Missing ')'

[valkum]

Hello again, this time with a real bug, I think.

/virtual/main.c:66:83: error: expected ')'
        bpf_dins_pkt(skb, (u64)ip+12, 0, 32, ibpf_dext_pkt(skb, (u64)ip+16, 0, 32);
                                                                                  ^
/virtual/main.c:66:21: note: to match this '('
        bpf_dins_pkt(skb, (u64)ip+12, 0, 32, ibpf_dext_pkt(skb, (u64)ip+16, 0, 32);

Here is a sample program that is causing the error.

#include <uapi/linux/bpf.h>
#include <uapi/linux/if_ether.h>
#include <uapi/linux/if_packet.h>
#include <uapi/linux/ip.h>
#include <uapi/linux/in.h>
#include <uapi/linux/udp.h>
#include <bcc/proto.h>

#define ETH_LEN 14

struct dns_hdr_t
{
    uint16_t id;
    uint16_t flags;
    uint16_t qdcount;
    uint16_t ancount;
    uint16_t nscount;
    uint16_t arcount;
} BPF_PACKET_HEADER;


struct dns_query_flags_t
{
  uint16_t qtype;
  uint16_t qclass;
} BPF_PACKET_HEADER;

struct dns_char_t
{
    char c;
} BPF_PACKET_HEADER;

struct Key {
  unsigned char p[32];
};

struct Leaf {
  // Not really needed in this example
  unsigned char p[4];
};

BPF_TABLE("hash", struct Key, struct Leaf, cache, 128);

int dns_test(struct __sk_buff *skb)
{
  u8 *cursor = 0;
  struct Key key = {};
  struct ethernet_t *ethernet = cursor_advance(cursor, sizeof(*ethernet));
  if(ethernet->type == ETH_P_IP) {

    struct ip_t *ip = cursor_advance(cursor, sizeof(*ip));
    u16 hlen_bytes = ip->hlen << 2;
    if(ip->nextp == IPPROTO_UDP) {

      struct udp_t *udp = cursor_advance(cursor, sizeof(*udp));
      if(udp->dport == 53){
        u8 *sentinel = cursor + udp->length - sizeof(*udp) - 4;

        struct dns_hdr_t *dns_hdr = cursor_advance(cursor, sizeof(*dns_hdr));
        if((dns_hdr->flags >>15) != 0) {
          return -1;
        }
         // Error is happening here.
          ip->src = ip->dst;
          return 0;
      }
    }
  }

  return -1;
}

[drzaeus77]

This is a rewriter deficiency. Workaround until this issue is fixed should be to do:

u32 tmp_dst = ip->dst;
ip->src = tmp_dst;

[mauriciovasquezbernal]

Hi @drzaeus77, there are also some issues when defines are used:

#include <bcc/proto.h>

#define ETHERNET_BROADCAST 0xffffffffffff

int http_filter(struct __sk_buff *skb) {
    u8 *cursor = 0;
    struct ethernet_t *ethernet = cursor_advance(cursor, sizeof(*ethernet));
    ethernet->src = ETHERNET_BROADCAST;
    return -1;
}

fails to compile with the following error:

/virtual/main.c:14:44: error: expected expression
        bpf_dins_pkt(skb, (u64)ethernet+6, 0, 48, )     ethernet->src = ETHERNET_BROADCAST;

A workaround is to enclose the define in parentheses:

ethernet->src = (ETHERNET_BROADCAST);

PD: please ignore any possible verifier error on this example, I created it only to illustrate this bcc issue.

[tu-nv]

Today I faced the error when both rewriter and macro are used. Master branch, ubuntu 19.10, kernel 5.3. I think this is an error, although the parentheses trick does work.

#include <linux/if_ether.h>
#include <bcc/proto.h>
#include <uapi/linux/pkt_cls.h>

#define PKT_LEN_ADD 1


int mon_ingress(struct __sk_buff *skb) {

    u8 *cursor = 0;

    struct ethernet_t *eth = cursor_advance(cursor, sizeof(*eth));
    if (eth->type != ETH_P_IP) {
        return TC_ACT_OK;
    }

    struct ip_t *ip = cursor_advance(cursor, sizeof(*ip));

    ip->tlen += PKT_LEN_ADD;

    return TC_ACT_OK;
}

and the error:

/virtual/main.c:31:52: error: expected ')'
    bpf_dins_pkt(skb, (u64)ip+2, 0, 16, PKT_LEN_ADD;
                                                   ^
/virtual/main.c:31:17: note: to match this '('
    bpf_dins_pkt(skb, (u64)ip+2, 0, 16, PKT_LEN_ADD;
                ^
1 error generated.

[yonghong-song]

The bcc does not have enough information to do rewrite. So let us explicitly warn user the issues instead of silently generating incorrect code.
#2936