Re: [PATCH] fcntl: make F_DUPFD_QUERY associative

[Jeff Layton <jlayton@kernel.org>]

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?