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@xxxxxxxxxxxxxxx>
> Reported-by: Lennart Poettering <lennart@xxxxxxxxxxxxxx>
> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
...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?
