Re: [PATCH v11 net-next 00/12] eBPF syscall, verifier, testsuite

[Alexei Starovoitov <ast@plumgrid.com>]

On Wed, Sep 10, 2014 at 2:03 AM, Daniel Borkmann <dborkman@redhat.com> wrote:
>>             struct { /* anonymous struct used by BPF_PROG_LOAD command */
>>                 enum bpf_prog_type    prog_type;
>>                 __u32                 insn_cnt;
>>                 const struct bpf_insn *insns;
>>                 const char            *license;
>>                 __u32                 log_level; /* verbosity level of
>> eBPF verifier */
>>                 __u32                 log_size;  /* size of user buffer */
>>                 void                  *log_buf;  /* user supplied buffer
>> */
>
>
> What is log buffer? Would that mean the verifier will return an error
> string if the program will not pass it, or if not, what other data?
> I think the man page is missing how to examine the returned verifier
> log buffer data.

yes. it's an error log (as text string for humans) from verifier.
It is briefly explained in the man page:
       EACCES For BPF_PROG_LOAD, though program has valid instructions, it was
              rejected, since it was deemed unsafe (may access disallowed mem-
              ory region or  uninitialized  stack/register  or  function  con-
              straints don't match actual types or misaligned access). In such
              case it is recommended to call bpf() again with  log_level  =  1
              and examine log_buf for specific reason provided by verifier.
and there is a whole section about it in
Documentation/networking/filter.txt
called "Understanding eBPF verifier messages".