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Date: Thu, 6 Sep 2018 17:18:06 -0700
From: Petar Penkov <ppenkov@...gle.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Petar Penkov <peterpenkov96@...il.com>,
Networking <netdev@...r.kernel.org>,
"David S . Miller" <davem@...emloft.net>,
Alexei Starovoitov <ast@...nel.org>,
simon.horman@...ronome.com, ecree@...arflare.com,
songliubraving@...com, Tom Herbert <tom@...bertland.com>,
Willem de Bruijn <willemb@...gle.com>
Subject: Re: [bpf-next 1/3] flow_dissector: implements flow dissector BPF hook
On Mon, Sep 3, 2018 at 1:54 PM, Petar Penkov <ppenkov@...gle.com> wrote:
>
> On Sun, Sep 2, 2018 at 2:03 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> > On 08/30/2018 08:22 PM, Petar Penkov wrote:
> >> From: Petar Penkov <ppenkov@...gle.com>
> >>
> >> Adds a hook for programs of type BPF_PROG_TYPE_FLOW_DISSECTOR and
> >> attach type BPF_FLOW_DISSECTOR that is executed in the flow dissector
> >> path. The BPF program is per-network namespace.
> >>
> >> Signed-off-by: Petar Penkov <ppenkov@...gle.com>
> >> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > [...]
> >> + err = check_flow_keys_access(env, off, size);
> >> + if (!err && t == BPF_READ && value_regno >= 0)
> >> + mark_reg_unknown(env, regs, value_regno);
> >> } else {
> >> verbose(env, "R%d invalid mem access '%s'\n", regno,
> >> reg_type_str[reg->type]);
> >> @@ -1925,6 +1954,8 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
> >> case PTR_TO_PACKET_META:
> >> return check_packet_access(env, regno, reg->off, access_size,
> >> zero_size_allowed);
> >> + case PTR_TO_FLOW_KEYS:
> >> + return check_flow_keys_access(env, reg->off, access_size);
> >> case PTR_TO_MAP_VALUE:
> >> return check_map_access(env, regno, reg->off, access_size,
> >> zero_size_allowed);
> >> @@ -3976,6 +4007,7 @@ static bool may_access_skb(enum bpf_prog_type type)
> >> case BPF_PROG_TYPE_SOCKET_FILTER:
> >> case BPF_PROG_TYPE_SCHED_CLS:
> >> case BPF_PROG_TYPE_SCHED_ACT:
> >> + case BPF_PROG_TYPE_FLOW_DISSECTOR:
> >> return true;
> >
> > This one should not be added here. It would allow for LD_ABS to be used, but
> > you already have direct packet access as well as bpf_skb_load_bytes() helper
> > enabled. Downside on LD_ABS is that error path will exit the BPF prog with
> > return 0 for historical reasons w/o user realizing (here: to BPF_OK mapping).
> > So we should not encourage use of LD_ABS/IND anymore in eBPF context and
> > avoid surprises.
> >
> >> default:
> >> return false;
> >> @@ -4451,6 +4483,7 @@ static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
> >> case PTR_TO_CTX:
> >> case CONST_PTR_TO_MAP:
> >> case PTR_TO_PACKET_END:
> >> + case PTR_TO_FLOW_KEYS:
> >> /* Only valid matches are exact, which memcmp() above
> >> * would have accepted
> >> */
> >> diff --git a/net/core/filter.c b/net/core/filter.c
> >> index c25eb36f1320..0143b9c0c67e 100644
> >> --- a/net/core/filter.c
> >> +++ b/net/core/filter.c
> >> @@ -5092,6 +5092,17 @@ sk_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >> }
> >> }
> >>
> >> +static const struct bpf_func_proto *
> >> +flow_dissector_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >> +{
> >> + switch (func_id) {
> >> + case BPF_FUNC_skb_load_bytes:
> >> + return &bpf_skb_load_bytes_proto;
> >
> > Probably makes sense to also enable bpf_skb_pull_data helper for direct packet
> > access use to fetch non-linear data from here once.
On closer look, it turns out that __skb_flow_dissect takes a const skb
pointer, which conflicts with the realloc in bpf_skb_pull_data. But,
we also don't need it if I change bpf_flow_dissect_get_header to try
bpf_skb_load_bytes when direct packet access fails. This is very
similar to the existing use of __skb_header_pointer. See the below
snippet:
static __always_inline void *bpf_flow_dissect_get_header(struct __sk_buff *skb,
__u16 hdr_size,
void *buffer)
{
struct bpf_dissect_cb *cb = (struct bpf_dissect_cb *)(skb->cb);
void *data_end = (__u8 *)(long)skb->data_end;
void *data = (__u8 *)(long)skb->data;
__u8 *hdr;
/* Verifies this variable offset does not overflow */
if (cb->nhoff > (USHRT_MAX - hdr_size))
return NULL;
hdr = data + cb->nhoff;
if (hdr + hdr_size <= data_end)
return hdr;
if (bpf_skb_load_bytes(skb, cb->nhoff, buffer, hdr_size))
return NULL;
return buffer;
}
> >
> >> + default:
> >> + return bpf_base_func_proto(func_id);
> >> + }
> >> +}
> >> +
> >> static const struct bpf_func_proto *
> >> lwt_out_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >> {
> >> @@ -5207,6 +5218,7 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
> >> case bpf_ctx_range(struct __sk_buff, data):
> >> case bpf_ctx_range(struct __sk_buff, data_meta):
> >> case bpf_ctx_range(struct __sk_buff, data_end):
> >> + case bpf_ctx_range(struct __sk_buff, flow_keys):
> >> if (size != size_default)
> >> return false;
> >> break;
> >> @@ -5235,6 +5247,7 @@ static bool sk_filter_is_valid_access(int off, int size,
> >> case bpf_ctx_range(struct __sk_buff, data):
> >> case bpf_ctx_range(struct __sk_buff, data_meta):
> >> case bpf_ctx_range(struct __sk_buff, data_end):
> >> + case bpf_ctx_range(struct __sk_buff, flow_keys):
> >> case bpf_ctx_range_till(struct __sk_buff, family, local_port):
> > [...]
> > Thanks,
> > Daniel
>
> Thank you for your feedback, Daniel! I'll make these changes and submit a v2.
> Petar
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