[PATCHv6 5/6] fanotify: enable close-on-exec on events' fd when requested in fanotify_init()

[Yann Droneaud]

According to commit 80af258867648, file descriptor 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().

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).

Reading article "Botching up ioctls"[1] by Daniel Vetter might make one
feel uncomfortable when trying to add extension to an API that doesn't
check for unrecognized values.

But it seems logical to set close-on-exec flag on the file descriptor
if userspace is allowed to request it with O_CLOEXEC.

In fact, according to some lookup on http://codesearch.debian.net/ and
various search engine, there's already some userspace code requesting it:

- in systemd's readahead[2]:

    fanotify_fd = fanotify_init(FAN_CLOEXEC|FAN_NONBLOCK, O_RDONLY|O_LARGEFILE|O_CLOEXEC|O_NOATIME);

- in clsync[3]:

    #define FANOTIFY_EVFLAGS (O_LARGEFILE|O_RDONLY|O_CLOEXEC)

    int fanotify_d = fanotify_init(FANOTIFY_FLAGS, FANOTIFY_EVFLAGS);

- in examples [4] from "Filesystem monitoring in the Linux kernel"
  article[5] by Aleksander Morgado:

    if ((fanotify_fd = fanotify_init (FAN_CLOEXEC,
                                      O_RDONLY | O_CLOEXEC | O_LARGEFILE)) < 0)

Lookup also returned some wrong usage of the syscall:

- in Gonk HAL from Mozilla Firefox OS sources[6]:

    mFd = fanotify_init(FAN_CLASS_NOTIF, FAN_CLOEXEC);

Adding support for O_CLOEXEC in fanotify won't magically enable it for
Gonk since FAN_CLOEXEC is defined as 0x1, which is likely equal to
O_WRONLY when used in open flag context. In the other hand, it won't
hurt it either.

So this patch replaces call to macro get_unused_fd() by a call to
function get_unused_fd_flags() with event_f_flags value as argument.
This way O_CLOEXEC flag in the second argument of fanotify_init(2)
syscall is interpreted so that close-on-exec get enabled.

[1] http://blog.ffwll.ch/2013/11/botching-up-ioctls.html
[2] http://cgit.freedesktop.org/systemd/systemd/tree/src/readahead/readahead-collect.c?id=v208#n294
[3] https://github.com/xaionaro/clsync/blob/v0.2.1/sync.c#L1631
    https://github.com/xaionaro/clsync/blob/v0.2.1/configuration.h#L38
[4] http://www.lanedo.com/~aleksander/fanotify/fanotify-example.c
[5] http://www.lanedo.com/2013/filesystem-monitoring-linux-kernel/
[6] http://hg.mozilla.org/mozilla-central/file/325c74addeba/hal/gonk/GonkDiskSpaceWatcher.cpp#l167

Link: http://lkml.kernel.org/r/cover.1394532336.git.ydroneaud@xxxxxxxxxx
Cc: Eric Paris <eparis@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yann Droneaud <ydroneaud@xxxxxxxxxx>
---
 fs/notify/fanotify/fanotify_user.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 4e565c814309..31bc27ed5bcf 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -65,7 +65,7 @@ static int create_fd(struct fsnotify_group *group,
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 
-	client_fd = get_unused_fd();
+	client_fd = get_unused_fd_flags(group->fanotify_data.f_flags);
 	if (client_fd < 0)
 		return client_fd;
 
-- 
1.8.5.3

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