Re: [PATCH v6 4/5] seccomp: add support for passing fds via USER_NOTIF

[Jann Horn]

On Thu, Sep 6, 2018 at 5:29 PM Tycho Andersen <tycho@xxxxxxxx> wrote:
> The idea here is that the userspace handler should be able to pass an fd
> back to the trapped task, for example so it can be returned from socket().
[...]
> diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
> index d1498885c1c7..1c0aab306426 100644
> --- a/Documentation/userspace-api/seccomp_filter.rst
> +++ b/Documentation/userspace-api/seccomp_filter.rst
> @@ -235,6 +235,9 @@ The interface for a seccomp notification fd consists of two structures:
>          __u64 id;
>          __s32 error;
>          __s64 val;
> +        __u8 return_fd;
> +        __u32 fd;
> +        __u32 fd_flags;

Normally,  syscalls that take an optional file descriptor accept a
signed 32-bit number, with -1 standing for "no file descriptor". Is
there a reason why this uses a separate variable to signal whether an
fd was provided?

Apart from that, this patch looks good to me.