[PATCH 3/4] inotify_user: add system call inotify_add_watch_at()

[Max Kellermann <max.kellermann@ionos.com>]

This implements a missing piece in the inotify API: referring to a
file by a directory file descriptor and a path name.  This can be
solved in userspace currently only by doing something similar to:

  int old = open(".");
  fchdir(dfd);
  inotify_add_watch(....);
  fchdir(old);

Support for LOOKUP_EMPTY is still missing.  We could add another IN_*
flag for that (which would clutter the IN_* flags list further) or
add a "flags" parameter to the new system call (which would however
duplicate features already present via special IN_* flags).

To: Jan Kara <jack@suse.cz>
Cc: Amir Goldstein <amir73il@gmail.com>
To: linux-fsdevel@vger.kernel.org
To: linux-kernel@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 fs/notify/inotify/inotify_user.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index b6e6f6ab21f8..8a9096c5ebb1 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -797,6 +797,12 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname,
 	return do_inotify_add_watch(fd, AT_FDCWD, pathname, mask);
 }
 
+SYSCALL_DEFINE4(inotify_add_watch_at, int, fd, int, dfd, const char __user *, pathname,
+		u32, mask)
+{
+	return do_inotify_add_watch(fd, dfd, pathname, mask);
+}
+
 SYSCALL_DEFINE2(inotify_rm_watch, int, fd, __s32, wd)
 {
 	struct fsnotify_group *group;
-- 
2.39.2