[PATCH] fcntl: make F_DUPFD_QUERY associative

[Christian Brauner <brauner@kernel.org>]

Currently when passing a closed file descriptor to
fcntl(fd, F_DUPFD_QUERY, fd_dup) the order matters:

    fd = open("/dev/null");
    fd_dup = dup(fd);

When we now close one of the file descriptors we get:

    (1) fcntl(fd, fd_dup) // -EBADF
    (2) fcntl(fd_dup, fd) // 0 aka not equal

depending on which file descriptor is passed first. That's not a huge
deal but it gives the api I slightly weird feel. Make it so that the
order doesn't matter by requiring that both file descriptors are valid:

(1') fcntl(fd, fd_dup) // -EBADF
(2') fcntl(fd_dup, fd) // -EBADF

Fixes: c62b758bae6a ("fcntl: add F_DUPFD_QUERY fcntl()")
Cc: <stable@vger.kernel.org>
Reported-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/fcntl.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 22dd9dcce7ec..3d89de31066a 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -397,6 +397,9 @@ static long f_dupfd_query(int fd, struct file *filp)
 {
 	CLASS(fd_raw, f)(fd);
 
+	if (fd_empty(f))
+		return -EBADF;
+
 	/*
 	 * We can do the 'fdput()' immediately, as the only thing that
 	 * matters is the pointer value which isn't changed by the fdput.
-- 
2.45.2