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Date: Wed, 15 Jul 2020 19:23:09 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
kernel-team <kernel-team@...udflare.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Marek Majkowski <marek@...udflare.com>
Subject: Re: [PATCH bpf-next v4 04/16] inet: Run SK_LOOKUP BPF program on
socket lookup
On Mon, Jul 13, 2020 at 10:47 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> Run a BPF program before looking up a listening socket on the receive path.
> Program selects a listening socket to yield as result of socket lookup by
> calling bpf_sk_assign() helper and returning SK_PASS code. Program can
> revert its decision by assigning a NULL socket with bpf_sk_assign().
>
> Alternatively, BPF program can also fail the lookup by returning with
> SK_DROP, or let the lookup continue as usual with SK_PASS on return, when
> no socket has not been selected with bpf_sk_assign(). Other return values
you probably meant "no socket has been selected"?
> are treated the same as SK_DROP.
Why not enforce it instead? Check check_return_code() in verifier.c,
it's trivial to do it for SK_LOOKUP.
>
> This lets the user match packets with listening sockets freely at the last
> possible point on the receive path, where we know that packets are destined
> for local delivery after undergoing policing, filtering, and routing.
>
> With BPF code selecting the socket, directing packets destined to an IP
> range or to a port range to a single socket becomes possible.
>
> In case multiple programs are attached, they are run in series in the order
> in which they were attached. The end result is determined from return codes
> of all the programs according to following rules:
>
> 1. If any program returned SK_PASS and selected a valid socket, the socket
> is used as result of socket lookup.
> 2. If more than one program returned SK_PASS and selected a socket,
> last selection takes effect.
> 3. If any program returned SK_DROP or an invalid return code, and no
> program returned SK_PASS and selected a socket, socket lookup fails
> with -ECONNREFUSED.
> 4. If all programs returned SK_PASS and none of them selected a socket,
> socket lookup continues to htable-based lookup.
>
> Suggested-by: Marek Majkowski <marek@...udflare.com>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>
> Notes:
> v4:
> - Reduce BPF sk_lookup prog return codes to SK_PASS/SK_DROP. (Lorenz)
your description above still assumes prog can return something besides
SK_PASS and SK_DROP?
> - Default to drop & warn on illegal return value from BPF prog. (Lorenz)
> - Rename netns_bpf_attach_type_enable/disable to _need/unneed. (Lorenz)
> - Export bpf_sk_lookup_enabled symbol for CONFIG_IPV6=m (kernel test robot)
> - Invert return value from bpf_sk_lookup_run_v4 to true on skip reuseport.
> - Move dedicated prog_array runner close to its callers in filter.h.
>
> v3:
> - Use a static_key to minimize the hook overhead when not used. (Alexei)
> - Adapt for running an array of attached programs. (Alexei)
> - Adapt for optionally skipping reuseport selection. (Martin)
>
> include/linux/filter.h | 102 +++++++++++++++++++++++++++++++++++++
> kernel/bpf/net_namespace.c | 32 +++++++++++-
> net/core/filter.c | 3 ++
> net/ipv4/inet_hashtables.c | 31 +++++++++++
> 4 files changed, 167 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 380746f47fa1..b9ad0fdabca5 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -1295,4 +1295,106 @@ struct bpf_sk_lookup_kern {
> bool no_reuseport;
> };
>
> +extern struct static_key_false bpf_sk_lookup_enabled;
> +
> +/* Runners for BPF_SK_LOOKUP programs to invoke on socket lookup.
> + *
> + * Allowed return values for a BPF SK_LOOKUP program are SK_PASS and
> + * SK_DROP. Any other return value is treated as SK_DROP. Their
> + * meaning is as follows:
> + *
> + * SK_PASS && ctx.selected_sk != NULL: use selected_sk as lookup result
> + * SK_PASS && ctx.selected_sk == NULL: continue to htable-based socket lookup
> + * SK_DROP : terminate lookup with -ECONNREFUSED
> + *
> + * This macro aggregates return values and selected sockets from
> + * multiple BPF programs according to following rules:
> + *
> + * 1. If any program returned SK_PASS and a non-NULL ctx.selected_sk,
> + * macro result is SK_PASS and last ctx.selected_sk is used.
> + * 2. If any program returned non-SK_PASS return value,
> + * macro result is the last non-SK_PASS return value.
> + * 3. Otherwise result is SK_PASS and ctx.selected_sk is NULL.
> + *
> + * Caller must ensure that the prog array is non-NULL, and that the
> + * array as well as the programs it contains remain valid.
> + */
> +#define BPF_PROG_SK_LOOKUP_RUN_ARRAY(array, ctx, func) \
> + ({ \
> + struct bpf_sk_lookup_kern *_ctx = &(ctx); \
> + struct bpf_prog_array_item *_item; \
> + struct sock *_selected_sk; \
> + struct bpf_prog *_prog; \
> + u32 _ret, _last_ret; \
> + bool _no_reuseport; \
> + \
> + migrate_disable(); \
> + _last_ret = SK_PASS; \
> + _selected_sk = NULL; \
> + _no_reuseport = false; \
these three could be moved before migrate_disable(), or even better
just initialize corresponding variables above?
> + _item = &(array)->items[0]; \
> + while ((_prog = READ_ONCE(_item->prog))) { \
> + /* restore most recent selection */ \
> + _ctx->selected_sk = _selected_sk; \
> + _ctx->no_reuseport = _no_reuseport; \
> + \
> + _ret = func(_prog, _ctx); \
> + if (_ret == SK_PASS) { \
> + /* remember last non-NULL socket */ \
> + if (_ctx->selected_sk) { \
> + _selected_sk = _ctx->selected_sk; \
> + _no_reuseport = _ctx->no_reuseport; \
> + } \
> + } else { \
> + /* remember last non-PASS ret code */ \
> + _last_ret = _ret; \
> + } \
> + _item++; \
> + } \
> + _ctx->selected_sk = _selected_sk; \
> + _ctx->no_reuseport = _no_reuseport; \
> + migrate_enable(); \
> + _ctx->selected_sk ? SK_PASS : _last_ret; \
> + })
> +
[...]
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