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

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Message-ID: <475c2645-01fc-7f25-da78-7ff180760b97@fb.com>
Date:   Wed, 13 Dec 2017 07:50:38 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>, Yonghong Song <yhs@...com>,
        <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 7:44 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.