bpf: allow programs to write to certain skb fields

allow programs read/write skb->mark, tc_index fields and
((struct qdisc_skb_cb *)cb)->data.

mark and tc_index are generically useful in TC.
cb[0]-cb[4] are primarily used to pass arguments from one
program to another called via bpf_tail_call() which can
be seen in sockex3_kern.c example.

All fields of 'struct __sk_buff' are readable to socket and tc_cls_act progs.
mark, tc_index are writeable from tc_cls_act only.
cb[0]-cb[4] are writeable by both sockets and tc_cls_act.

Add verifier tests and improve sample code.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index ca854e5bb2f7..2235aee8096a 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -105,7 +105,8 @@ struct bpf_verifier_ops {
 	 */
 	bool (*is_valid_access)(int off, int size, enum bpf_access_type type);
 
-	u32 (*convert_ctx_access)(int dst_reg, int src_reg, int ctx_off,
+	u32 (*convert_ctx_access)(enum bpf_access_type type, int dst_reg,
+				  int src_reg, int ctx_off,
 				  struct bpf_insn *insn);
 };