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

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Date:	Wed, 10 Sep 2014 10:32:40 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...uxfoundation.org>,
	Kees Cook <keescook@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 net-next 00/12] eBPF syscall, verifier, testsuite

On Wed, Sep 10, 2014 at 2:03 AM, Daniel Borkmann <dborkman@...hat.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".
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