netdev - Re: [PATCH bpf-next 0/2] bpf: remove incorrect 'verifier bug' warning

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Date:   Wed, 20 Mar 2019 23:31:54 +0000
From:   Yonghong Song <yhs@...com>
To:     Paul Chaignon <paul.chaignon@...nge.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
CC:     "xiao.han@...nge.com" <xiao.han@...nge.com>,
        "paul.chaignon@...il.com" <paul.chaignon@...il.com>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>
Subject: Re: [PATCH bpf-next 0/2] bpf: remove incorrect 'verifier bug' warning



On 3/20/19 5:57 AM, Paul Chaignon wrote:
> The BPF verifier checks the maximum number of call stack frames twice,
> first in the main CFG traversal (do_check) and then in a subsequent
> traversal (check_max_stack_depth).  If the second check fails, it logs a
> 'verifier bug' warning and errors out, as the number of call stack frames
> should have been verified already.
> 
> However, the second check may fail without indicating a verifier bug: if
> the excessive function calls reside in dead code, the main CFG traversal
> may not visit them; the subsequent traversal visits all instructions,
> including dead code.
> 
> This case raises the question of how invalid dead code should be treated.

Maybe we should do this check after dead code elimination to be 
consistent with do_check? There could some other kinds of illegal stuff
in the dead code, e.g., illegal/unsupported helpers, etc. I suppose we 
did not warn or reject the program, right?

> The first patch implements the conservative option and rejects such code;
> the second adds a test case.
> 
> Paul Chaignon (2):
>    bpf: remove incorrect 'verifier bug' warning
>    selftests/bpf: test case for invalid call stack in dead code
> 
>   kernel/bpf/verifier.c                        |  5 +--
>   tools/testing/selftests/bpf/verifier/calls.c | 38 ++++++++++++++++++++
>   2 files changed, 41 insertions(+), 2 deletions(-)
>