Re: [PATCH v2 net-next 1/2] bpf: allow extended BPF programs access skb fields
From: Daniel Borkmann <hidden>
Date: 2015-03-14 01:46:42
Also in:
lkml, netdev
On 03/13/2015 07:57 PM, Alexei Starovoitov wrote:
introduce user accessible mirror of in-kernel 'struct sk_buff':
struct __sk_buff {
__u32 len;
__u32 pkt_type;
__u32 mark;
__u32 queue_mapping;
};
bpf programs can do:
int bpf_prog(struct __sk_buff *skb)
{
__u32 var = skb->pkt_type;
which will be compiled to bpf assembler as:
dst_reg = *(u32 *)(src_reg + 4) // 4 == offsetof(struct __sk_buff, pkt_type)
bpf verifier will check validity of access and will convert it to:
dst_reg = *(u8 *)(src_reg + offsetof(struct sk_buff, __pkt_type_offset))
dst_reg &= 7
since skb->pkt_type is a bitfield.
Signed-off-by: Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/w@public.gmane.org>...
quoted hunk ↗ jump to hunk
+static u32 convert_skb_access(int skb_field, int dst_reg, int src_reg, + struct bpf_insn *insn_buf) +{ + struct bpf_insn *insn = insn_buf; + + switch (skb_field) { + case SKF_AD_MARK: + BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, mark) != 4); + + *insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg, + offsetof(struct sk_buff, mark)); + break; + + case SKF_AD_PKTTYPE: + *insn++ = BPF_LDX_MEM(BPF_B, dst_reg, src_reg, PKT_TYPE_OFFSET()); + *insn++ = BPF_ALU32_IMM(BPF_AND, dst_reg, PKT_TYPE_MAX); +#ifdef __BIG_ENDIAN_BITFIELD + *insn++ = BPF_ALU32_IMM(BPF_RSH, dst_reg, 5); +#endif + break; + + case SKF_AD_QUEUE: + BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, queue_mapping) != 2); + + *insn++ = BPF_LDX_MEM(BPF_H, dst_reg, src_reg, + offsetof(struct sk_buff, queue_mapping)); + break; + } + + return insn - insn_buf; +} + static bool convert_bpf_extensions(struct sock_filter *fp, struct bpf_insn **insnp) { struct bpf_insn *insn = *insnp; + u32 cnt; switch (fp->k) { case SKF_AD_OFF + SKF_AD_PROTOCOL:@@ -167,13 +200,8 @@ static bool convert_bpf_extensions(struct sock_filter *fp, break; case SKF_AD_OFF + SKF_AD_PKTTYPE: - *insn++ = BPF_LDX_MEM(BPF_B, BPF_REG_A, BPF_REG_CTX, - PKT_TYPE_OFFSET()); - *insn = BPF_ALU32_IMM(BPF_AND, BPF_REG_A, PKT_TYPE_MAX); -#ifdef __BIG_ENDIAN_BITFIELD - insn++; - *insn = BPF_ALU32_IMM(BPF_RSH, BPF_REG_A, 5); -#endif + cnt = convert_skb_access(SKF_AD_PKTTYPE, BPF_REG_A, BPF_REG_CTX, insn); + insn += cnt - 1; break; case SKF_AD_OFF + SKF_AD_IFINDEX:@@ -197,10 +225,8 @@ static bool convert_bpf_extensions(struct sock_filter *fp, break; case SKF_AD_OFF + SKF_AD_MARK: - BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, mark) != 4); - - *insn = BPF_LDX_MEM(BPF_W, BPF_REG_A, BPF_REG_CTX, - offsetof(struct sk_buff, mark)); + cnt = convert_skb_access(SKF_AD_MARK, BPF_REG_A, BPF_REG_CTX, insn); + insn += cnt - 1; break; case SKF_AD_OFF + SKF_AD_RXHASH:@@ -211,10 +237,8 @@ static bool convert_bpf_extensions(struct sock_filter *fp, break; case SKF_AD_OFF + SKF_AD_QUEUE: - BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, queue_mapping) != 2); - - *insn = BPF_LDX_MEM(BPF_H, BPF_REG_A, BPF_REG_CTX, - offsetof(struct sk_buff, queue_mapping)); + cnt = convert_skb_access(SKF_AD_QUEUE, BPF_REG_A, BPF_REG_CTX, insn); + insn += cnt - 1; break; case SKF_AD_OFF + SKF_AD_VLAN_TAG:@@ -1147,13 +1171,55 @@ sk_filter_func_proto(enum bpf_func_id func_id)
...
+static u32 sk_filter_convert_ctx_access(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 __sk_buff, len):
+ BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, len) != 4);
+
+ *insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
+ offsetof(struct sk_buff, len));
+ break;
+
+ case offsetof(struct __sk_buff, mark):
+ return convert_skb_access(SKF_AD_MARK, dst_reg, src_reg, insn);
+
+ case offsetof(struct __sk_buff, pkt_type):
+ return convert_skb_access(SKF_AD_PKTTYPE, dst_reg, src_reg, insn);
+
+ case offsetof(struct __sk_buff, queue_mapping):
+ return convert_skb_access(SKF_AD_QUEUE, dst_reg, src_reg, insn);
+ }
+
+ return insn - insn_buf;
}Hmm, I actually liked the previous version much better. :( Now, some members use convert_skb_access() and some skb members are converted directly in-place in both, convert_bpf_extensions() _and_ in sk_filter_convert_ctx_access(). Previously, it was much more consistent, which I like better. And only because of the simple BUILD_BUG_ON()? :/