Re: [PATCH 5/6] seccomp: add a way to attach a filter via eBPF fd

Alexei Starovoitov
2015-09-04T20:40:30Z

On Fri, Sep 04, 2015 at 10:04:23AM -0600, Tycho Andersen wrote:
> This is the final bit needed to support seccomp filters created via the bpf
> syscall.
>
> One concern with this patch is exactly what the interface should look like
> for users, since seccomp()'s second argument is a pointer, we could ask
> people to pass a pointer to the fd, but implies we might write to it which
> seems impolite. Right now we cast the pointer (and force the user to cast
> it), which generates ugly warnings. I'm not sure what the right answer is
> here.

I think passing &fd is fine. setsockopt does similar things.

> -#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC)
> +#define SECCOMP_FILTER_FLAG_MASK (\
> + SECCOMP_FILTER_FLAG_TSYNC | SECCOMP_FILTER_FLAG_EBPF)
>
> #ifdef CONFIG_SECCOMP
>
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 0f238a4..c29a423 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -16,6 +16,7 @@
>
> /* Valid flags for SECCOMP_SET_MODE_FILTER */
> #define SECCOMP_FILTER_FLAG_TSYNC 1
> +#define SECCOMP_FILTER_FLAG_EBPF (1 << 1)
...
> - prepared = seccomp_prepare_user_filter(filter);
> + if (flags & SECCOMP_FILTER_FLAG_EBPF)
> + prepared = seccomp_prepare_ebpf(filter);
> + else
> + prepared = seccomp_prepare_user_filter(filter);
> +

I think instead of flag for existing SECCOMP_SET_MODE_FILTER
command, it would have been cleaner to add new command
SECCOMP_SET_MODE_BPF
and pass &fd to it.
Both kernel implementation and user side would look better ?