mark.2
> ...
>> +.B FAN_MARK_IGNORED_SURV_MODIFY
>> +The ignore mask shall survive modify events.
>> +If this flag is not set, the ignore mask is cleared when a modify event occurs
>> +for the ignored file or directory.
>   This is correct but maybe we should explicitely stress that the modify
> event doesn't have to be delivered to the fanotify group? I.e. the
> clearing happens even if the mark doesn't have FAN_MODIFY set... But maybe
> it would just unecessarily complicate the explanation. Just a suggestion.

I think it's an important detail, worth clarifying.

>> diff --git a/man7/fanotify.7 b/man7/fanotify.7
>> new file mode 100644
>> index 0000000..083244f
>> --- /dev/null
>> +++ b/man7/fanotify.7
> ...
>> +A possible usage of the ignore mask is for a file cache.
>> +Events of interest for a file cache are modification of a file and closing
>> +of the same.
>> +Hence, the cached directory or mount point is to be marked to receive these
>> +events.
>> +After receiving the first event informing that a file has been modified, the
>> +corresponding cache entry will be invalidated.
>> +No further modification events for this file are of interest until the file is
>> +closed.
>> +Hence, the modify event can be added to the ignore mask.
>> +Upon receiving the closed event, the modify event can be removed from the
>                       ^^^^ close

Thanks. Fixed.

>> +ignore mask and the file cache entry can be updated.
>> +.PP
>> +The entries in the fanotify notification groups refer to files and directories
>> +via their inode number and to mounts via their mount ID.
>> +If files or directories are renamed or moved, the respective entries survive.
>> +If files or directories are deleted or mounts are unmounted, the corresponding
>> +entries are deleted.
>> +.SS The event queue
>> +As events occur on the filesystem objects monitired by a notification group,
                                              ^^^ monitored
Already fixed.
 
>> +the fanotify system generates events that are collected in a queue.
>> +These events can then be read (using
>> +.BR read (2)
>> +or similar)
>> +from the fanotify file descriptor
>> +returned by
>> +.BR fanotify_init (2).
>> +
>> +Two types of events are generated:
>> +notification events and permission events.
>> +Notification events are merely informative
>> +and require no action to be taken by
>> +the receiving application except for closing the file descriptor passed in the
>> +event.
>> +Permission events are requests to the receiving application to decide whether
>> +permission for a file access shall be granted.
>> +For these events, the recipient must write a response which decides whether
>> +access is granted or not.
>> +
>> +Queue entries for notification events are removed when the event has been
>> +read.
>> +Queue entries for permission events are removed when the permission
>> +decision has been taken by writing to the fanotify file descriptor.
>   This is true however it may be slightly misleading - the permission
> events are removed from the queue as soon as they are read. But they are
> destroyed only after the permission response is received. So from
> userspace POV dequeued events are invisible...

Probably also worth clarifying.

>> +A program listening to fanotify events can compare this PID to the PID returned
>> +by
>> +.BR getpid (2),
>> +to determine whether the event is caused by the listener itself, or is due to a
>> +file access by another program.
>> +.PP
>> +The bit mask in
>> +.I mask
>> +signals which events have occurred for a single filesystem object.
>> +Multiple bits may be set in this mask,
>> +if more than one event occurred for the monitored filesystem obect.
>                                                                 ^^ object

Already fixed.

>> +.B ENOENT
>> +The file descriptor
>> +.I fd
>> +in the response structure is not valid.
>> +This might occur because the file was already deleted by another thread or
>> +process.
>   This isn't quite true. Another thread of the same process may have already
> closed the 'fd'. But it has nothing to do with a file being deleted.
> fanotify uses file descriptors exactly so that renames or deletes don't
> make the event invalid.

Okay -- also needs fixing.

Heinrich, do you want to patch the above open points, or shall I take a shot?

Thanks,

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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