netdev - Re: [PATCH 1/2] bpf: Remove struct bpf_verifier_env argument from print_bpf

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Date:   Thu, 22 Mar 2018 10:34:18 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jiri Olsa <jolsa@...hat.com>,
        Quentin Monnet <quentin.monnet@...ronome.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] bpf: Remove struct bpf_verifier_env argument from
 print_bpf_insn

On 03/21/2018 07:37 PM, Jiri Olsa wrote:
> On Wed, Mar 21, 2018 at 05:25:33PM +0000, Quentin Monnet wrote:
>> 2018-03-21 16:02 UTC+0100 ~ Jiri Olsa <jolsa@...nel.org>
>>> We use print_bpf_insn in user space (bpftool and soon perf),
>>> so it'd be nice to keep it generic and strip it off the kernel
>>> struct bpf_verifier_env argument.
>>>
>>> This argument can be safely removed, because its users can
>>> use the struct bpf_insn_cbs::private_data to pass it.
>>>
>>> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
>>> ---
>>>  kernel/bpf/disasm.c   | 52 +++++++++++++++++++++++++--------------------------
>>>  kernel/bpf/disasm.h   |  5 +----
>>>  kernel/bpf/verifier.c |  6 +++---
>>>  3 files changed, 30 insertions(+), 33 deletions(-)
>>
>> [...]
>>
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index c6eff108aa99..9f27d3fa7259 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -202,8 +202,7 @@ EXPORT_SYMBOL_GPL(bpf_verifier_log_write);
>>>   * generic for symbol export. The function was renamed, but not the calls in
>>>   * the verifier to avoid complicating backports. Hence the alias below.
>>>   */
>>> -static __printf(2, 3) void verbose(struct bpf_verifier_env *env,
>>> -				   const char *fmt, ...)
>>> +static __printf(2, 3) void verbose(void *private_data, const char *fmt, ...)
>>>  	__attribute__((alias("bpf_verifier_log_write")));
>>
>> Just as a note, verbose() will be aliased to a function whose prototype
>> differs (bpf_verifier_log_write() still expects a struct
>> bpf_verifier_env as its first argument). I am not so familiar with
>> function aliases, could this change be a concern?
> 
> yea, but as it was pointer for pointer switch I did not
> see any problem with that.. I'll check more

Ok, holding off for now until we have clarification. Other option could also
be to make it void *private_data everywhere and for the kernel writer then
do struct bpf_verifier_env *env = private_data.