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

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Date:   Wed, 13 Dec 2017 16:44:30 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, ast@...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 12/13/2017 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)
[...]

> ---
>  include/linux/bpf.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 93e15b9..8dbbfd7 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -254,7 +254,6 @@ typedef unsigned long (*bpf_ctx_copy_t)(void *dst, const void *src,
>  
>  u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
>  		     void *ctx, u64 ctx_size, bpf_ctx_copy_t ctx_copy);
> -int bpf_event_query_prog_array(struct perf_event *event, void __user *info);
>  
>  int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>  			  union bpf_attr __user *uattr);
> @@ -558,6 +557,15 @@ static inline int sock_map_prog(struct bpf_map *map,
>  }
>  #endif
>  
> +#if defined(CONFIG_EVENT_TRACING) && defined(CONFIG_BPF_SYSCALL)
> +int bpf_event_query_prog_array(struct perf_event *event, void __user *info);
> +#else
> +static inline int bpf_event_query_prog_array(struct perf_event *event, void __user *info)
> +{
> +	return -EOPNOTSUPP;
> +}
> +#endif
> +
>  /* verifier prototypes for helper functions called from eBPF programs */
>  extern const struct bpf_func_proto bpf_map_lookup_elem_proto;
>  extern const struct bpf_func_proto bpf_map_update_elem_proto;
>