bpf verifier log is truncated, how to get the full log?

Question

As the following, bpf verifier log is truncated at the last. How could I get the full log ?

368=mmmmmmmm fp-376=mmmmmmmm fp-432=mmmmmmmm fp-440=inv fp-448=inv fp-456=map_value fp-464=inv
389: (73) *(u8 *)(r3 +322) = r1
390: (71) r1 = *(u8 *)(r2 +713)
 R0=inv(id=0,umax_value=9223372036854775807,var_off=(0x0; 0x7fffffffffffffff)) R1_w=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R2_w=map_value(id=0,off=0,ks=260,vs=904,imm=0) R3=pkt(id=0,off=42,r=398,imm=0) R4_w=inv0 R6=invP0 R7=ctx(id=0,off=0,imm=0) R8=inv(id=0) R9=inv(id=9) R10=fp0 fp-32=????mmmm fp-40=mmmmmmmm 
fp-48=mmmmmmmm fp-56=mmmmmmmm fp-64=mmmmmmmm fp-72=mmmmmmmm fp-80=mmmmmmmm fp-88=mmmmmmmm fp-96=mmmmmmmm fp-104=mmmmmmmm fp-112=mmmmmmmm fp-120=mmmmmmmm fp-128=mmmmmmmm fp-136=mmmmmmmm fp-144=mmmmmmmm 
fp-152=mmmmmmmm fp-160=mmmmmmmm fp-168=mmmmmmmm fp-176=mmmmmmmm fp-184=mmmmmmmm fp-192=mmmmmmmm fp-200=mmmmmmmm fp-208=mmmmmmmm fp-216=mmmmmmmm fp-224=mmmmmmmm fp-232=mmmmmmmm fp-240=mmmmmmmm fp-248=mmmmmmmm fp-256=mmmmmmmm fp-264=mmmmmmmm fp-272=mmmmmmmm fp-280=mmmmmmmm fp-288=mmmmmmmm fp-296=mmmm???? 
fp-304=??mmmmmm fp-312=mmmmmmmm fp-320=mmmmmmmm fp-328=?mmmmmmm fp-336=mmmmmmmm fp-344= (truncated...)

supplement：

Under the guide of @Qeole, I have solved the problem.

cilium/ebpf implement，It can be used as a reference.

https://github.com/cilium/ebpf/commit/f365a1e12f0a2477c41ee907a917db6f9bd9cf72

Answer 1

You need to pass a larger buffer (and to indicate its length accordingly) to the verifier when you load your program.

The kernel receives a pointer to a union bpf_attr, which for loading programs starts like this:

    struct { /* anonymous struct used by BPF_PROG_LOAD command */
        __u32       prog_type;  /* one of enum bpf_prog_type */
        __u32       insn_cnt;
        __aligned_u64   insns;
        __aligned_u64   license;
        __u32       log_level;  /* verbosity level of verifier */
        __u32       log_size;   /* size of user buffer */
        __aligned_u64   log_buf;    /* user supplied buffer */

log_buf, of size log_size, is the buffer filled by the verifier. You don't usually set up these parameters yourself, how you should do it depends on what you use to load your program. Most loaders rely on libbpf, and in recent versions, they should automatically attempt to reload the program with a larger buffer size in case of error and if the verifier output is truncated.