Subject: Re: [PATCH v2 net-next 1/2] bpf: allow extended BPF programs access skb fields

From: Alexei Starovoitov <[email-masked]>
Date: Fri, 13 Mar 2015 19:07:15 -0700

On 3/13/15 6:46 PM, Daniel Borkmann wrote:
> 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 <[email-masked]>
> ...
>> +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()? :/

not because of single build_bug_on, but because of having
a single place to adjust offsets and sizes when location of
sk_buff fields changes. that's the main advantage and it's a big one.
imo it's much cleaner than previous approach.