On Tue, Oct 08, 2024 at 07:42:51AM GMT, Jeff Layton wrote:

On Tue, 2024-10-08 at 13:30 +0200, Christian Brauner wrote:

...

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.

Consistency is good, so:

Reviewed-by: Jeff Layton <jlayton@kernel.org>

...that said, we should document that -EBADF means that at least one of the fd's is bogus, but this API doesn't tell you which ones those are. To figure that out, I guess you'd need to do something like issue F_GETFD against each and see which ones return -EBADF?

It's actually worse because fcntl() can also give you EBADF if you have an O_PATH file descriptor and you request an option that won't work on an O_PATH file descriptor. It's complete nonsense.

So the most reliable way to figure out whether the fd is valid, is to use a really really old fcntl() like idk F_GETFD and call it. Because that should always work (ignoring really stupid things such as using seccomp or an LSM to block F_GETFD) and if you get EBADF it must be because the file descriptor isn't valid. Obviously that's racy if the fdtable is shared but I don't think it's a big problem.

So if you get EBADF from F_DUPFD_QUERY and you really really need to know whether the kernel supports it or any of the two fds was invalid then yes, you need to follow this up with a F_GETFD. Again, racy but won't matter most of the time.

Really, we should have returned something like EOPNOTSUPP from fcntl() for the O_PATH case that would've meant that it's easy to detect new flags.