netdev - Re: [PATCH net-next] bpf/tracing: fix kernel/events/core.c compilation error

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Date:   Wed, 13 Dec 2017 09:26:05 -0800
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, <sfr@...b.auug.org.au>,
        <netdev@...r.kernel.org>
CC:     <kernel-team@...com>
Subject: Re: [PATCH net-next] bpf/tracing: fix kernel/events/core.c
 compilation error



On 12/13/17 7:50 AM, Alexei Starovoitov wrote:
> On 12/13/17 7:44 AM, Daniel Borkmann wrote:
>> On 12/13/2017 08:42 AM, Yonghong Song wrote:
>>> Commit f371b304f12e ("bpf/tracing: allow user space to
>>> query prog array on the same tp") introduced a perf
>>> ioctl command to query prog array attached to the
>>> same perf tracepoint. The commit introduced a
>>> compilation error when either CONFIG_BPF_SYSCALL or
>>> CONFIG_EVENT_TRACING is not defined:
>>>   kernel/events/core.o: In function `perf_ioctl':
>>>   core.c:(.text+0x98c4): undefined reference to 
>>> `bpf_event_query_prog_array'
>>>
>>> This patch fixed this error.
>>>
>>> Fixes: f371b304f12e ("bpf/tracing: allow user space to query prog 
>>> array on the same tp")
>>> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
>>> Signed-off-by: Yonghong Song <yhs@...com>
>>
>> Looking at _perf_ioctl(), we also have perf_event_set_bpf_prog()
>> there. It's basically under CONFIG_EVENT_TRACING, which later calls
>> perf_event_attach_bpf_prog() which is under CONFIG_BPF_EVENTS, so
>> where we have the dummy handler returning -EOPNOTSUPP when BPF
>> events is not set. bpf_trace.c is only built when CONFIG_BPF_EVENTS
>> is set and that by itself depends on BPF_SYSCALL already. So it would
>> be more correct to do the same thing here ...
>>
>> #if defined(CONFIG_EVENT_TRACING) && defined(CONFIG_BPF_EVENTS)
>> [...]
> 
> +1
> #ifdef CONFIG_BPF_EVENTS
> works, whereas CONFIG_EVENT_TRACING probably not, since kprobe
> can be disabled independently which will turn off BPF_EVENTS
> and body of bpf_event_query_prog_array() will be gone.

I tested to enable/disable uprobe/kprobe/both and my patch works.
But I did not test  enable a non uprobe/kprobe tracing event
(e.g., CONFIG_FUNCTION_TRACER) where CONFIG_TRACING and 
CONFIG_EVENT_TRACING is on but CONFIG_UPROBES_EVENT/CONFIG_KPROBES_EVENT
is off and then my patch breaks.

Looks like
#ifdef CONFIG_BPF_EVENTS
is suffice.
This config will enable to include bpf_trace.c with the real definition. 
It will depend on KPROBE_EVENTS or UPROBE_EVENTS and either
of them will enable CONFIG_TRACING and then CONFIG_EVENT_TRACING.

Will resubmit the patch after testing.