linux-kernel - Re: [PATCHv8 1/6] fanotify: enable close-on-exec on events' fd when requested in fanotify

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Date:	Thu, 25 Sep 2014 22:57:23 +0200
From:	Heinrich Schuchardt <xypron.debian@....de>
To:	Yann Droneaud <ydroneaud@...eya.com>
CC:	Eric Paris <eparis@...hat.com>,
	Richard Guy Briggs <rgb@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	stable@...r.kernel.org, linux-api@...r.kernel.org,
	Jan Kara <jack@...e.cz>
Subject: Re: [PATCHv8 1/6] fanotify: enable close-on-exec on events' fd when
 requested in fanotify_init()

On 24.09.2014 15:11, Yann Droneaud wrote:
> According to commit 80af258867648 ('fanotify: groups can specify
> their f_flags for new fd'), file descriptors created as part of
> file access notification events inherit flags from the
> event_f_flags argument passed to syscall fanotify_init(2).
>
> So while it is legal for userspace to call fanotify_init() with
> O_CLOEXEC as part of its second argument, O_CLOEXEC is currently
> silently ignored.
>
> Indeed event_f_flags are only given to dentry_open(), which only
> seems to care about O_ACCMODE and O_PATH in do_dentry_open(),
> O_DIRECT in open_check_o_direct() and O_LARGEFILE in
> generic_file_open().

I tested on kernel 3.17.0-rc5. I passed O_CLOEXEC in event_f_flags.
When I called fcnt(event_metadata->fd, F_GETFD) it did not return 
FD_CLOEXEC. So I can confirm your observation that O_CLOEXEC is not 
working as expected.

I found this definition
#define get_unused_fd() get_unused_fd_flags(0)

So essentially when get_unused_fd() is called for a fanotify event 
O_CLOEXEC is ignored.

This is what your patch fixes.

>
> More, there's no effective check on event_f_flags value that
> would catch unknown / unsupported values, unlike the one on
> f_flags argument of the syscall (see FAN_ALL_INIT_FLAGS in
> include/uapi/linux/fanotify.h).

The fanotify_init(2) man page describes which flags are allowable in 
event_f_flags.

Could you, please, explain why the following code in fanotify_user.c is 
not to be considered an effective check:

     if (event_f_flags & ~FANOTIFY_INIT_ALL_EVENT_F_BITS)
         return -EINVAL;

     switch (event_f_flags & O_ACCMODE) {
     case O_RDONLY:
     case O_RDWR:
     case O_WRONLY:
         break;
     default:
         return -EINVAL;
     }

I CC Jan Kara as he reviewed the code.

Best regards

Heinrich Schuchardt
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