Re: [PATCH 1/6] ebpf: add a seccomp program type

Alexei Starovoitov
2015-09-04T20:17:53Z

On Fri, Sep 04, 2015 at 10:04:19AM -0600, Tycho Andersen wrote:
> seccomp uses eBPF as its underlying storage and execution format, and eBPF
> has features that seccomp would like to make use of in the future. This
> patch adds a formal seccomp type to the eBPF verifier.
>
> The current implementation of the seccomp eBPF type is very limited, and
> doesn't support some interesting features (notably, maps) of eBPF. However,
> the primary motivation for this patchset is to enable checkpoint/restore
> for seccomp filters later in the series, to this limited feature set is ok
> for now.

yes. good compromise to start.

> +static const struct bpf_func_proto *
> +seccomp_func_proto(enum bpf_func_id func_id)
> +{
> + /* Right now seccomp eBPF loading doesn't support maps; seccomp filters
> + * are considered to be read-only after they're installed, so map fds
> + * probably need to be invalidated when a seccomp filter with maps is
> + * installed.

Just disabling bpf_map_lookup/update() helpers (the way you did here)
is enough. The prorgram can still have references to maps, but since they
won't be accessed it's safe.

> + *
> + * The rest of these might be reasonable to call from seccomp, so we
> + * export them.
> + */
> + switch (func_id) {
> + case BPF_FUNC_ktime_get_ns:
> + return &bpf_ktime_get_ns_proto;
> + case BPF_FUNC_trace_printk:
> + return bpf_get_trace_printk_proto();
> + case BPF_FUNC_get_prandom_u32:
> + return &bpf_get_prandom_u32_proto;
> + case BPF_FUNC_get_smp_processor_id:
> + return &bpf_get_smp_processor_id_proto;
> + case BPF_FUNC_tail_call:
> + return &bpf_tail_call_proto;
> + case BPF_FUNC_get_current_pid_tgid:
> + return &bpf_get_current_pid_tgid_proto;
> + case BPF_FUNC_get_current_uid_gid:
> + return &bpf_get_current_uid_gid_proto;
> + case BPF_FUNC_get_current_comm:
> + return &bpf_get_current_comm_proto;

the list looks good to start with.

>
> +static u32 seccomp_convert_ctx_access(enum bpf_access_type type, int dst_reg,
> + int src_reg, int ctx_off,
> + struct bpf_insn *insn_buf)
> +{
> + struct bpf_insn *insn = insn_buf;
> +
> + switch (ctx_off) {
> + case offsetof(struct seccomp_data, nr):

the conversion of seccomp_data fields is unnecessary.
We're doing conversion for sk_buff, because sk_buff and __sk_buff aree two
different structures. __sk_buff is user ABI with its own fields that losely
correspond to in-kernel struct sk_buff.
seccomp_data is already part of user ABI, so it's ok to access as-is.